Does it matter that autotuning grows the socket buffers on a request/response test?

I was getting ready to do some aggregate netperf request/response tests, using the bits that will be the 2.5.0 release of netperf, where the "omni" tests are the default. This means that rather than seeing the initial socket buffer sizes I started seeing the final socket buffer sizes.

Previously I'd explicitly looked at the final socket buffer sizes during TCP_STREAM tests, and emails about that are burried in the archive. But I'd never looked explicitly for request/response tests.

What surprised me was that a TCP request/response test with single-byte requests and responses, and TCP_NODELAY set, could have its socket buffers grown with say no more than 31 transactions outstanding at one time - ie no more than 31 bytes outstanding on the connection in any one direction at any one time.

It does seem repeatable

# HDR="-P 1";for b in 28 29 30 31; do netperf -t omni $HDR -H 15.184.3.62 -- -r 1 -b $b -D -O foo; HDR="-P 0"; done OMNI Send|Recv TEST from 0.0.0.0 (0.0.0.0) port 0 AF_INET to 15.184.3.62 (15.184.3.62) port 0 AF_INET : nodelay : histogram
Local Local Remote Remote Request Response Initial
Elapsed Throughput Throughput
Send Socket Recv Socket Send Socket Recv Socket Size Size Burst
Time Units

Size        Size        Size        Size        Bytes   Bytes Requests (sec)
Final       Final       Final       Final 

16384 87380 16384 87380 1 1 28
10.00 200464.51 Trans/s
16384 87380 16384 87380 1 1 29
10.00 204136.24 Trans/s
121200 87380 121200 87380 1 1 30
10.00 198229.08 Trans/s
121200 87380 121200 87380 1 1 31
10.00 196986.98 Trans/s

# HDR="-P 1";for b in 28 29 30 31; do netperf -t omni $HDR -H 15.184.3.62 -- -r 1 -b $b -D -O foo; HDR="-P 0"; done OMNI Send|Recv TEST from 0.0.0.0 (0.0.0.0) port 0 AF_INET to 15.184.3.62 (15.184.3.62) port 0 AF_INET : nodelay : histogram
Local Local Remote Remote Request Response Initial
Elapsed Throughput Throughput
Send Socket Recv Socket Send Socket Recv Socket Size Size Burst
Time Units

Size        Size        Size        Size        Bytes   Bytes Requests (sec)
Final       Final       Final       Final 

16384 87380 16384 87380 1 1 28
10.00 202550.00 Trans/s
16384 87380 16384 87380 1 1 29
10.00 194460.50 Trans/s
121200 87380 121200 87380 1 1 30
10.00 199372.34 Trans/s
121200 87380 121200 87380 1 1 31
10.00 196089.33 Trans/s

The initial burst code does try to "walk up" to the number of outstanding requests to avoid getting things lumped together thanks to cwnd (*). Though, a tcpdump trace does show the occasional segment of length > 1:

# tcpdump -r /tmp/trans.pcap  tcp and not port 12865 | awk '{print $NF}' | sort -n | uniq -c
reading from file /tmp/trans.pcap, link-type EN10MB (Ethernet)
     17 0
1903752 1
     28 2
     29 3
     10 4
     11 5
      9 6
     14 7
     18 8
      9 9
     12 10
      3 11

Still, should that have caused the socket buffers to grow? FWIW, it isn't all single-byte transactions for a burst size of 29 either:

# tcpdump -r /tmp/trans_29.pcap  tcp and not port 12865 | awk '{print $NF}' | sort -n | uniq -c
reading from file /tmp/trans_29.pcap, link-type EN10MB (Ethernet)
     13 0
1771215 1
      4 2
      2 3
      3 4
      2 5
      2 6
      1 7
      2 8
      1 9
      1 11

but that does not seem to grow the socket buffers. 2.6.38-8-server on both sides through a Mellanox MT26438 operating as 10GbE.

rick jones

* #ifdef WANT_FIRST_BURST

	/* so, since we've gotten a response back, update the
	   bookkeeping accordingly.  there is one less request
	   outstanding and we can put one more out there than before. */
	requests_outstanding -= 1;
	if ((request_cwnd < first_burst_size) &&
	    (NETPERF_IS_RR(direction))) {
	  request_cwnd += 1;
	  if (debug) {
	    fprintf(where,
		    "incr req_cwnd to %d first_burst %d reqs_outstndng %d\n",
		    request_cwnd,
		    first_burst_size,
		    requests_outstanding);
	  }
	}

#endif

Also, some larger burst sizes also cause the receive socket buffer to increase:

# HDR="-P 1";for b in 0 1 2 4 16 64 128 256; do netperf -t omni $HDR -H 15.184.3.62 -- -r 1 -b $b -D -O foo; HDR="-P 0"; done OMNI Send|Recv TEST from 0.0.0.0 (0.0.0.0) port 0 AF_INET to 15.184.3.62 (15.184.3.62) port 0 AF_INET : nodelay : histogram
Local Local Remote Remote Request Response Initial
Elapsed Throughput Throughput
Send Socket Recv Socket Send Socket Recv Socket Size Size Burst
Time Units

Size        Size        Size        Size        Bytes   Bytes Requests (sec)
Final       Final       Final       Final 

16384 87380 16384 87380 1 1 0
10.00 20838.10 Trans/s
16384 87380 16384 87380 1 1 1
10.00 38204.89 Trans/s
16384 87380 16384 87380 1 1 2
10.00 52497.02 Trans/s
16384 87380 16384 87380 1 1 4
10.00 70641.97 Trans/s
16384 87380 16384 87380 1 1 16
10.00 136965.24 Trans/s
121200 87380 121200 87380 1 1 64
10.00 197037.63 Trans/s
121200 87380 16384 87380 1 1 128
10.00 203092.56 Trans/s
121200 313248 121200 349392 1 1 256
10.00 163766.32 Trans/s