Date: Tue, 20 Feb 2007 01:18:07 -0500 From: Andre Guibert de Bruet <andy@siliconlandmark.com> To: Andrew Gallatin <gallatin@cs.duke.edu> Cc: current@freebsd.org Subject: Re: excessive TCP duplicate acks? Message-ID: <2FE2BC67-A829-4BF9-B606-65DE1393E8DE@siliconlandmark.com> In-Reply-To: <17850.13146.266196.499166@grasshopper.cs.duke.edu> References: <17850.13146.266196.499166@grasshopper.cs.duke.edu>
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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@bling.properkernel.com:/usr/obj/usr/src/sys/BLING i386 Andy /* Andre Guibert de Bruet * 6f43 6564 7020 656f 2e74 4220 7469 6a20 */ /* Code poet / Sysadmin * 636f 656b 2e79 5320 7379 6461 696d 2e6e */ /* GSM: +1 734 846 8758 * 5520 494e 2058 6c73 7565 6874 002e 0000 */ /* WWW: siliconlandmark.com * C/C++, Java, Perl, PHP, SQL, XHTML, XML */
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