CONFIG_PACKET_MMAP help
Hi,
I upgrading an application to the CONFIG_PACKET_MMAP interface, and was trying to figure out how the api works. I 'RTFS' But had a few questions:
1. for tp_frame_size, I dont want to truncate any data on ethernet, I need 1514 bytes, is this the best way to do it and not waste space?
static const int TURBO_FRAME_SIZE=TPACKET_ALIGN(TPACKET_ALIGN(sizeof(tpacket_hdr))+TPACKET_ALIGN(sizeof(struct sockaddr_ll)+ETH_HLEN) + 1500);
2. what is tp_block_nr for? I dont understand it, I just set it to 1 and make tp_block_size big enough for all the frames I need, so its just one contiguous space, all I need is about a megabyte I think.
3. is this the general approach for the api?
open socket
set ring sizemmap()
h starts at frame[0] of the mmaped area
while(1) {
if (tp->status == 0) poll() for pollin on the socket /* is there a race here? */
parse/copy out what I want from h + h->tp_mac
set tp->status to 0 when I am done
h = next packet in ring, or wraps
}
4. what does the copy threshold setsockopt tuning accomplish? doesnt it always have to copy anyway, to the mmaped area?
Thanks,c.c. me por favor...
K. Lohan
------------------------
kambo at home dawt com
Hi,
Quoted 7 lines · [email protected] wrote:
1. for
tp_frame_size, I dont want to truncate any data on ethernet, I need 1514 bytes, is this the best way to do it and not waste space?static const int TURBO_FRAME_SIZE=
TPACKET_ALIGN(TPACKET_ALIGN(sizeof(tpacket_hdr)) + TPACKET_ALIGN(sizeof(struct sockaddr_ll)+ETH_HLEN) + 1500);
Looks OK. Maybe instead of ETH_HLEN min(ETH_HLEN,16)? The framesize calculation is really strange...
Quoted 3 lines
2. what is
tp_block_nrfor? I dont understand it, I just set it to 1 and maketp_block_sizebig enough for all the frames I need, so its just one contiguous space, all I need is about a megabyte I think.
Better go the other way around - set tb_block_size to PAGE_SIZE and tb_block_nr appropriate. tb_block_size is the contiguous physical memory the kernel tries to allocate. Anything above PAGE_SIZE is likely to fail. For you that would mean only 2 packets per 4k-page. You could try to start with bigger (power of 2) block sizes and go down to smaller ones if it fails (ENOMEM). [1]. Btw, there's in implicit limit on tb_block_nr. The vector to manage the blocks is kmalloc'ed and may not be larger than 128kb giving max 32768 blocks. Hmm... moment... seems there's a similar limit for tp_frame_nr (max 32768 frames). I'm pretty sure _that_ limit was not there when I worked with this during 2.3. Not so nice on gigabit ethernet :-(
Quoted 2 lines
3. is this the general approach for the api?
[...]
Looks OK too.
Quoted 2 lines
if (
tp->status== 0)poll()for pollin on the socket /* is there a race here? */
No race.
Quoted 2 lines
4. what does the copy threshold setsockopt tuning accomplish? doesnt it always have to copy anyway, to the mmaped area?
I haven't used it myself. Reading the sources it does something different. Afaics when active if there's a packet that has been truncated by the framesize it is additionally stored in the socket's receive queue to be fetched by a normal read/recv. It notifies you about this by setting the TP_STATUS_COPY bit. So it seems to mean: copy to socket if threshold (framesize) exceeded.
Ciao, ET.
[1] The PACKET_RX_RING sockopt accepts all block sizes that are a multiple of PAGE_SIZE but always allocates a power of 2 size chunk. So using non power of 2 sizes will waste locked kernel memory.
Hello!
Quoted 2 lines
1. for
tp_frame_size, I dont want to truncate any data on ethernet, I need 1514 bytes, is this the best way to do it and not waste space?
To select small snapsize (obtained from later experiments), to set PACKET_COPY_THRESH to read larger packets via recvmsg().
Quoted 3 lines
2. what is
tp_block_nrfor? I dont understand it, I just set it to 1 and maketp_block_sizebig enough for all the frames I need, so its just one contiguous space, all I need is about a megabyte I think.
Kernel has problems with allocating large chunks of memory. If you see problems with allocating large chuns, split them to less ones.
Quoted 3 lines
while(1) {
if (tp->status== 0)poll()for pollin on the socket /* is there a race here? */
No. poll returns, when new frame appears.
Quoted 2 lines
4. what does the copy threshold setsockopt tuning accomplish? doesnt it always have to copy anyway, to the mmaped area?
see anser to question 1. It has a sense when size of chunk is small enough. Small packets are copied to ring, large ones (which are truncated) are queued to socket to be received via recvmsg().
Alexey