SUNRPC: Add sysctl variables for server TCP snd/rcv buffer values

The motivation for this patch is improved WAN write performance plus greater user control on the server of the TCP buffer values (window size). The TCP window determines the amount of outstanding data that a client can have on the wire and should be large enough that a NFS client can fill up the pipe (the bandwidth * delay product). Currently the TCP receive buffer size (used for client writes) is set very low, which prevents a client from filling up a network pipe with a large bandwidth * delay product.

Currently, the server TCP send window is set to accommodate the maximum number of outstanding NFSD read requests (# nfsds * maxiosize), while the server TCP receive window is set to a fixed value which can hold a few requests. While these values set a TCP window size that is fine in LAN environments with a small BDP, WAN environments can require a much larger TCP window size, e.g., 10GigE transatlantic link with a rtt of 120 ms has a BDP of approx 60MB.

I have a patch to net/svc/svcsock.c that allows a user to manually set the server TCP send and receive buffer through the sysctl interface. to suit the required TCP window of their network architecture. It adds two /proc entries, one for the receive buffer size and one for the send buffer size:
/proc/sys/sunrpc/tcp_sndbuf
/proc/sys/sunrpc/tcp_rcvbuf

The uses the current buffer sizes in the code are as minimum values, which the user cannot decrease. If the user sets a value of 0 in either /proc entry, it resets the buffer size to the default value. The set /proc values are utilized when the TCP connection is initialized (mount time). The values are bounded above by the *minimum* of the /proc values and the network TCP sysctls.

To demonstrate the usefulness of this patch, details of an experiment between 2 computers with a rtt of 30ms is provided below. In this experiment, increasing the server /proc/sys/sunrpc/tcp_rcvbuf value doubles write performance.

EXPERIMENT
==========
This experiment simulates a WAN by using tc together with netem to add a 30 ms delay to all packets on a nfs client. The goal is to show that by only changing tcp_rcvbuf, the nfs client can increase write performance in the WAN. To verify the patch has the desired effect on the TCP window, I created two tcptrace plots that show the difference in tcp window behaviour before and after the server TCP rcvbuf size is increased. When using the default server tcpbuf value of 6M, we can see the TCP window top out around 4.6 M, whereas increasing the server tcpbuf value to 32M, we can see that the TCP window tops out around 13M. Performance jumps from 43 MB/s to 90 MB/s.

Hardware:
2 dual-core opteron blades
GigE, Broadcom NetXtreme II BCM57065 cards
A single gigabit switch in the middle
1500 MTU
8 GB memory

Software:
Kernel: Bruce's 2.6.25-rc9-CITI_NFS4_ALL-1 tree
RHEL4

NFS Configuration:
64 rpc slots
32 nfsds
Export ext3 file system. This disk is quite slow, I therefore exported using async to reduce the effect of the disk on the back end. This way, the experiments record the time it takes for the data to get to the server (not to the disk).

# exportfs -v
/export     <world>(rw,async,wdelay,nohide,insecure,no_root_squash,fsid=0)

# cat /proc/mounts
bear109:/export /mnt nfs rw,vers=3,rsize=1048576,wsize=1048576,namlen=255,hard,nointr,proto=tcp,timeo=600,retrans=2,sec=sys,mountproto=udp,addr=9.1.74.144 0 0

fs.nfs.nfs_congestion_kb = 91840
net.ipv4.tcp_congestion_control = cubic

Network tc Command executed on client:
tc qdisc add dev eth0 root netem delay 30ms
rtt from client (bear108) to server (bear109)
#ping bear109
PING bear109.almaden.ibm.com (9.1.74.144) 56(84) bytes of data.
64 bytes from bear109.almaden.ibm.com (9.1.74.144): icmp_seq=0 ttl=64 time=31.4 ms 64 bytes from bear109.almaden.ibm.com (9.1.74.144): icmp_seq=1 ttl=64 time=32.0 ms

TCP Configuration on client and server:
# Controls IP packet forwarding
net.ipv4.ip_forward = 0
# Controls source route verification net.ipv4.conf.default.rp_filter = 1
# Do not accept source routing net.ipv4.conf.default.accept_source_route = 0
# Controls the System Request debugging functionality of the kernel kernel.sysrq = 0
# Controls whether core dumps will append the PID to the core filename # Useful for debugging multi-threaded applications kernel.core_uses_pid = 1
# Controls the use of TCP syncookies net.ipv4.tcp_syncookies = 1
# Controls the maximum size of a message, in bytes kernel.msgmnb = 65536
# Controls the default maxmimum size of a mesage queue kernel.msgmax = 65536
# Controls the maximum shared segment size, in bytes kernel.shmmax = 68719476736
# Controls the maximum number of shared memory segments, in pages kernel.shmall = 4294967296
### IPV4 specific settings
net.ipv4.tcp_timestamps = 0
net.ipv4.tcp_sack = 1

# on systems with a VERY fast bus -> memory interface this is the big gainer
net.ipv4.tcp_rmem = 4096 16777216 16777216
net.ipv4.tcp_wmem = 4096 16777216 16777216
net.ipv4.tcp_mem = 4096 16777216 16777216
### CORE settings (mostly for socket and UDP effect)
net.core.rmem_max = 16777216
net.core.wmem_max = 16777216
net.core.rmem_default = 16777216
net.core.wmem_default = 16777216
net.core.optmem_max =  16777216
net.core.netdev_max_backlog = 300000
# Don't cache ssthresh from previous connection
net.ipv4.tcp_no_metrics_save = 1
# make sure we don't run out of memory
vm.min_free_kbytes = 32768

Experiments:

On Server: (note that the real tcp buffer size is double tcp_rcvbuf) [root@bear109 ~]# echo 0 > /proc/sys/sunrpc/tcp_rcvbuf [root@bear109 ~]# cat /proc/sys/sunrpc/tcp_rcvbuf
3158016

On Client:
mount -t nfs bear109:/export /mnt
[root@bear108 ~]# iozone -aec -i 0 -+n -f /mnt/test -r 1M -s 500M
...
KB reclen write
512000 1024 43252 umount /mnt

On server:
[root@bear109 ~]# echo 16777216 > /proc/sys/sunrpc/tcp_rcvbuf [root@bear109 ~]# cat /proc/sys/sunrpc/tcp_rcvbuf 16777216

On Client:
mount -t nfs bear109:/export /mnt
[root@bear108 ~]# iozone -aec -i 0 -+n -f /mnt/test -r 1M -s 500M
...
KB reclen write
512000 1024 90396

There are also fairly unsophisticated ways to add longer delays on your test network, and turning up the latency knob would be a useful test.

Howdy Dean-

Quoted 3 lines · Dean Hildebrand

The motivation for this patch is improved WAN write performance plus greater user control on the server of the TCP buffer values (window size). The TCP window determines the amount of outstanding data that a client can have on the wire and should be large enough that a NFS client can fill up the pipe (the bandwidth * delay product). Currently the TCP receive buffer size (used for client writes) is set very low, which prevents a client from filling up a network pipe with a large bandwidth * delay product.

Currently, the server TCP send window is set to accommodate the maximum number of outstanding NFSD read requests (# nfsds * maxiosize), while the server TCP receive window is set to a fixed value which can hold a few requests. While these values set a TCP window size that is fine in LAN environments with a small BDP, WAN environments can require a much larger TCP window size, e.g., 10GigE transatlantic link with a rtt of 120 ms has a BDP of approx 60MB.

Was the receive buffer size computation adjusted when support for large transfer sizes was recently added to the NFS server?

Quoted 4 lines

I have a patch to net/svc/svcsock.c that allows a user to manually set the server TCP send and receive buffer through the sysctl interface. to suit the required TCP window of their network architecture. It adds two /proc entries, one for the receive buffer size and one for the send buffer size:
/proc/sys/sunrpc/tcp_sndbuf
/proc/sys/sunrpc/tcp_rcvbuf

What I'm wondering is if we can find some algorithm to set the buffer and window sizes *automatically*. Why can't the NFS server select an appropriately large socket buffer size by default?

Since the socket buffer size is just a limit (no memory is allocated) why, for example, shouldn't the buffer size be large for all environments that have sufficient physical memory?

Quoted 101 lines

The uses the current buffer sizes in the code are as minimum values, which the user cannot decrease. If the user sets a value of 0 in either /proc entry, it resets the buffer size to the default value. The set /proc values are utilized when the TCP connection is initialized (mount time). The values are bounded above by the *minimum* of the /proc values and the network TCP sysctls.

To demonstrate the usefulness of this patch, details of an experiment between 2 computers with a rtt of 30ms is provided below. In this experiment, increasing the server /proc/sys/sunrpc/tcp_rcvbuf value doubles write performance.

EXPERIMENT
==========
This experiment simulates a WAN by using tc together with netem to add a 30 ms delay to all packets on a nfs client. The goal is to show that by only changing tcp_rcvbuf, the nfs client can increase write performance in the WAN. To verify the patch has the desired effect on the TCP window, I created two tcptrace plots that show the difference in tcp window behaviour before and after the server TCP rcvbuf size is increased. When using the default server tcpbuf value of 6M, we can see the TCP window top out around 4.6 M, whereas increasing the server tcpbuf value to 32M, we can see that the TCP window tops out around 13M. Performance jumps from 43 MB/s to 90 MB/s.

Hardware:
2 dual-core opteron blades
GigE, Broadcom NetXtreme II BCM57065 cards
A single gigabit switch in the middle
1500 MTU
8 GB memory

Software:
Kernel: Bruce's 2.6.25-rc9-CITI_NFS4_ALL-1 tree
RHEL4

NFS Configuration:
64 rpc slots
32 nfsds
Export ext3 file system. This disk is quite slow, I therefore exported using async to reduce the effect of the disk on the back end. This way, the experiments record the time it takes for the data to get to the server (not to the disk).

# exportfs -v
/export     <world>(rw,async,wdelay,nohide,insecure,no_root_squash,fsid=0)

# cat /proc/mounts
bear109:/export /mnt nfs rw,vers=3,rsize=1048576,wsize=1048576,namlen=255,hard,nointr,proto=tcp,timeo=600,retrans=2,sec=sys,mountproto=udp,addr=9.1.74.144 0 0

fs.nfs.nfs_congestion_kb = 91840
net.ipv4.tcp_congestion_control = cubic

Network tc Command executed on client:
tc qdisc add dev eth0 root netem delay 30ms
rtt from client (bear108) to server (bear109)
#ping bear109
PING bear109.almaden.ibm.com (9.1.74.144) 56(84) bytes of data.
64 bytes from bear109.almaden.ibm.com (9.1.74.144): icmp_seq=0 ttl=64 time=31.4 ms 64 bytes from bear109.almaden.ibm.com (9.1.74.144): icmp_seq=1 ttl=64 time=32.0 ms

TCP Configuration on client and server:
# Controls IP packet forwarding
net.ipv4.ip_forward = 0
# Controls source route verification net.ipv4.conf.default.rp_filter = 1
# Do not accept source routing net.ipv4.conf.default.accept_source_route = 0
# Controls the System Request debugging functionality of the kernel kernel.sysrq = 0
# Controls whether core dumps will append the PID to the core filename # Useful for debugging multi-threaded applications kernel.core_uses_pid = 1
# Controls the use of TCP syncookies net.ipv4.tcp_syncookies = 1
# Controls the maximum size of a message, in bytes kernel.msgmnb = 65536
# Controls the default maxmimum size of a mesage queue kernel.msgmax = 65536
# Controls the maximum shared segment size, in bytes kernel.shmmax = 68719476736
# Controls the maximum number of shared memory segments, in pages kernel.shmall = 4294967296
### IPV4 specific settings
net.ipv4.tcp_timestamps = 0
net.ipv4.tcp_sack = 1

# on systems with a VERY fast bus -> memory interface this is the big gainer
net.ipv4.tcp_rmem = 4096 16777216 16777216
net.ipv4.tcp_wmem = 4096 16777216 16777216
net.ipv4.tcp_mem = 4096 16777216 16777216
### CORE settings (mostly for socket and UDP effect)
net.core.rmem_max = 16777216
net.core.wmem_max = 16777216
net.core.rmem_default = 16777216
net.core.wmem_default = 16777216
net.core.optmem_max =  16777216
net.core.netdev_max_backlog = 300000
# Don't cache ssthresh from previous connection
net.ipv4.tcp_no_metrics_save = 1
# make sure we don't run out of memory
vm.min_free_kbytes = 32768

Experiments:

On Server: (note that the real tcp buffer size is double tcp_rcvbuf) [root@bear109 ~]# echo 0 > /proc/sys/sunrpc/tcp_rcvbuf [root@bear109 ~]# cat /proc/sys/sunrpc/tcp_rcvbuf
3158016

On Client:
mount -t nfs bear109:/export /mnt
[root@bear108 ~]# iozone -aec -i 0 -+n -f /mnt/test -r 1M -s 500M
...
KB reclen write
512000 1024 43252 umount /mnt

On server:
[root@bear109 ~]# echo 16777216 > /proc/sys/sunrpc/tcp_rcvbuf [root@bear109 ~]# cat /proc/sys/sunrpc/tcp_rcvbuf 16777216

Quoted 6 lines

On Client:
mount -t nfs bear109:/export /mnt
[root@bear108 ~]# iozone -aec -i 0 -+n -f /mnt/test -r 1M -s 500M
...
KB reclen write
512000 1024 90396

The numbers you have here are averages over the whole run. Performing these tests using a variety of record lengths and file sizes (up to several tens of gigabytes) would be useful to see where different memory and network latencies kick in.

In addition, have you looked at network traces to see if the server's TCP implementation is behaving optimally (or near optimally)? Have you tried using some of the more esoteric TCP congestion algorithms available in 2.6 kernels?

There are also fairly unsophisticated ways to add longer delays on your test network, and turning up the latency knob would be a useful test.

Signature · 3 lines

-- 
Chuck Lever
chuck[dot]lever[at]oracle[dot]com

Hi Chuck,

Quoted 8 lines · Chuck Lever

Howdy Dean-

Quoted 3 lines · Dean Hildebrand

The motivation for this patch is improved WAN write performance plus greater user control on the server of the TCP buffer values (window size). The TCP window determines the amount of outstanding data that a client can have on the wire and should be large enough that a NFS client can fill up the pipe (the bandwidth * delay product). Currently the TCP receive buffer size (used for client writes) is set very low, which prevents a client from filling up a network pipe with a large bandwidth * delay product.

Currently, the server TCP send window is set to accommodate the maximum number of outstanding NFSD read requests (# nfsds * maxiosize), while the server TCP receive window is set to a fixed value which can hold a few requests. While these values set a TCP window size that is fine in LAN environments with a small BDP, WAN environments can require a much larger TCP window size, e.g., 10GigE transatlantic link with a rtt of 120 ms has a BDP of approx 60MB.

Was the receive buffer size computation adjusted when support for large transfer sizes was recently added to the NFS server?

Yes, it is based on the transfer size. So in the current code, having a larger transfer size can improve efficiency PLUS help create a larger possible TCP window. The issue seems to be that tcp window, # of NFSDs, and transfer size are all independent variables that need to be tuned individually depending on rtt, network bandwidth, disk bandwidth, etc etc... We can adjust the last 2, so this patch helps adjust the first (tcp window).

Quoted 5 lines

Quoted 4 lines

I have a patch to net/svc/svcsock.c that allows a user to manually set the server TCP send and receive buffer through the sysctl interface. to suit the required TCP window of their network architecture. It adds two /proc entries, one for the receive buffer size and one for the send buffer size:
/proc/sys/sunrpc/tcp_sndbuf
/proc/sys/sunrpc/tcp_rcvbuf

Quoted 4 lines

What I'm wondering is if we can find some algorithm to set the buffer and window sizes *automatically*. Why can't the NFS server select an appropriately large socket buffer size by default?

Since the socket buffer size is just a limit (no memory is allocated) why, for example, shouldn't the buffer size be large for all environments that have sufficient physical memory?

I think the problem there is that the only way to set the buffer size automatically would be to know the rtt and bandwidth of the network connection. Excessive numbers of packets can get dropped if the TCP buffer is set too large for a specific network connection.

In this case, the window opens too wide and lets too many packets out into the system, somewhere along the path buffers start overflowing and packets are lost, TCP congestion avoidance kicks in and cuts the window size dramatically and performance along with it. This type of behaviour creates a sawtooth pattern for the TCP window, which is less favourable than a more steady state pattern that is created if the TCP buffer size is set appropriately.

Another point is that setting the buffer size isn't always a straightforward process. All papers I've read on the subject, and my experience confirms this, is that setting tcp buffer sizes is more of an art.

So having the server set a good default value is half the battle, but allowing users to twiddle with this value is vital.

Quoted 107 lines

Quoted 106 lines

The uses the current buffer sizes in the code are as minimum values, which the user cannot decrease. If the user sets a value of 0 in either /proc entry, it resets the buffer size to the default value. The set /proc values are utilized when the TCP connection is initialized (mount time). The values are bounded above by the *minimum* of the /proc values and the network TCP sysctls.

To demonstrate the usefulness of this patch, details of an experiment between 2 computers with a rtt of 30ms is provided below. In this experiment, increasing the server /proc/sys/sunrpc/tcp_rcvbuf value doubles write performance.

EXPERIMENT
==========
This experiment simulates a WAN by using tc together with netem to add a 30 ms delay to all packets on a nfs client. The goal is to show that by only changing tcp_rcvbuf, the nfs client can increase write performance in the WAN. To verify the patch has the desired effect on the TCP window, I created two tcptrace plots that show the difference in tcp window behaviour before and after the server TCP rcvbuf size is increased. When using the default server tcpbuf value of 6M, we can see the TCP window top out around 4.6 M, whereas increasing the server tcpbuf value to 32M, we can see that the TCP window tops out around 13M. Performance jumps from 43 MB/s to 90 MB/s.

Hardware:
2 dual-core opteron blades
GigE, Broadcom NetXtreme II BCM57065 cards
A single gigabit switch in the middle
1500 MTU
8 GB memory

Software:
Kernel: Bruce's 2.6.25-rc9-CITI_NFS4_ALL-1 tree
RHEL4

NFS Configuration:
64 rpc slots
32 nfsds
Export ext3 file system. This disk is quite slow, I therefore exported using async to reduce the effect of the disk on the back end. This way, the experiments record the time it takes for the data to get to the server (not to the disk).

# exportfs -v
/export     <world>(rw,async,wdelay,nohide,insecure,no_root_squash,fsid=0)

# cat /proc/mounts
bear109:/export /mnt nfs rw,vers=3,rsize=1048576,wsize=1048576,namlen=255,hard,nointr,proto=tcp,timeo=600,retrans=2,sec=sys,mountproto=udp,addr=9.1.74.144 0 0

fs.nfs.nfs_congestion_kb = 91840
net.ipv4.tcp_congestion_control = cubic

Network tc Command executed on client:
tc qdisc add dev eth0 root netem delay 30ms
rtt from client (bear108) to server (bear109)
#ping bear109
PING bear109.almaden.ibm.com (9.1.74.144) 56(84) bytes of data.
64 bytes from bear109.almaden.ibm.com (9.1.74.144): icmp_seq=0 ttl=64 time=31.4 ms 64 bytes from bear109.almaden.ibm.com (9.1.74.144): icmp_seq=1 ttl=64 time=32.0 ms

TCP Configuration on client and server:
# Controls IP packet forwarding
net.ipv4.ip_forward = 0
# Controls source route verification net.ipv4.conf.default.rp_filter = 1
# Do not accept source routing net.ipv4.conf.default.accept_source_route = 0
# Controls the System Request debugging functionality of the kernel kernel.sysrq = 0
# Controls whether core dumps will append the PID to the core filename # Useful for debugging multi-threaded applications kernel.core_uses_pid = 1
# Controls the use of TCP syncookies net.ipv4.tcp_syncookies = 1
# Controls the maximum size of a message, in bytes kernel.msgmnb = 65536
# Controls the default maxmimum size of a mesage queue kernel.msgmax = 65536
# Controls the maximum shared segment size, in bytes kernel.shmmax = 68719476736
# Controls the maximum number of shared memory segments, in pages kernel.shmall = 4294967296
### IPV4 specific settings
net.ipv4.tcp_timestamps = 0
net.ipv4.tcp_sack = 1

# on systems with a VERY fast bus -> memory interface this is the big gainer
net.ipv4.tcp_rmem = 4096 16777216 16777216
net.ipv4.tcp_wmem = 4096 16777216 16777216
net.ipv4.tcp_mem = 4096 16777216 16777216
### CORE settings (mostly for socket and UDP effect)
net.core.rmem_max = 16777216
net.core.wmem_max = 16777216
net.core.rmem_default = 16777216
net.core.wmem_default = 16777216
net.core.optmem_max =  16777216
net.core.netdev_max_backlog = 300000
# Don't cache ssthresh from previous connection
net.ipv4.tcp_no_metrics_save = 1
# make sure we don't run out of memory
vm.min_free_kbytes = 32768

Experiments:

On Server: (note that the real tcp buffer size is double tcp_rcvbuf) [root@bear109 ~]# echo 0 > /proc/sys/sunrpc/tcp_rcvbuf [root@bear109 ~]# cat /proc/sys/sunrpc/tcp_rcvbuf
3158016

On Client:
mount -t nfs bear109:/export /mnt
[root@bear108 ~]# iozone -aec -i 0 -+n -f /mnt/test -r 1M -s 500M
...
KB reclen write
512000 1024 43252 umount /mnt

On server:
[root@bear109 ~]# echo 16777216 > /proc/sys/sunrpc/tcp_rcvbuf [root@bear109 ~]# cat /proc/sys/sunrpc/tcp_rcvbuf 16777216

On Client:
mount -t nfs bear109:/export /mnt
[root@bear108 ~]# iozone -aec -i 0 -+n -f /mnt/test -r 1M -s 500M
...
KB reclen write
512000 1024 90396

Quoted 1 line

The numbers you have here are averages over the whole run. Performing these tests using a variety of record lengths and file sizes (up to several tens of gigabytes) would be useful to see where different memory and network latencies kick in.

Definitely useful, although I'm not sure how this relates to this patch.

This patch isn't trying to alter default values, or predict buffer sizes based on rtt values, or dynamically alter the tcp window based on dropped packets, etc, it is just giving users the ability to customize the server tcp buffer size.

The information you are curious about is more relevant to creating better default values of the tcp buffer size. This could be useful, but would be a long process and there are so many variables that I'm not sure that you could pick proper default values anyways. The important thing is that the client can currently set its tcp buffer size via the sysctl's, this is useless if the server is stuck at a fixed value since the tcp window will be the minimum of the client and server's tcp buffer sizes.

The server cannot do just the same thing as the client since it cannot just rely on the tcp sysctl's since it also needs to ensure it has enough buffer space for each NFSD.

My goal with this patch is to provide users with the same flexibility that the client has regarding tcp buffer sizes, but also ensure that the minimum amount of buffer space that the NFSDs require is allocated.

Quoted 5 lines · Dean Hildebrand

I think the problem there is that the only way to set the buffer size automatically would be to know the rtt and bandwidth of the network connection. Excessive numbers of packets can get dropped if the TCP buffer is set too large for a specific network connection.

Quoted 7 lines

In this case, the window opens too wide and lets too many packets out into the system, somewhere along the path buffers start overflowing and packets are lost, TCP congestion avoidance kicks in and cuts the window size dramatically and performance along with it. This type of behaviour creates a sawtooth pattern for the TCP window, which is less favourable than a more steady state pattern that is created if the TCP buffer size is set appropriately.

Agreed it is a performance problem, but I thought some of the newer TCP congestion algorithms were specifically designed to address this by not closing the window as aggressively.

Once the window is wide open, then, it would appear that choosing a good congestion avoidance algorithm is also important.

Quoted 9 lines · Dean Hildebrand

Hi Chuck,

Quoted 6 lines · Chuck Lever

Howdy Dean-

Quoted 3 lines · Dean Hildebrand

The motivation for this patch is improved WAN write performance=20 plus greater user control on the server of the TCP buffer values=20 (window size). The TCP window determines the amount of outstanding=

Quoted 1 line

Quoted 1 line

Quoted 1 line

data that a client can have on the wire and should be large enough=

Quoted 3 lines

Quoted 3 lines

Quoted 3 lines

that a NFS client can fill up the pipe (the bandwidth * delay=20 product). Currently the TCP receive buffer size (used for client=20 writes) is set very low, which prevents a client from filling up a=

Quoted 7 lines

Quoted 7 lines

Quoted 7 lines

network pipe with a large bandwidth * delay product.

Currently, the server TCP send window is set to accommodate the=20 maximum number of outstanding NFSD read requests (# nfsds *=20 maxiosize), while the server TCP receive window is set to a fixed=20 value which can hold a few requests. While these values set a TCP=20 window size that is fine in LAN environments with a small BDP, WAN=

Quoted 2 lines

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environments can require a much larger TCP window size, e.g.,=20 10GigE transatlantic link with a rtt of 120 ms has a BDP of approx=

Quoted 5 lines

Quoted 4 lines

Quoted 1 line

60MB.

Was the receive buffer size computation adjusted when support for=20 large transfer sizes was recently added to the NFS server?

Yes, it is based on the transfer size. So in the current code, havin=

Quoted 2 lines

a larger transfer size can improve efficiency PLUS help create a=20 larger possible TCP window. The issue seems to be that tcp window, #=

Quoted 6 lines

of NFSDs, and transfer size are all independent variables that need=20 to be tuned individually depending on rtt, network bandwidth, disk=20 bandwidth, etc etc... We can adjust the last 2, so this patch helps=20 adjust the first (tcp window).

Quoted 2 lines

Quoted 2 lines

I have a patch to net/svc/svcsock.c that allows a user to manually=

Quoted 3 lines

Quoted 3 lines

Quoted 3 lines

set the server TCP send and receive buffer through the sysctl=20 interface. to suit the required TCP window of their network=20 architecture. It adds two /proc entries, one for the receive buffe=

Quoted 11 lines

Quoted 11 lines

Quoted 3 lines

size and one for the send buffer size:
/proc/sys/sunrpc/tcp_sndbuf
/proc/sys/sunrpc/tcp_rcvbuf

What I'm wondering is if we can find some algorithm to set the=20 buffer and window sizes *automatically*. Why can't the NFS server=20 select an appropriately large socket buffer size by default?

Since the socket buffer size is just a limit (no memory is=20 allocated) why, for example, shouldn't the buffer size be large for=

Quoted 2 lines

Quoted 1 line

all environments that have sufficient physical memory?

I think the problem there is that the only way to set the buffer siz=

Quoted 5 lines

automatically would be to know the rtt and bandwidth of the network=20 connection. Excessive numbers of packets can get dropped if the TCP=20 buffer is set too large for a specific network connection.

In this case, the window opens too wide and lets too many packets ou=

Quoted 2 lines

into the system, somewhere along the path buffers start overflowing=20 and packets are lost, TCP congestion avoidance kicks in and cuts the=

Quoted 1 line

window size dramatically and performance along with it. This type of=

Quoted 3 lines

behaviour creates a sawtooth pattern for the TCP window, which is=20 less favourable than a more steady state pattern that is created if=20 the TCP buffer size is set appropriately.

Agreed it is a performance problem, but I thought some of the newer=20 TCP congestion algorithms were specifically designed to address this=20 by not closing the window as aggressively.

Once the window is wide open, then, it would appear that choosing a=20 good congestion avoidance algorithm is also important.

Quoted 3 lines

Another point is that setting the buffer size isn't always a=20 straightforward process. All papers I've read on the subject, and my=

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experience confirms this, is that setting tcp buffer sizes is more o=

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an art.

So having the server set a good default value is half the battle, bu=

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allowing users to twiddle with this value is vital.

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The uses the current buffer sizes in the code are as minimum=20 values, which the user cannot decrease. If the user sets a value o=

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0 in either /proc entry, it resets the buffer size to the default=20 value. The set /proc values are utilized when the TCP connection i=

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initialized (mount time). The values are bounded above by the=20 *minimum* of the /proc values and the network TCP sysctls.

To demonstrate the usefulness of this patch, details of an=20 experiment between 2 computers with a rtt of 30ms is provided=20 below. In this experiment, increasing the server=20 /proc/sys/sunrpc/tcp_rcvbuf value doubles write performance.

EXPERIMENT
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
This experiment simulates a WAN by using tc together with netem to=

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add a 30 ms delay to all packets on a nfs client. The goal is to=20 show that by only changing tcp_rcvbuf, the nfs client can increase=

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write performance in the WAN. To verify the patch has the desired=20 effect on the TCP window, I created two tcptrace plots that show=20 the difference in tcp window behaviour before and after the server=

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TCP rcvbuf size is increased. When using the default server tcpbuf=

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value of 6M, we can see the TCP window top out around 4.6 M,=20 whereas increasing the server tcpbuf value to 32M, we can see that=

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the TCP window tops out around 13M. Performance jumps from 43 MB/s=

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to 90 MB/s.

Hardware:
2 dual-core opteron blades
GigE, Broadcom NetXtreme II BCM57065 cards
A single gigabit switch in the middle
1500 MTU
8 GB memory

Software:
Kernel: Bruce's 2.6.25-rc9-CITI_NFS4_ALL-1 tree
RHEL4

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NFS Configuration:
64 rpc slots
32 nfsds
Export ext3 file system. This disk is quite slow, I therefore=20 exported using async to reduce the effect of the disk on the back=20 end. This way, the experiments record the time it takes for the=20 data to get to the server (not to the disk).

# exportfs -v
/export <world>(rw,async,wdelay,nohide,insecure,no_root_squash,fsi=

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# cat /proc/mounts
bear109:/export /mnt nfs=20
rw,vers=3D3,rsize=3D1048576,wsize=3D1048576,namlen=3D255,hard,noin=

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0 0

fs.nfs.nfs_congestion_kb =3D 91840
net.ipv4.tcp_congestion_control =3D cubic

Network tc Command executed on client:
tc qdisc add dev eth0 root netem delay 30ms
rtt from client (bear108) to server (bear109)
#ping bear109
PING bear109.almaden.ibm.com (9.1.74.144) 56(84) bytes of data. 64 bytes from bear109.almaden.ibm.com (9.1.74.144): icmp_seq=3D0=20 ttl=3D64 time=3D31.4 ms
64 bytes from bear109.almaden.ibm.com (9.1.74.144): icmp_seq=3D1=20 ttl=3D64 time=3D32.0 ms

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TCP Configuration on client and server:
# Controls IP packet forwarding
net.ipv4.ip_forward =3D 0
# Controls source route verification net.ipv4.conf.default.rp_filter =3D 1
# Do not accept source routing net.ipv4.conf.default.accept_source_route =3D 0
# Controls the System Request debugging functionality of the kerne=

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kernel.sysrq =3D 0
# Controls whether core dumps will append the PID to the core file=

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# Useful for debugging multi-threaded applications kernel.core_uses_pid =3D 1
# Controls the use of TCP syncookies net.ipv4.tcp_syncookies =3D 1
# Controls the maximum size of a message, in bytes kernel.msgmnb =3D 65536
# Controls the default maxmimum size of a mesage queue kernel.msgmax =3D 65536
# Controls the maximum shared segment size, in bytes kernel.shmmax =3D 68719476736
# Controls the maximum number of shared memory segments, in pages kernel.shmall =3D 4294967296
### IPV4 specific settings
net.ipv4.tcp_timestamps =3D 0
net.ipv4.tcp_sack =3D 1

# on systems with a VERY fast bus -> memory interface this is the=20
big gainer
net.ipv4.tcp_rmem =3D 4096 16777216 16777216
net.ipv4.tcp_wmem =3D 4096 16777216 16777216
net.ipv4.tcp_mem =3D 4096 16777216 16777216
### CORE settings (mostly for socket and UDP effect)
net.core.rmem_max =3D 16777216
net.core.wmem_max =3D 16777216
net.core.rmem_default =3D 16777216
net.core.wmem_default =3D 16777216
net.core.optmem_max =3D 16777216
net.core.netdev_max_backlog =3D 300000
# Don't cache ssthresh from previous connection
net.ipv4.tcp_no_metrics_save =3D 1
# make sure we don't run out of memory
vm.min_free_kbytes =3D 32768

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Experiments:

On Server: (note that the real tcp buffer size is double tcp_rcvbu=

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[root@bear109 ~]# echo 0 > /proc/sys/sunrpc/tcp_rcvbuf [root@bear109 ~]# cat /proc/sys/sunrpc/tcp_rcvbuf 3158016

On Client:
mount -t nfs bear109:/export /mnt
[root@bear108 ~]# iozone -aec -i 0 -+n -f /mnt/test -r 1M -s 500M
...
KB reclen write
512000 1024 43252 umount /mnt

On server:
[root@bear109 ~]# echo 16777216 > /proc/sys/sunrpc/tcp_rcvbuf [root@bear109 ~]# cat /proc/sys/sunrpc/tcp_rcvbuf 16777216

On Client:
mount -t nfs bear109:/export /mnt
[root@bear108 ~]# iozone -aec -i 0 -+n -f /mnt/test -r 1M -s 500M
...
KB reclen write
512000 1024 90396

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The numbers you have here are averages over the whole run.=20 Performing these tests using a variety of record lengths and file=20 sizes (up to several tens of gigabytes) would be useful to see wher=

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different memory and network latencies kick in.

Definitely useful, although I'm not sure how this relates to this pa=

It relates to the whole idea that this is a valid and useful paramete=

to tweak.

What your experiment shows is that there is some improvement when the=

TCP window is allowed to expand. It does not demonstrate that the=20 *best* way to provide this facility is to allow administrators to tun=

the server's TCP buffer sizes.

A single average number can hide a host of underlying sins. This=20 simple experiment, for example, does not demonstrate that TCP window=20 size is the most significant issue here.

It does not show that it is more or less effective to adjust the=20 window size than to select an appropriate congestion control algorith=

(say, BIC).

It does not show whether the client and server are using TCP optimall=

It does not expose problems related to having a single data stream=20 with one blocking head (eg SCTP can allow multiple streams over the=20 same connection; or better performance might be achieved with multipl=

TCP connections, even if they allow only small windows).

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This patch isn't trying to alter default values, or predict buffer=20 sizes based on rtt values, or dynamically alter the tcp window based=

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on dropped packets, etc, it is just giving users the ability to=20 customize the server tcp buffer size.

I know you posted this patch because of the experiments at CITI with=20 long-run 10GbE, and it's handy to now have this to experiment with.

It might also be helpful if we had a patch that made the server=20 perform better in common environments, so a better default setting it=

seems to me would have greater value than simply creating a new tunin=

knob.

Would it be hard to add a metric or two with this tweak that would=20 allow admins to see how often a socket buffer was completely full,=20 completely empty, or how often the window size is being aggressively =

While we may not be able to determine a single optimal buffer size fo=

all BDPs, are there diminishing returns in most common cases for=20 increasing the buffer size past, say, 16MB?

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The information you are curious about is more relevant to creating=20 better default values of the tcp buffer size. This could be useful,=20 but would be a long process and there are so many variables that I'm=

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not sure that you could pick proper default values anyways. The=20 important thing is that the client can currently set its tcp buffer=20 size via the sysctl's, this is useless if the server is stuck at a=20 fixed value since the tcp window will be the minimum of the client=20 and server's tcp buffer sizes.

Well, Linux servers are not the only servers that a Linux client will=

ever encounter, so the client-side sysctl isn't as bad as useless. Bu=

one can argue whether that knob is ever tweaked by client=20 administrators, and how useful it is.

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The server cannot do just the same thing as the client since it=20 cannot just rely on the tcp sysctl's since it also needs to ensure i=

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has enough buffer space for each NFSD.

I agree the server's current logic is too conservative.

However, the server has an automatic load-leveling feature -- it can=20 close sockets if it notices it is running out of resources, and the=20 Linux server does this already. I don't think it would be terribly=20 harmful to overcommit the socket buffer space since we have such a=20 safety valve.

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My goal with this patch is to provide users with the same flexibilit=

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that the client has regarding tcp buffer sizes, but also ensure that=

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the minimum amount of buffer space that the NFSDs require is allocat=

What is the formula you used to determine the value to poke into the=20 sysctl, btw?

What is an appropriate setting for a server that has to handle a mix=20 of local and remote clients, for example, or a client that has to=20 connect to a mix of local and remote servers?

Quoted 24 lines · Chuck Lever

Quoted 22 lines · Dean Hildebrand

Hi Chuck,

Quoted 8 lines · Chuck Lever

Howdy Dean-

Quoted 3 lines · Dean Hildebrand

The motivation for this patch is improved WAN write performance plus greater user control on the server of the TCP buffer values (window size). The TCP window determines the amount of outstanding data that a client can have on the wire and should be large enough that a NFS client can fill up the pipe (the bandwidth * delay product). Currently the TCP receive buffer size (used for client writes) is set very low, which prevents a client from filling up a network pipe with a large bandwidth * delay product.

Currently, the server TCP send window is set to accommodate the maximum number of outstanding NFSD read requests (# nfsds * maxiosize), while the server TCP receive window is set to a fixed value which can hold a few requests. While these values set a TCP window size that is fine in LAN environments with a small BDP, WAN environments can require a much larger TCP window size, e.g., 10GigE transatlantic link with a rtt of 120 ms has a BDP of approx 60MB.

Was the receive buffer size computation adjusted when support for large transfer sizes was recently added to the NFS server?

Yes, it is based on the transfer size. So in the current code, having a larger transfer size can improve efficiency PLUS help create a larger possible TCP window. The issue seems to be that tcp window, # of NFSDs, and transfer size are all independent variables that need to be tuned individually depending on rtt, network bandwidth, disk bandwidth, etc etc... We can adjust the last 2, so this patch helps adjust the first (tcp window).

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I have a patch to net/svc/svcsock.c that allows a user to manually set the server TCP send and receive buffer through the sysctl interface. to suit the required TCP window of their network architecture. It adds two /proc entries, one for the receive buffer size and one for the send buffer size:
/proc/sys/sunrpc/tcp_sndbuf
/proc/sys/sunrpc/tcp_rcvbuf

What I'm wondering is if we can find some algorithm to set the buffer and window sizes *automatically*. Why can't the NFS server select an appropriately large socket buffer size by default?

Since the socket buffer size is just a limit (no memory is allocated) why, for example, shouldn't the buffer size be large for all environments that have sufficient physical memory?

I think the problem there is that the only way to set the buffer size automatically would be to know the rtt and bandwidth of the network connection. Excessive numbers of packets can get dropped if the TCP buffer is set too large for a specific network connection.

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In this case, the window opens too wide and lets too many packets out into the system, somewhere along the path buffers start overflowing and packets are lost, TCP congestion avoidance kicks in and cuts the window size dramatically and performance along with it. This type of behaviour creates a sawtooth pattern for the TCP window, which is less favourable than a more steady state pattern that is created if the TCP buffer size is set appropriately.

Agreed it is a performance problem, but I thought some of the newer TCP congestion algorithms were specifically designed to address this by not closing the window as aggressively.

Yes, every tcp algorithm seems to have its own niche. Personally, I have found bic the best in the WAN as it is pretty aggressive at returning to the original window size. Since cubic is now the Linux default, and changing the tcp cong control algorithm is done for an entire system (meaning local clients could be adversely affected by choosing one designed for specialized networks), I think we should try to optimize cubic.

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Once the window is wide open, then, it would appear that choosing a good congestion avoidance algorithm is also important.

Yes, but it is always important to avoid ever letting the window get too wide, as this will cause a hiccup every single time you try to send a bunch of data (a tcp window closes very quickly after data is transmitted, so waiting 1 second causing you to start from the beginning with a small window)

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Another point is that setting the buffer size isn't always a straightforward process. All papers I've read on the subject, and my experience confirms this, is that setting tcp buffer sizes is more of an art.

So having the server set a good default value is half the battle, but allowing users to twiddle with this value is vital.

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The uses the current buffer sizes in the code are as minimum values, which the user cannot decrease. If the user sets a value of 0 in either /proc entry, it resets the buffer size to the default value. The set /proc values are utilized when the TCP connection is initialized (mount time). The values are bounded above by the *minimum* of the /proc values and the network TCP sysctls.

To demonstrate the usefulness of this patch, details of an experiment between 2 computers with a rtt of 30ms is provided below. In this experiment, increasing the server /proc/sys/sunrpc/tcp_rcvbuf value doubles write performance.

EXPERIMENT
==========
This experiment simulates a WAN by using tc together with netem to add a 30 ms delay to all packets on a nfs client. The goal is to show that by only changing tcp_rcvbuf, the nfs client can increase write performance in the WAN. To verify the patch has the desired effect on the TCP window, I created two tcptrace plots that show the difference in tcp window behaviour before and after the server TCP rcvbuf size is increased. When using the default server tcpbuf value of 6M, we can see the TCP window top out around 4.6 M, whereas increasing the server tcpbuf value to 32M, we can see that the TCP window tops out around 13M. Performance jumps from 43 MB/s to 90 MB/s.

Hardware:
2 dual-core opteron blades
GigE, Broadcom NetXtreme II BCM57065 cards
A single gigabit switch in the middle
1500 MTU
8 GB memory

Software:
Kernel: Bruce's 2.6.25-rc9-CITI_NFS4_ALL-1 tree
RHEL4

NFS Configuration:
64 rpc slots
32 nfsds
Export ext3 file system. This disk is quite slow, I therefore exported using async to reduce the effect of the disk on the back end. This way, the experiments record the time it takes for the data to get to the server (not to the disk).

# exportfs -v
/export <world>(rw,async,wdelay,nohide,insecure,no_root_squash,fsid=0)

# cat /proc/mounts
bear109:/export /mnt nfs rw,vers=3,rsize=1048576,wsize=1048576,namlen=255,hard,nointr,proto=tcp,timeo=600,retrans=2,sec=sys,mountproto=udp,addr=9.1.74.144 0 0

fs.nfs.nfs_congestion_kb = 91840
net.ipv4.tcp_congestion_control = cubic

Network tc Command executed on client:
tc qdisc add dev eth0 root netem delay 30ms
rtt from client (bear108) to server (bear109)
#ping bear109
PING bear109.almaden.ibm.com (9.1.74.144) 56(84) bytes of data.
64 bytes from bear109.almaden.ibm.com (9.1.74.144): icmp_seq=0 ttl=64 time=31.4 ms 64 bytes from bear109.almaden.ibm.com (9.1.74.144): icmp_seq=1 ttl=64 time=32.0 ms

TCP Configuration on client and server:
# Controls IP packet forwarding
net.ipv4.ip_forward = 0
# Controls source route verification net.ipv4.conf.default.rp_filter = 1
# Do not accept source routing net.ipv4.conf.default.accept_source_route = 0
# Controls the System Request debugging functionality of the kernel kernel.sysrq = 0
# Controls whether core dumps will append the PID to the core filename # Useful for debugging multi-threaded applications kernel.core_uses_pid = 1
# Controls the use of TCP syncookies net.ipv4.tcp_syncookies = 1
# Controls the maximum size of a message, in bytes kernel.msgmnb = 65536
# Controls the default maxmimum size of a mesage queue kernel.msgmax = 65536
# Controls the maximum shared segment size, in bytes kernel.shmmax = 68719476736
# Controls the maximum number of shared memory segments, in pages kernel.shmall = 4294967296
### IPV4 specific settings
net.ipv4.tcp_timestamps = 0
net.ipv4.tcp_sack = 1

# on systems with a VERY fast bus -> memory interface this is the big gainer
net.ipv4.tcp_rmem = 4096 16777216 16777216
net.ipv4.tcp_wmem = 4096 16777216 16777216
net.ipv4.tcp_mem = 4096 16777216 16777216
### CORE settings (mostly for socket and UDP effect)
net.core.rmem_max = 16777216
net.core.wmem_max = 16777216
net.core.rmem_default = 16777216
net.core.wmem_default = 16777216
net.core.optmem_max = 16777216
net.core.netdev_max_backlog = 300000
# Don't cache ssthresh from previous connection
net.ipv4.tcp_no_metrics_save = 1
# make sure we don't run out of memory
vm.min_free_kbytes = 32768

Experiments:

On Server: (note that the real tcp buffer size is double tcp_rcvbuf) [root@bear109 ~]# echo 0 > /proc/sys/sunrpc/tcp_rcvbuf [root@bear109 ~]# cat /proc/sys/sunrpc/tcp_rcvbuf
3158016

On Client:
mount -t nfs bear109:/export /mnt
[root@bear108 ~]# iozone -aec -i 0 -+n -f /mnt/test -r 1M -s 500M
...
KB reclen write
512000 1024 43252 umount /mnt

On server:
[root@bear109 ~]# echo 16777216 > /proc/sys/sunrpc/tcp_rcvbuf [root@bear109 ~]# cat /proc/sys/sunrpc/tcp_rcvbuf 16777216

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On Client:
mount -t nfs bear109:/export /mnt
[root@bear108 ~]# iozone -aec -i 0 -+n -f /mnt/test -r 1M -s 500M
...
KB reclen write
512000 1024 90396

The numbers you have here are averages over the whole run. Performing these tests using a variety of record lengths and file sizes (up to several tens of gigabytes) would be useful to see where different memory and network latencies kick in.

Definitely useful, although I'm not sure how this relates to this patch.

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It relates to the whole idea that this is a valid and useful parameter to tweak.

What your experiment shows is that there is some improvement when the TCP window is allowed to expand. It does not demonstrate that the *best* way to provide this facility is to allow administrators to tune the server's TCP buffer sizes.

By definition of how TCP is designed, tweaking the send and receive buffer sizes is a useful. Please see the tcp tuning guides in my other post. I would characterize tweaking the buffers as a necessary condition but not a sufficient condition to achieve good throughput with tcp over long distances.

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A single average number can hide a host of underlying sins. This simple experiment, for example, does not demonstrate that TCP window size is the most significant issue here.

I would say it slightly differently, that it demonstrates that it is significant, but maybe not the *most* significant. There are many possible bottlenecks and possible knobs to tweak. For example, I'm still not achieving link speeds, so I'm sure there are other bottlenecks that are causing reduced performance.

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It does not show that it is more or less effective to adjust the window size than to select an appropriate congestion control algorithm (say, BIC).

Any tcp cong. control algorithm is highly dependent on the tcp buffer size. The choice of algorithm changes the behaviour when packets are dropped and in the initial opening of the window, but once the window is open and no packets are being dropped, the algorithm is irrelevant. So BIC, or westwood, or highspeed might do better in the face of dropped packets, but since the current receive buffer is so small, dropped packets are not the problem. Once we can use the sysctl's to tweak the server buffer size, only then is the choice of algorithm going to be important.

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It does not show whether the client and server are using TCP optimally.

I'm not sure what you mean by *optimally*. They use tcp the only way they know how non?

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It does not expose problems related to having a single data stream with one blocking head (eg SCTP can allow multiple streams over the same connection; or better performance might be achieved with multiple TCP connections, even if they allow only small windows).

Yes, using multiple tcp connections might be useful, but that doesn't mean you wouldn't want to adjust the tcp window of each one using my patch. Actually, I can't seem to find the quote, but I read somewhere that achieving performance in the WAN can be done 2 different ways: a) If you can tune the buffer sizes that is the best way to go, but b) if you don't have root access to change the linux tcp settings then using multiple tcp streams can compensate for small buffer sizes.

Andy has/had a patch to add multiple tcp streams to NFS. I think his patch and my patch work in collaboration to improve wan performance.

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This patch isn't trying to alter default values, or predict buffer sizes based on rtt values, or dynamically alter the tcp window based on dropped packets, etc, it is just giving users the ability to customize the server tcp buffer size.

I know you posted this patch because of the experiments at CITI with long-run 10GbE, and it's handy to now have this to experiment with.

Actually at IBM we have our own reasons for using NFS over the WAN. I would like to get these 2 knobs into the kernel as it is hard to tell customers to apply kernel patches....

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It might also be helpful if we had a patch that made the server perform better in common environments, so a better default setting it seems to me would have greater value than simply creating a new tuning knob.

I think there are possibly 2 (or more) patches. One that improves the default buffer sizes and one that lets sysadmins tweak the value. I don't see why they are mutually exclusive.

My patch is a first step towards allowing NFS into WAN environments. Linux currently has sysctl values for the TCP parameters for exactly this reason, it is impossible to predict the network environment of a linux machine.

If the Linux nfs server isn't going to build off of the existing Linux TCP values (which all sysadmins know how to tweak), then it must allow sysadmins to tweak the NFS server tcp values, either using my patch or some other related patch. I'm open to how the server tcp buffers are tweaked, they just need to be able to be tweaked. For example, if all tcp buffer values in linux were taken out of the /proc file system and hardcoded, I think there would be a revolt.

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Would it be hard to add a metric or two with this tweak that would allow admins to see how often a socket buffer was completely full, completely empty, or how often the window size is being aggressively cut?

So I've done this using tcpdump in combination with tcptrace. I've shown people at citi how the tcp window grows in the experiment I describe.

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While we may not be able to determine a single optimal buffer size for all BDPs, are there diminishing returns in most common cases for increasing the buffer size past, say, 16MB?

Good question. It all depends on how much data you are transferring. In order to fully open a 128MB tcp window over a very long WAN, you will need to transfer at least a few gigabytes of data. If you only transfer 100 MB at a time, then you will probably be fine with a 16 MB window as you are not transferring enough data to open the window anyways. In our environment, we are expecting to transfer 100s of GB if not even more, so the 16 MB window would be very limiting.

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The information you are curious about is more relevant to creating better default values of the tcp buffer size. This could be useful, but would be a long process and there are so many variables that I'm not sure that you could pick proper default values anyways. The important thing is that the client can currently set its tcp buffer size via the sysctl's, this is useless if the server is stuck at a fixed value since the tcp window will be the minimum of the client and server's tcp buffer sizes.

Well, Linux servers are not the only servers that a Linux client will ever encounter, so the client-side sysctl isn't as bad as useless. But one can argue whether that knob is ever tweaked by client administrators, and how useful it is.

Definitely not useless. Doing a google search for 'tcp_rmem' returns over 11000 hits describing how to configure tcp settings. (ok, I didn't review every result, but the first few pages of results are telling) It doesn't really matter what OS the client and server use, as long as both have the ability to tweak the tcp buffer size.

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What is an appropriate setting for a server that has to handle a mix of local and remote clients, for example, or a client that has to connect to a mix of local and remote servers?

Yes, this is a tricky one. I believe the best way to handle it is to set the server tcp buffer to the MAX(local, remote) and then let the local client set a smaller tcp buffer and the remote client set a larger tcp buffer. The problem there is that then what if the local client is also a remote client of another nfs server?? At this point there seems to be some limitations.....

Since what we really want to limit is the maximum size of the TCP receive window, it would be more precise to change the name of the new sysctl to something like nfs_tcp_max_window_size.

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Another point is that setting the buffer size isn't always a straightforward process. All papers I've read on the subject, and my experience confirms this, is that setting tcp buffer sizes is more of an art.

So having the server set a good default value is half the battle, but allowing users to twiddle with this value is vital.

Quoted 100 lines

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The uses the current buffer sizes in the code are as minimum values, which the user cannot decrease. If the user sets a value of 0 in either /proc entry, it resets the buffer size to the default value. The set /proc values are utilized when the TCP connection is initialized (mount time). The values are bounded above by the *minimum* of the /proc values and the network TCP sysctls.

To demonstrate the usefulness of this patch, details of an experiment between 2 computers with a rtt of 30ms is provided below. In this experiment, increasing the server /proc/sys/sunrpc/tcp_rcvbuf value doubles write performance.

EXPERIMENT
==========
This experiment simulates a WAN by using tc together with netem to add a 30 ms delay to all packets on a nfs client. The goal is to show that by only changing tcp_rcvbuf, the nfs client can increase write performance in the WAN. To verify the patch has the desired effect on the TCP window, I created two tcptrace plots that show the difference in tcp window behaviour before and after the server TCP rcvbuf size is increased. When using the default server tcpbuf value of 6M, we can see the TCP window top out around 4.6 M, whereas increasing the server tcpbuf value to 32M, we can see that the TCP window tops out around 13M. Performance jumps from 43 MB/s to 90 MB/s.

Hardware:
2 dual-core opteron blades
GigE, Broadcom NetXtreme II BCM57065 cards
A single gigabit switch in the middle
1500 MTU
8 GB memory

Software:
Kernel: Bruce's 2.6.25-rc9-CITI_NFS4_ALL-1 tree
RHEL4

NFS Configuration:
64 rpc slots
32 nfsds
Export ext3 file system. This disk is quite slow, I therefore exported using async to reduce the effect of the disk on the back end. This way, the experiments record the time it takes for the data to get to the server (not to the disk).

# exportfs -v
/export <world>(rw,async,wdelay,nohide,insecure,no_root_squash,fsid=0)

# cat /proc/mounts
bear109:/export /mnt nfs rw,vers=3,rsize=1048576,wsize=1048576,namlen=255,hard,nointr,proto=tcp,timeo=600,retrans=2,sec=sys,mountproto=udp,addr=9.1.74.144 0 0

fs.nfs.nfs_congestion_kb = 91840
net.ipv4.tcp_congestion_control = cubic

Network tc Command executed on client:
tc qdisc add dev eth0 root netem delay 30ms
rtt from client (bear108) to server (bear109)
#ping bear109
PING bear109.almaden.ibm.com (9.1.74.144) 56(84) bytes of data.
64 bytes from bear109.almaden.ibm.com (9.1.74.144): icmp_seq=0 ttl=64 time=31.4 ms 64 bytes from bear109.almaden.ibm.com (9.1.74.144): icmp_seq=1 ttl=64 time=32.0 ms

TCP Configuration on client and server:
# Controls IP packet forwarding
net.ipv4.ip_forward = 0
# Controls source route verification net.ipv4.conf.default.rp_filter = 1
# Do not accept source routing net.ipv4.conf.default.accept_source_route = 0
# Controls the System Request debugging functionality of the kernel kernel.sysrq = 0
# Controls whether core dumps will append the PID to the core filename # Useful for debugging multi-threaded applications kernel.core_uses_pid = 1
# Controls the use of TCP syncookies net.ipv4.tcp_syncookies = 1
# Controls the maximum size of a message, in bytes kernel.msgmnb = 65536
# Controls the default maxmimum size of a mesage queue kernel.msgmax = 65536
# Controls the maximum shared segment size, in bytes kernel.shmmax = 68719476736
# Controls the maximum number of shared memory segments, in pages kernel.shmall = 4294967296
### IPV4 specific settings
net.ipv4.tcp_timestamps = 0
net.ipv4.tcp_sack = 1

# on systems with a VERY fast bus -> memory interface this is the big gainer
net.ipv4.tcp_rmem = 4096 16777216 16777216
net.ipv4.tcp_wmem = 4096 16777216 16777216
net.ipv4.tcp_mem = 4096 16777216 16777216
### CORE settings (mostly for socket and UDP effect)
net.core.rmem_max = 16777216
net.core.wmem_max = 16777216
net.core.rmem_default = 16777216
net.core.wmem_default = 16777216
net.core.optmem_max = 16777216
net.core.netdev_max_backlog = 300000
# Don't cache ssthresh from previous connection
net.ipv4.tcp_no_metrics_save = 1
# make sure we don't run out of memory
vm.min_free_kbytes = 32768

Experiments:

On Server: (note that the real tcp buffer size is double tcp_rcvbuf) [root@bear109 ~]# echo 0 > /proc/sys/sunrpc/tcp_rcvbuf [root@bear109 ~]# cat /proc/sys/sunrpc/tcp_rcvbuf
3158016

On Client:
mount -t nfs bear109:/export /mnt
[root@bear108 ~]# iozone -aec -i 0 -+n -f /mnt/test -r 1M -s 500M
...
KB reclen write
512000 1024 43252 umount /mnt

On server:
[root@bear109 ~]# echo 16777216 > /proc/sys/sunrpc/tcp_rcvbuf [root@bear109 ~]# cat /proc/sys/sunrpc/tcp_rcvbuf 16777216

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On Client:
mount -t nfs bear109:/export /mnt
[root@bear108 ~]# iozone -aec -i 0 -+n -f /mnt/test -r 1M -s 500M
...
KB reclen write
512000 1024 90396

The numbers you have here are averages over the whole run. Performing these tests using a variety of record lengths and file sizes (up to several tens of gigabytes) would be useful to see where different memory and network latencies kick in.

Definitely useful, although I'm not sure how this relates to this patch.

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It relates to the whole idea that this is a valid and useful parameter to tweak.

What your experiment shows is that there is some improvement when the TCP window is allowed to expand. It does not demonstrate that the *best* way to provide this facility is to allow administrators to tune the server's TCP buffer sizes.

By definition of how TCP is designed, tweaking the send and receive buffer sizes is a useful. Please see the tcp tuning guides in my other post. I would characterize tweaking the buffers as a necessary condition but not a sufficient condition to achieve good throughput with tcp over long distances.

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A single average number can hide a host of underlying sins. This simple experiment, for example, does not demonstrate that TCP window size is the most significant issue here.

I would say it slightly differently, that it demonstrates that it is significant, but maybe not the *most* significant. There are many possible bottlenecks and possible knobs to tweak. For example, I'm still not achieving link speeds, so I'm sure there are other bottlenecks that are causing reduced performance.

I think that's my basic point. We don't have the full picture yet. There are benefits to adjusting the maximum window size, but as we learn more it may turn out that we want an entirely different knob or knobs.

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It does not show that it is more or less effective to adjust the window size than to select an appropriate congestion control algorithm (say, BIC).

Any tcp cong. control algorithm is highly dependent on the tcp buffer size. The choice of algorithm changes the behaviour when packets are dropped and in the initial opening of the window, but once the window is open and no packets are being dropped, the algorithm is irrelevant. So BIC, or westwood, or highspeed might do better in the face of dropped packets, but since the current receive buffer is so small, dropped packets are not the problem. Once we can use the sysctl's to tweak the server buffer size, only then is the choice of algorithm going to be important.

Maybe my use of the terminology is imprecise, but clearly the congestion control algorithm matters for determining the TCP window size, which is exactly what we're discussing here.

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It does not show whether the client and server are using TCP optimally.

I'm not sure what you mean by *optimally*. They use tcp the only way they know how non?

I'm talking about whether they use Nagle, when they PUSH, how they use the window (servers can close a window when they are busy, for example), and of course whether they can or should use multiple connections.

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It does not expose problems related to having a single data stream with one blocking head (eg SCTP can allow multiple streams over the same connection; or better performance might be achieved with multiple TCP connections, even if they allow only small windows).

Yes, using multiple tcp connections might be useful, but that doesn't mean you wouldn't want to adjust the tcp window of each one using my patch. Actually, I can't seem to find the quote, but I read somewhere that achieving performance in the WAN can be done 2 different ways: a) If you can tune the buffer sizes that is the best way to go, but b) if you don't have root access to change the linux tcp settings then using multiple tcp streams can compensate for small buffer sizes.

Andy has/had a patch to add multiple tcp streams to NFS. I think his patch and my patch work in collaboration to improve wan performance.

Yep, I've discussed this work with him several times. This might be a more practical solution than allowing larger window sizes (one reason being the dangers of allowing the window to get too large).

While the use of multiple streams has benefits besides increasing the effective TCP window size, only the client side controls the number of connections. The server wouldn't have much to say about it.

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This patch isn't trying to alter default values, or predict buffer sizes based on rtt values, or dynamically alter the tcp window based on dropped packets, etc, it is just giving users the ability to customize the server tcp buffer size.

I know you posted this patch because of the experiments at CITI with long-run 10GbE, and it's handy to now have this to experiment with.

Actually at IBM we have our own reasons for using NFS over the WAN. I would like to get these 2 knobs into the kernel as it is hard to tell customers to apply kernel patches....

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It might also be helpful if we had a patch that made the server perform better in common environments, so a better default setting it seems to me would have greater value than simply creating a new tuning knob.

I think there are possibly 2 (or more) patches. One that improves the default buffer sizes and one that lets sysadmins tweak the value. I don't see why they are mutually exclusive.

They are not. I'm OK with studying the problem and adjusting the defaults appropriately.

The issue is whether adding this knob is the right approach to adjusting the server. I don't think we have enough information to understand if this is the most useful approach. In other words, it seems like a band-aid right now, but in the long run it might be the correct answer.

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My patch is a first step towards allowing NFS into WAN environments. Linux currently has sysctl values for the TCP parameters for exactly this reason, it is impossible to predict the network environment of a linux machine.

If the Linux nfs server isn't going to build off of the existing Linux TCP values (which all sysadmins know how to tweak), then it must allow sysadmins to tweak the NFS server tcp values, either using my patch or some other related patch. I'm open to how the server tcp buffers are tweaked, they just need to be able to be tweaked. For example, if all tcp buffer values in linux were taken out of the /proc file system and hardcoded, I think there would be a revolt.

I'm not arguing for no tweaking. What I'm saying is we should provide knobs that are as useful as possible, and include metrics and clear instructions for when and how to set the knob.

You've shown there is improvement, but not that this is the best solution. It just feels like the work isn't done yet.

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Would it be hard to add a metric or two with this tweak that would allow admins to see how often a socket buffer was completely full, completely empty, or how often the window size is being aggressively cut?

So I've done this using tcpdump in combination with tcptrace. I've shown people at citi how the tcp window grows in the experiment I describe.

No, I mean as a part of the patch that adds the tweak, it should report various new statistics that can allow admins to see that they need adjustment, or that there isn't a problem at all in this area.

Scientific system tuning means assessing the problem, trying a change, then measuring to see if it was effective, or if it caused more trouble. Lather, rinse, repeat.

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While we may not be able to determine a single optimal buffer size for all BDPs, are there diminishing returns in most common cases for increasing the buffer size past, say, 16MB?

Good question. It all depends on how much data you are transferring. In order to fully open a 128MB tcp window over a very long WAN, you will need to transfer at least a few gigabytes of data. If you only transfer 100 MB at a time, then you will probably be fine with a 16 MB window as you are not transferring enough data to open the window anyways. In our environment, we are expecting to transfer 100s of GB if not even more, so the 16 MB window would be very limiting.

What about for a fast LAN?

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The information you are curious about is more relevant to creating better default values of the tcp buffer size. This could be useful, but would be a long process and there are so many variables that I'm not sure that you could pick proper default values anyways. The important thing is that the client can currently set its tcp buffer size via the sysctl's, this is useless if the server is stuck at a fixed value since the tcp window will be the minimum of the client and server's tcp buffer sizes.

Well, Linux servers are not the only servers that a Linux client will ever encounter, so the client-side sysctl isn't as bad as useless. But one can argue whether that knob is ever tweaked by client administrators, and how useful it is.

Definitely not useless. Doing a google search for 'tcp_rmem' returns over 11000 hits describing how to configure tcp settings. (ok, I didn't review every result, but the first few pages of results are telling) It doesn't really matter what OS the client and server use, as long as both have the ability to tweak the tcp buffer size.

The number of hits may reflect the desperation that many have had over the years to get better performance from the Linux NFS implementation. These days we have better performance out of the box, so there is less need for this kind of after-market tweaking.

I think we would be in a much better place if the client and server implementations worked "well enough" in nearly any network or environment. That's been my goal since I started working on Linux NFS seven years ago.

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What is an appropriate setting for a server that has to handle a mix of local and remote clients, for example, or a client that has to connect to a mix of local and remote servers?

Yes, this is a tricky one. I believe the best way to handle it is to set the server tcp buffer to the MAX(local, remote) and then let the local client set a smaller tcp buffer and the remote client set a larger tcp buffer. The problem there is that then what if the local client is also a remote client of another nfs server?? At this point there seems to be some limitations.....

Using multiple connections solves this problem pretty well, I think.

Signature · 2 lines

Chuck Lever
chuck[dot]lever[at]oracle[dot]com

We have a full picture of TCP. TCP is well known, there are lots of papers/info on it, I have no doubt on what is occurring with TCP as I have traces that clearly show what is happening. All documents and information clearly state that the buffer size is a critical part of improving TCP performance in the WAN. In addition, the congestion control algorithm does NOT control the maximum size of the TCP window. The CCA controls how quickly the window reaches the maximum size, what happens when a packet is dropped and when to close the window. The only item that controls the maximum size of the TCP window is the buffer values that I want a sysctl to tweak (just to be in line with the existing tcp buffer sysctls in Documentation/networking/ip-sysctl.txt)

What we don't have is a full picture of the other parts of transferring data from client to server, e.g., Trond just fixed a bug with regards to the writeback cache which should help write performance, that was an unknown up until this point.

Multiple TCP Streams
===============
There is a really big downside to multiple TCP streams: you have multiple TCP streams :) Each one has its own overhead, setup connection cost, and of course TCP window. With a WAN rtt of 200 ms (typical over satellite) and the current buffer size of 4MB, the nfs client would need 50+ TCP connections to achieve the correct performance. That is a
lot of overhead when comparing it with simply following the standard TCP tuning knowhow of increasing the buffer sizes.

The main documentation the show that multiple tcp streams helps over the WAN is from GridFTP experiments. They go over the pos and neg of the approach, but also talk about how tcp buffer size is also very important. Multiple tcp streams is not a replacement for a proper buffer size (http://www.globus.org/alliance/publications/clusterworld/0904GridFinal.pdf)

If you have documentation counteracting these experiments I would be very interested to see them.

One Variable or Two
===============
I'd be happy with using a single variable for both the send and receive buffers, but since we are essentially doing the same thing as the net.ipv4.tcp_wmem/rmem variables, I think nfsd_tcp_max_mem would be more in line with existing Linux terminology. (also, we are talking about nfsd, not nfs, so I'd prefer to make that clear in the variable name)

Summary
=======
I'm providing you with all the information I have with regards to my experiments with NFS and TCP. I agree that a better default is needed and my patch allows further experimentation to get to that value. My patch does not add modify current NFS behaviour. It changes a hard coded value for the server buffer size to be a variable in /proc. Blocking a method to modify this hard coded value means blocking further experimentation to find a better default value. My patch is a first step toward trying to find a good default tcp server buffer value.

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Since what we really want to limit is the maximum size of the TCP receive window, it would be more precise to change the name of the new sysctl to something like nfs_tcp_max_window_size.

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Another point is that setting the buffer size isn't always a straightforward process. All papers I've read on the subject, and my experience confirms this, is that setting tcp buffer sizes is more of an art.

So having the server set a good default value is half the battle, but allowing users to twiddle with this value is vital.

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The uses the current buffer sizes in the code are as minimum values, which the user cannot decrease. If the user sets a value of 0 in either /proc entry, it resets the buffer size to the default value. The set /proc values are utilized when the TCP connection is initialized (mount time). The values are bounded above by the *minimum* of the /proc values and the network TCP sysctls.

To demonstrate the usefulness of this patch, details of an experiment between 2 computers with a rtt of 30ms is provided below. In this experiment, increasing the server /proc/sys/sunrpc/tcp_rcvbuf value doubles write performance.

EXPERIMENT
==========
This experiment simulates a WAN by using tc together with netem to add a 30 ms delay to all packets on a nfs client. The goal is to show that by only changing tcp_rcvbuf, the nfs client can increase write performance in the WAN. To verify the patch has the desired effect on the TCP window, I created two tcptrace plots that show the difference in tcp window behaviour before and after the server TCP rcvbuf size is increased. When using the default server tcpbuf value of 6M, we can see the TCP window top out around 4.6 M, whereas increasing the server tcpbuf value to 32M, we can see that the TCP window tops out around 13M. Performance jumps from 43 MB/s to 90 MB/s.

Hardware:
2 dual-core opteron blades
GigE, Broadcom NetXtreme II BCM57065 cards
A single gigabit switch in the middle
1500 MTU
8 GB memory

Software:
Kernel: Bruce's 2.6.25-rc9-CITI_NFS4_ALL-1 tree
RHEL4

NFS Configuration:
64 rpc slots
32 nfsds
Export ext3 file system. This disk is quite slow, I therefore exported using async to reduce the effect of the disk on the back end. This way, the experiments record the time it takes for the data to get to the server (not to the disk).

# exportfs -v
/export <world>(rw,async,wdelay,nohide,insecure,no_root_squash,fsid=0)

# cat /proc/mounts
bear109:/export /mnt nfs rw,vers=3,rsize=1048576,wsize=1048576,namlen=255,hard,nointr,proto=tcp,timeo=600,retrans=2,sec=sys,mountproto=udp,addr=9.1.74.144 0 0

fs.nfs.nfs_congestion_kb = 91840
net.ipv4.tcp_congestion_control = cubic

Network tc Command executed on client:
tc qdisc add dev eth0 root netem delay 30ms
rtt from client (bear108) to server (bear109)
#ping bear109
PING bear109.almaden.ibm.com (9.1.74.144) 56(84) bytes of data.
64 bytes from bear109.almaden.ibm.com (9.1.74.144): icmp_seq=0 ttl=64 time=31.4 ms 64 bytes from bear109.almaden.ibm.com (9.1.74.144): icmp_seq=1 ttl=64 time=32.0 ms

TCP Configuration on client and server:
# Controls IP packet forwarding
net.ipv4.ip_forward = 0
# Controls source route verification net.ipv4.conf.default.rp_filter = 1
# Do not accept source routing net.ipv4.conf.default.accept_source_route = 0
# Controls the System Request debugging functionality of the kernel kernel.sysrq = 0
# Controls whether core dumps will append the PID to the core filename # Useful for debugging multi-threaded applications kernel.core_uses_pid = 1
# Controls the use of TCP syncookies net.ipv4.tcp_syncookies = 1
# Controls the maximum size of a message, in bytes kernel.msgmnb = 65536
# Controls the default maxmimum size of a mesage queue kernel.msgmax = 65536
# Controls the maximum shared segment size, in bytes kernel.shmmax = 68719476736
# Controls the maximum number of shared memory segments, in pages kernel.shmall = 4294967296
### IPV4 specific settings
net.ipv4.tcp_timestamps = 0
net.ipv4.tcp_sack = 1

# on systems with a VERY fast bus -> memory interface this is the big gainer
net.ipv4.tcp_rmem = 4096 16777216 16777216
net.ipv4.tcp_wmem = 4096 16777216 16777216
net.ipv4.tcp_mem = 4096 16777216 16777216
### CORE settings (mostly for socket and UDP effect)
net.core.rmem_max = 16777216
net.core.wmem_max = 16777216
net.core.rmem_default = 16777216
net.core.wmem_default = 16777216
net.core.optmem_max = 16777216
net.core.netdev_max_backlog = 300000
# Don't cache ssthresh from previous connection
net.ipv4.tcp_no_metrics_save = 1
# make sure we don't run out of memory
vm.min_free_kbytes = 32768

Experiments:

On Server: (note that the real tcp buffer size is double tcp_rcvbuf) [root@bear109 ~]# echo 0 > /proc/sys/sunrpc/tcp_rcvbuf [root@bear109 ~]# cat /proc/sys/sunrpc/tcp_rcvbuf
3158016

On Client:
mount -t nfs bear109:/export /mnt
[root@bear108 ~]# iozone -aec -i 0 -+n -f /mnt/test -r 1M -s 500M
...
KB reclen write
512000 1024 43252 umount /mnt

On server:
[root@bear109 ~]# echo 16777216 > /proc/sys/sunrpc/tcp_rcvbuf [root@bear109 ~]# cat /proc/sys/sunrpc/tcp_rcvbuf 16777216

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On Client:
mount -t nfs bear109:/export /mnt
[root@bear108 ~]# iozone -aec -i 0 -+n -f /mnt/test -r 1M -s 500M
...
KB reclen write
512000 1024 90396

The numbers you have here are averages over the whole run. Performing these tests using a variety of record lengths and file sizes (up to several tens of gigabytes) would be useful to see where different memory and network latencies kick in.

Definitely useful, although I'm not sure how this relates to this patch.

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It relates to the whole idea that this is a valid and useful parameter to tweak.

What your experiment shows is that there is some improvement when the TCP window is allowed to expand. It does not demonstrate that the *best* way to provide this facility is to allow administrators to tune the server's TCP buffer sizes.

By definition of how TCP is designed, tweaking the send and receive buffer sizes is a useful. Please see the tcp tuning guides in my other post. I would characterize tweaking the buffers as a necessary condition but not a sufficient condition to achieve good throughput with tcp over long distances.

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A single average number can hide a host of underlying sins. This simple experiment, for example, does not demonstrate that TCP window size is the most significant issue here.

I would say it slightly differently, that it demonstrates that it is significant, but maybe not the *most* significant. There are many possible bottlenecks and possible knobs to tweak. For example, I'm still not achieving link speeds, so I'm sure there are other bottlenecks that are causing reduced performance.

I think that's my basic point. We don't have the full picture yet. There are benefits to adjusting the maximum window size, but as we learn more it may turn out that we want an entirely different knob or knobs.

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It does not show that it is more or less effective to adjust the window size than to select an appropriate congestion control algorithm (say, BIC).

Any tcp cong. control algorithm is highly dependent on the tcp buffer size. The choice of algorithm changes the behaviour when packets are dropped and in the initial opening of the window, but once the window is open and no packets are being dropped, the algorithm is irrelevant. So BIC, or westwood, or highspeed might do better in the face of dropped packets, but since the current receive buffer is so small, dropped packets are not the problem. Once we can use the sysctl's to tweak the server buffer size, only then is the choice of algorithm going to be important.

Maybe my use of the terminology is imprecise, but clearly the congestion control algorithm matters for determining the TCP window size, which is exactly what we're discussing here.

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It does not show whether the client and server are using TCP optimally.

I'm not sure what you mean by *optimally*. They use tcp the only way they know how non?

I'm talking about whether they use Nagle, when they PUSH, how they use the window (servers can close a window when they are busy, for example), and of course whether they can or should use multiple connections.

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It does not expose problems related to having a single data stream with one blocking head (eg SCTP can allow multiple streams over the same connection; or better performance might be achieved with multiple TCP connections, even if they allow only small windows).

Yes, using multiple tcp connections might be useful, but that doesn't mean you wouldn't want to adjust the tcp window of each one using my patch. Actually, I can't seem to find the quote, but I read somewhere that achieving performance in the WAN can be done 2 different ways: a) If you can tune the buffer sizes that is the best way to go, but b) if you don't have root access to change the linux tcp settings then using multiple tcp streams can compensate for small buffer sizes.

Andy has/had a patch to add multiple tcp streams to NFS. I think his patch and my patch work in collaboration to improve wan performance.

Yep, I've discussed this work with him several times. This might be a more practical solution than allowing larger window sizes (one reason being the dangers of allowing the window to get too large).

While the use of multiple streams has benefits besides increasing the effective TCP window size, only the client side controls the number of connections. The server wouldn't have much to say about it.

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This patch isn't trying to alter default values, or predict buffer sizes based on rtt values, or dynamically alter the tcp window based on dropped packets, etc, it is just giving users the ability to customize the server tcp buffer size.

I know you posted this patch because of the experiments at CITI with long-run 10GbE, and it's handy to now have this to experiment with.

Actually at IBM we have our own reasons for using NFS over the WAN. I would like to get these 2 knobs into the kernel as it is hard to tell customers to apply kernel patches....

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It might also be helpful if we had a patch that made the server perform better in common environments, so a better default setting it seems to me would have greater value than simply creating a new tuning knob.

I think there are possibly 2 (or more) patches. One that improves the default buffer sizes and one that lets sysadmins tweak the value. I don't see why they are mutually exclusive.

They are not. I'm OK with studying the problem and adjusting the defaults appropriately.

The issue is whether adding this knob is the right approach to adjusting the server. I don't think we have enough information to understand if this is the most useful approach. In other words, it seems like a band-aid right now, but in the long run it might be the correct answer.

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My patch is a first step towards allowing NFS into WAN environments. Linux currently has sysctl values for the TCP parameters for exactly this reason, it is impossible to predict the network environment of a linux machine.

If the Linux nfs server isn't going to build off of the existing Linux TCP values (which all sysadmins know how to tweak), then it must allow sysadmins to tweak the NFS server tcp values, either using my patch or some other related patch. I'm open to how the server tcp buffers are tweaked, they just need to be able to be tweaked. For example, if all tcp buffer values in linux were taken out of the /proc file system and hardcoded, I think there would be a revolt.

I'm not arguing for no tweaking. What I'm saying is we should provide knobs that are as useful as possible, and include metrics and clear instructions for when and how to set the knob.

You've shown there is improvement, but not that this is the best solution. It just feels like the work isn't done yet.

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Would it be hard to add a metric or two with this tweak that would allow admins to see how often a socket buffer was completely full, completely empty, or how often the window size is being aggressively cut?

So I've done this using tcpdump in combination with tcptrace. I've shown people at citi how the tcp window grows in the experiment I describe.

No, I mean as a part of the patch that adds the tweak, it should report various new statistics that can allow admins to see that they need adjustment, or that there isn't a problem at all in this area.

Scientific system tuning means assessing the problem, trying a change, then measuring to see if it was effective, or if it caused more trouble. Lather, rinse, repeat.

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While we may not be able to determine a single optimal buffer size for all BDPs, are there diminishing returns in most common cases for increasing the buffer size past, say, 16MB?

Good question. It all depends on how much data you are transferring. In order to fully open a 128MB tcp window over a very long WAN, you will need to transfer at least a few gigabytes of data. If you only transfer 100 MB at a time, then you will probably be fine with a 16 MB window as you are not transferring enough data to open the window anyways. In our environment, we are expecting to transfer 100s of GB if not even more, so the 16 MB window would be very limiting.

What about for a fast LAN?

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The information you are curious about is more relevant to creating better default values of the tcp buffer size. This could be useful, but would be a long process and there are so many variables that I'm not sure that you could pick proper default values anyways. The important thing is that the client can currently set its tcp buffer size via the sysctl's, this is useless if the server is stuck at a fixed value since the tcp window will be the minimum of the client and server's tcp buffer sizes.

Well, Linux servers are not the only servers that a Linux client will ever encounter, so the client-side sysctl isn't as bad as useless. But one can argue whether that knob is ever tweaked by client administrators, and how useful it is.

Definitely not useless. Doing a google search for 'tcp_rmem' returns over 11000 hits describing how to configure tcp settings. (ok, I didn't review every result, but the first few pages of results are telling) It doesn't really matter what OS the client and server use, as long as both have the ability to tweak the tcp buffer size.

The number of hits may reflect the desperation that many have had over the years to get better performance from the Linux NFS implementation. These days we have better performance out of the box, so there is less need for this kind of after-market tweaking.

I think we would be in a much better place if the client and server implementations worked "well enough" in nearly any network or environment. That's been my goal since I started working on Linux NFS seven years ago.

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What is an appropriate setting for a server that has to handle a mix of local and remote clients, for example, or a client that has to connect to a mix of local and remote servers?

Yes, this is a tricky one. I believe the best way to handle it is to set the server tcp buffer to the MAX(local, remote) and then let the local client set a smaller tcp buffer and the remote client set a larger tcp buffer. The problem there is that then what if the local client is also a remote client of another nfs server?? At this point there seems to be some limitations.....

Using multiple connections solves this problem pretty well, I think.

Signature · 2 lines

Chuck Lever
chuck[dot]lever[at]oracle[dot]com

Quoted 1 line · Dean Hildebrand

We have a full picture of TCP. TCP is well known, there are lots of papers/info on it, I have no doubt on what is occurring with TCP as I have traces that clearly show what is happening. All documents and information clearly state that the buffer size is a critical part of improving TCP performance in the WAN. In addition, the congestion control algorithm does NOT control the maximum size of the TCP window. The CCA controls how quickly the window reaches the maximum size, what happens when a packet is dropped and when to close the window. The only item that controls the maximum size of the TCP window is the buffer values that I want a sysctl to tweak (just to be in line with the existing tcp buffer sysctls in Documentation/networking/ip-sysctl.txt)

IMO it's just plain broken that the application layer has to understand and manage this detail about TCP.

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What we don't have is a full picture of the other parts of transferring data from client to server, e.g., Trond just fixed a bug with regards to the writeback cache which should help write performance, that was an unknown up until this point.

Multiple TCP Streams
===============
There is a really big downside to multiple TCP streams: you have multiple TCP streams :) Each one has its own overhead, setup connection cost, and of course TCP window. With a WAN rtt of 200 ms (typical over satellite) and the current buffer size of 4MB, the nfs client would need 50+ TCP connections to achieve the correct performance. That is a
lot of overhead when comparing it with simply following the standard TCP tuning knowhow of increasing the buffer sizes.

I suspect that anyone operating NFS over a sat link would have an already lowered performance expectation.

If the maximum window size defaults to, say, 16MB, and you have a smaller RTT (which is typical of intercontinental 10GbE links which you might be more willing to pump huge amounts of data over than a sat link), you will need fewer concurrent connections to achieve optimal performance, and that becomes more practical.

For networks with a much smaller BDP (like, say, MOST OF THEM :-) you might be able to get away with only a few connections, or even one, if we decide to use a larger default maximum window size.

There are plenty of advantages to having multiple connections between client and server. The fact that it helps the large BDP case is just a bonus.

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The main documentation the show that multiple tcp streams helps over the WAN is from GridFTP experiments. They go over the pos and neg of the approach, but also talk about how tcp buffer size is also very important. Multiple tcp streams is not a replacement for a proper buffer size (http://www.globus.org/alliance/publications/clusterworld/0904GridFinal.pdf)

Here's something else to consider:

TCP is likely not the right transport protocol for networks with a large BDP. Perhaps SCTP, which embeds support for multiple streams in a single connection, is better for this case... and what we really want to do is create an SCTP-based transport capability for NFS. Or maybe we really want to use iWARP over SCTP.

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If you have documentation counteracting these experiments I would be very interested to see them.

I think you are willfully misinterpreting my objection to your sysctl patch.

I never said your experiments are incorrect; they are valuable. My point is that they don't demonstrate that this is a useful knob for our most common use cases, and that it is the correct and only way to get "good enough" performance for most common deployments of NFS. It helps the large BDP case, but as you said, it doesn't make all the problems go away there either.

Is it easy to get optimal results with this? How do admins evaluate the results of changing this value? Is it easy to get bad results with it? Can it result in bad behavior that results in problems for other users of the network?

I also never said "use multiple connections but leave the buffer size alone." I think we agree that a larger receive buffer size is a good idea for the NFS server, in general. The question is whether allowing admins to tune it is the most effective way to benefit performance for our user base, or can we get away with using a more optimal but fixed default size (which is simpler for admins to understand and for us to maintain)?

Or are we just working around what is effectively a defect in TCP itself?

I think we need to look at the bigger picture, which contains plenty of other interesting alternatives that may have larger benefit.

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One Variable or Two
===============
I'd be happy with using a single variable for both the send and receive buffers, but since we are essentially doing the same thing as the net.ipv4.tcp_wmem/rmem variables, I think nfsd_tcp_max_mem would be more in line with existing Linux terminology. (also, we are talking about nfsd, not nfs, so I'd prefer to make that clear in the variable name)

Summary
=======
I'm providing you with all the information I have with regards to my experiments with NFS and TCP. I agree that a better default is needed and my patch allows further experimentation to get to that value. My patch does not add modify current NFS behaviour. It changes a hard coded value for the server buffer size to be a variable in /proc. Blocking a method to modify this hard coded value means blocking further experimentation to find a better default value. My patch is a first step toward trying to find a good default tcp server buffer value.

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Since what we really want to limit is the maximum size of the TCP receive window, it would be more precise to change the name of the new sysctl to something like nfs_tcp_max_window_size.

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Another point is that setting the buffer size isn't always a straightforward process. All papers I've read on the subject, and my experience confirms this, is that setting tcp buffer sizes is more of an art.

So having the server set a good default value is half the battle, but allowing users to twiddle with this value is vital.

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The uses the current buffer sizes in the code are as minimum values, which the user cannot decrease. If the user sets a value of 0 in either /proc entry, it resets the buffer size to the default value. The set /proc values are utilized when the TCP connection is initialized (mount time). The values are bounded above by the *minimum* of the /proc values and the network TCP sysctls.

To demonstrate the usefulness of this patch, details of an experiment between 2 computers with a rtt of 30ms is provided below. In this experiment, increasing the server /proc/sys/sunrpc/tcp_rcvbuf value doubles write performance.

EXPERIMENT
==========
This experiment simulates a WAN by using tc together with netem to add a 30 ms delay to all packets on a nfs client. The goal is to show that by only changing tcp_rcvbuf, the nfs client can increase write performance in the WAN. To verify the patch has the desired effect on the TCP window, I created two tcptrace plots that show the difference in tcp window behaviour before and after the server TCP rcvbuf size is increased. When using the default server tcpbuf value of 6M, we can see the TCP window top out around 4.6 M, whereas increasing the server tcpbuf value to 32M, we can see that the TCP window tops out around 13M. Performance jumps from 43 MB/s to 90 MB/s.

Hardware:
2 dual-core opteron blades
GigE, Broadcom NetXtreme II BCM57065 cards
A single gigabit switch in the middle
1500 MTU
8 GB memory

Software:
Kernel: Bruce's 2.6.25-rc9-CITI_NFS4_ALL-1 tree
RHEL4

NFS Configuration:
64 rpc slots
32 nfsds
Export ext3 file system. This disk is quite slow, I therefore exported using async to reduce the effect of the disk on the back end. This way, the experiments record the time it takes for the data to get to the server (not to the disk).

# exportfs -v
/export <world>(rw,async,wdelay,nohide,insecure,no_root_squash,fsid=0)

# cat /proc/mounts
bear109:/export /mnt nfs rw,vers=3,rsize=1048576,wsize=1048576,namlen=255,hard,nointr,proto=tcp,timeo=600,retrans=2,sec=sys,mountproto=udp,addr=9.1.74.144 0 0

fs.nfs.nfs_congestion_kb = 91840
net.ipv4.tcp_congestion_control = cubic

Network tc Command executed on client:
tc qdisc add dev eth0 root netem delay 30ms
rtt from client (bear108) to server (bear109)
#ping bear109
PING bear109.almaden.ibm.com (9.1.74.144) 56(84) bytes of data.
64 bytes from bear109.almaden.ibm.com (9.1.74.144): icmp_seq=0 ttl=64 time=31.4 ms 64 bytes from bear109.almaden.ibm.com (9.1.74.144): icmp_seq=1 ttl=64 time=32.0 ms

TCP Configuration on client and server:
# Controls IP packet forwarding
net.ipv4.ip_forward = 0
# Controls source route verification net.ipv4.conf.default.rp_filter = 1
# Do not accept source routing net.ipv4.conf.default.accept_source_route = 0
# Controls the System Request debugging functionality of the kernel kernel.sysrq = 0
# Controls whether core dumps will append the PID to the core filename # Useful for debugging multi-threaded applications kernel.core_uses_pid = 1
# Controls the use of TCP syncookies net.ipv4.tcp_syncookies = 1
# Controls the maximum size of a message, in bytes kernel.msgmnb = 65536
# Controls the default maxmimum size of a mesage queue kernel.msgmax = 65536
# Controls the maximum shared segment size, in bytes kernel.shmmax = 68719476736
# Controls the maximum number of shared memory segments, in pages kernel.shmall = 4294967296
### IPV4 specific settings
net.ipv4.tcp_timestamps = 0
net.ipv4.tcp_sack = 1

# on systems with a VERY fast bus -> memory interface this is the big gainer
net.ipv4.tcp_rmem = 4096 16777216 16777216
net.ipv4.tcp_wmem = 4096 16777216 16777216
net.ipv4.tcp_mem = 4096 16777216 16777216
### CORE settings (mostly for socket and UDP effect)
net.core.rmem_max = 16777216
net.core.wmem_max = 16777216
net.core.rmem_default = 16777216
net.core.wmem_default = 16777216
net.core.optmem_max = 16777216
net.core.netdev_max_backlog = 300000
# Don't cache ssthresh from previous connection
net.ipv4.tcp_no_metrics_save = 1
# make sure we don't run out of memory
vm.min_free_kbytes = 32768

Experiments:

On Server: (note that the real tcp buffer size is double tcp_rcvbuf) [root@bear109 ~]# echo 0 > /proc/sys/sunrpc/tcp_rcvbuf [root@bear109 ~]# cat /proc/sys/sunrpc/tcp_rcvbuf
3158016

On Client:
mount -t nfs bear109:/export /mnt
[root@bear108 ~]# iozone -aec -i 0 -+n -f /mnt/test -r 1M -s 500M
...
KB reclen write
512000 1024 43252 umount /mnt

On server:
[root@bear109 ~]# echo 16777216 > /proc/sys/sunrpc/tcp_rcvbuf [root@bear109 ~]# cat /proc/sys/sunrpc/tcp_rcvbuf 16777216

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On Client:
mount -t nfs bear109:/export /mnt
[root@bear108 ~]# iozone -aec -i 0 -+n -f /mnt/test -r 1M -s 500M
...
KB reclen write
512000 1024 90396

The numbers you have here are averages over the whole run. Performing these tests using a variety of record lengths and file sizes (up to several tens of gigabytes) would be useful to see where different memory and network latencies kick in.

Definitely useful, although I'm not sure how this relates to this patch.

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It relates to the whole idea that this is a valid and useful parameter to tweak.

What your experiment shows is that there is some improvement when the TCP window is allowed to expand. It does not demonstrate that the *best* way to provide this facility is to allow administrators to tune the server's TCP buffer sizes.

By definition of how TCP is designed, tweaking the send and receive buffer sizes is a useful. Please see the tcp tuning guides in my other post. I would characterize tweaking the buffers as a necessary condition but not a sufficient condition to achieve good throughput with tcp over long distances.

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A single average number can hide a host of underlying sins. This simple experiment, for example, does not demonstrate that TCP window size is the most significant issue here.

I would say it slightly differently, that it demonstrates that it is significant, but maybe not the *most* significant. There are many possible bottlenecks and possible knobs to tweak. For example, I'm still not achieving link speeds, so I'm sure there are other bottlenecks that are causing reduced performance.

I think that's my basic point. We don't have the full picture yet. There are benefits to adjusting the maximum window size, but as we learn more it may turn out that we want an entirely different knob or knobs.

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It does not show that it is more or less effective to adjust the window size than to select an appropriate congestion control algorithm (say, BIC).

Any tcp cong. control algorithm is highly dependent on the tcp buffer size. The choice of algorithm changes the behaviour when packets are dropped and in the initial opening of the window, but once the window is open and no packets are being dropped, the algorithm is irrelevant. So BIC, or westwood, or highspeed might do better in the face of dropped packets, but since the current receive buffer is so small, dropped packets are not the problem. Once we can use the sysctl's to tweak the server buffer size, only then is the choice of algorithm going to be important.

Maybe my use of the terminology is imprecise, but clearly the congestion control algorithm matters for determining the TCP window size, which is exactly what we're discussing here.

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It does not show whether the client and server are using TCP optimally.

I'm not sure what you mean by *optimally*. They use tcp the only way they know how non?

I'm talking about whether they use Nagle, when they PUSH, how they use the window (servers can close a window when they are busy, for example), and of course whether they can or should use multiple connections.

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It does not expose problems related to having a single data stream with one blocking head (eg SCTP can allow multiple streams over the same connection; or better performance might be achieved with multiple TCP connections, even if they allow only small windows).

Yes, using multiple tcp connections might be useful, but that doesn't mean you wouldn't want to adjust the tcp window of each one using my patch. Actually, I can't seem to find the quote, but I read somewhere that achieving performance in the WAN can be done 2 different ways: a) If you can tune the buffer sizes that is the best way to go, but b) if you don't have root access to change the linux tcp settings then using multiple tcp streams can compensate for small buffer sizes.

Andy has/had a patch to add multiple tcp streams to NFS. I think his patch and my patch work in collaboration to improve wan performance.

Yep, I've discussed this work with him several times. This might be a more practical solution than allowing larger window sizes (one reason being the dangers of allowing the window to get too large).

While the use of multiple streams has benefits besides increasing the effective TCP window size, only the client side controls the number of connections. The server wouldn't have much to say about it.

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This patch isn't trying to alter default values, or predict buffer sizes based on rtt values, or dynamically alter the tcp window based on dropped packets, etc, it is just giving users the ability to customize the server tcp buffer size.

I know you posted this patch because of the experiments at CITI with long-run 10GbE, and it's handy to now have this to experiment with.

Actually at IBM we have our own reasons for using NFS over the WAN. I would like to get these 2 knobs into the kernel as it is hard to tell customers to apply kernel patches....

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It might also be helpful if we had a patch that made the server perform better in common environments, so a better default setting it seems to me would have greater value than simply creating a new tuning knob.

I think there are possibly 2 (or more) patches. One that improves the default buffer sizes and one that lets sysadmins tweak the value. I don't see why they are mutually exclusive.

They are not. I'm OK with studying the problem and adjusting the defaults appropriately.

The issue is whether adding this knob is the right approach to adjusting the server. I don't think we have enough information to understand if this is the most useful approach. In other words, it seems like a band-aid right now, but in the long run it might be the correct answer.

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My patch is a first step towards allowing NFS into WAN environments. Linux currently has sysctl values for the TCP parameters for exactly this reason, it is impossible to predict the network environment of a linux machine.

If the Linux nfs server isn't going to build off of the existing Linux TCP values (which all sysadmins know how to tweak), then it must allow sysadmins to tweak the NFS server tcp values, either using my patch or some other related patch. I'm open to how the server tcp buffers are tweaked, they just need to be able to be tweaked. For example, if all tcp buffer values in linux were taken out of the /proc file system and hardcoded, I think there would be a revolt.

I'm not arguing for no tweaking. What I'm saying is we should provide knobs that are as useful as possible, and include metrics and clear instructions for when and how to set the knob.

You've shown there is improvement, but not that this is the best solution. It just feels like the work isn't done yet.

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Would it be hard to add a metric or two with this tweak that would allow admins to see how often a socket buffer was completely full, completely empty, or how often the window size is being aggressively cut?

So I've done this using tcpdump in combination with tcptrace. I've shown people at citi how the tcp window grows in the experiment I describe.

No, I mean as a part of the patch that adds the tweak, it should report various new statistics that can allow admins to see that they need adjustment, or that there isn't a problem at all in this area.

Scientific system tuning means assessing the problem, trying a change, then measuring to see if it was effective, or if it caused more trouble. Lather, rinse, repeat.

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While we may not be able to determine a single optimal buffer size for all BDPs, are there diminishing returns in most common cases for increasing the buffer size past, say, 16MB?

Good question. It all depends on how much data you are transferring. In order to fully open a 128MB tcp window over a very long WAN, you will need to transfer at least a few gigabytes of data. If you only transfer 100 MB at a time, then you will probably be fine with a 16 MB window as you are not transferring enough data to open the window anyways. In our environment, we are expecting to transfer 100s of GB if not even more, so the 16 MB window would be very limiting.

What about for a fast LAN?

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The information you are curious about is more relevant to creating better default values of the tcp buffer size. This could be useful, but would be a long process and there are so many variables that I'm not sure that you could pick proper default values anyways. The important thing is that the client can currently set its tcp buffer size via the sysctl's, this is useless if the server is stuck at a fixed value since the tcp window will be the minimum of the client and server's tcp buffer sizes.

Well, Linux servers are not the only servers that a Linux client will ever encounter, so the client-side sysctl isn't as bad as useless. But one can argue whether that knob is ever tweaked by client administrators, and how useful it is.

Definitely not useless. Doing a google search for 'tcp_rmem' returns over 11000 hits describing how to configure tcp settings. (ok, I didn't review every result, but the first few pages of results are telling) It doesn't really matter what OS the client and server use, as long as both have the ability to tweak the tcp buffer size.

The number of hits may reflect the desperation that many have had over the years to get better performance from the Linux NFS implementation. These days we have better performance out of the box, so there is less need for this kind of after-market tweaking.

I think we would be in a much better place if the client and server implementations worked "well enough" in nearly any network or environment. That's been my goal since I started working on Linux NFS seven years ago.

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What is an appropriate setting for a server that has to handle a mix of local and remote clients, for example, or a client that has to connect to a mix of local and remote servers?

Yes, this is a tricky one. I believe the best way to handle it is to set the server tcp buffer to the MAX(local, remote) and then let the local client set a smaller tcp buffer and the remote client set a larger tcp buffer. The problem there is that then what if the local client is also a remote client of another nfs server?? At this point there seems to be some limitations.....

Using multiple connections solves this problem pretty well, I think.

Signature · 2 lines

Chuck Lever
chuck[dot]lever[at]oracle[dot]com

Another point is that setting the buffer size isn't always a straightforward process. All papers I've read on the subject, and my experience confirms this, is that setting tcp buffer sizes is more of an art.

Quoted 2 lines · Dean Hildebrand

Another point is that setting the buffer size isn't always a straightforward process. All papers I've read on the subject, and my experience confirms this, is that setting tcp buffer sizes is more of an art.

Aie. It's bad enough if we have a half-dozen or so sysctl's to set to get decent performance out of the nfs server. I don't like to hear that, on top of that, the choice of at least one of those variables is an art....

We can leave some knobs in there for the people that like to read that sort of paper, but the rest of the world will need *some* sort of heuristics.

The reason it is an art is that you don't know the hardware that exists between the client and server. Talking about things like BDP is fine, but in reality there are limited buffer sizes, flaky hardware, fluctuations in traffic, etc etc. Using the BDP as a starting point though seems like the best solution, but since the linux server doesn't know anything about what the BDP is, it is tough to hard code any value into the linux kernel. As you said, if we just give a reasonable default value and then ensure people can play with the knobs. Most people use NFS within a LAN, and to date there has been little if any discussion on using NFS over the WAN (hence my interest), so I would argue that the current values might not be all that bad with regards to defaults (at least we know the behaviour isn't horrible for most people).

Networks are messy. Anyone who wants to work in the WAN is going to have to read about such things, no way around it. A simple google search for 'tcp wan' or 'tcp wan linux' gives loads of suggestions on how to configure your network, so it really isn't a burden on sysadmins to do such a search and then use the given knobs to adjust the tcp buffer size appropriately. My patch gives sysadmins the ability to do the google search and then have some knobs to turn.

Some sample tcp tuning guides that I like:
http://acs.lbl.gov/TCP-tuning/tcp-wan-perf.pdf
http://acs.lbl.gov/TCP-tuning/linux.html
http://gentoo-wiki.com/HOWTO_TCP_Tuning (especially relevant is the part about the receive buffer)

http://www.linuxclustersinstitute.org/conferences/archive/2008/PDF/Hildebrand_98265.pdf
(our initial paper on pNFS tuning)

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The reason it is an art is that you don't know the hardware that exists between the client and server. Talking about things like BDP is fine, but in reality there are limited buffer sizes, flaky hardware, fluctuations in traffic, etc etc. Using the BDP as a starting point though seems like the best solution, but since the linux server doesn't know anything about what the BDP is, it is tough to hard code any value into the linux kernel. As you said, if we just give a reasonable default value and then ensure people can play with the knobs. Most people use NFS within a LAN, and to date there has been little if any discussion on using NFS over the WAN (hence my interest), so I would argue that the current values might not be all that bad with regards to defaults (at least we know the behaviour isn't horrible for most people).

Networks are messy. Anyone who wants to work in the WAN is going to have to read about such things, no way around it. A simple google search for 'tcp wan' or 'tcp wan linux' gives loads of suggestions on how to configure your network, so it really isn't a burden on sysadmins to do such a search and then use the given knobs to adjust the tcp buffer size appropriately. My patch gives sysadmins the ability to do the google search and then have some knobs to turn.

Some sample tcp tuning guides that I like:
http://acs.lbl.gov/TCP-tuning/tcp-wan-perf.pdf
http://acs.lbl.gov/TCP-tuning/linux.html
http://gentoo-wiki.com/HOWTO_TCP_Tuning (especially relevant is the part about the receive buffer)

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http://www.linuxclustersinstitute.org/conferences/archive/2008/PDF/Hildebrand_98265.pdf (our initial paper on pNFS tuning)

Several of those refer to problems that can happen when the receive buffer size is set unusually high, but none of them give a really detailed description of the behavior in that case--do you know of any?