On Thu, 1 Mar 2007, Andre Oppermann wrote:
With the TCP socket buffer autosizing and generally larger socket buffers
for high bandwidth and high delay connections tcp_output() has become
increasingly inefficient for sending segments. For every segment sent it is
traversing the entire socket buffer mbuf chain until it finds the offset to
continue from. Usually this is close to the end of the chain. Once it got
past a few dozen mbufs it starts to bust the CPU caches and performance
starts to fall off.
This patch solves the problem by maintaining an offset pointer in the socket
buffer to give tcp_output() the closest mbuf right away avoiding the
traversal from the beginning.
With this patch we should be able to compete nicely for the Internet land
speed record again.
The patch is here:
http://people.freebsd.org/~andre/sockbuf_sndptr-20070301.diff
Any testing, especially on 10Gig cards, and feedback appreciated.
I'm going to be on- and off-line from now through about the last week of March
on travel to LinuxForum.dk, AsiaBSDCon, and UKUUG. However, once I get back
online from that in late March through the end of April, I will be spending
quite a bit of time looking at high performance packet capture issues, so can
certainly spend some time looking at this. Per our earlier discussions and my
previous exploration of this issue, I see precisely this sort of problem with
TCP as socket buffer size grows, so should be set up to fairly easily test for
an improvement once I have my feet on the ground. If I'm lucky I may get a
breather for a few days from about 15 March - 18 March and could take an early
look at it then.
Thanks,
Robert N M Watson
Computer Laboratory
University of Cambridge
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