excessive TCP duplicate acks?
Andre Guibert de Bruet
andy at siliconlandmark.com
Tue Feb 20 06:30:30 UTC 2007
On Jan 26, 2007, at 11:59 AM, Andrew Gallatin wrote:
> When running some benchmarks, I noticed tons of duplicate acks showing
> up in systat -tcp (thousands, or tens of thousands per second).
> Taking a trace, I see that -current seems to send "lots" of duplicate
> acks. At first I thought this was a driver bug, but I've seen it with
> 3 different drivers (mxge, nve, xl) and at various network speeds. It
> seems to happen when the -current machine is the "sender" in a
> netperf, and seems to happen with both a linux a FreeBSD receiver,
> and is easy to reproduce using -current from yesterday (running
> on amd64 if it matters).
>
>> From my very naive tcpdump reading skills, it looks like the FreeBSD
> machine sends a full window with a partial payload and a push flag in
> the last segment. It ignores (or does not yet see the receiver's
> acks). It then spews tons of duplicate acks at the reciever until it
> notices the acks, and starts sending data again. This happens over
> and over again..
>
> Is this normal, or is there something wrong?
>
> In the appended tcpdump snippet taken at the receiver, 172.31.193.16
> was sending a netperf (netperf -H172.31.193.15 -- -s65535 -S32767) to
> 172.31.193.15. I can make a raw dump file available if anybody
> is interested.
>
> <..many packets omitted..>
I saw this behavior on an Intel gigabit NIC (em driver) with a kernel
from January 22nd. This problem still persists with a kernel from
today. Enabling/Disabling tx/rxcsum doesn't help.
Machine details can be found up at http://bling.properkernel.com/
freebsd/ (Which is incidentally the machine that is seeing these
issues). If you would like to see just what kind of traffic patterns
I am seeing, load up wireshark / tcpdump and download one of the
freebsd release images on the webserver.
uname -a: FreeBSD bling.properkernel.com 7.0-CURRENT FreeBSD 7.0-
CURRENT #8: Mon Feb 19 16:21:52 EST 2007
andy at bling.properkernel.com:/usr/obj/usr/src/sys/BLING i386
Andy
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