Date: Tue, 15 Oct 2002 18:30:44 -0700 From: Luigi Rizzo <rizzo@icir.org> To: Paul Herman <pherman@frenchfries.net> Cc: Lars Eggert <larse@ISI.EDU>, Steve Francis <steve@expertcity.com>, Kirill Ponomarew <ponomarew@oberon.net>, freebsd-net@FreeBSD.ORG Subject: Re: delayed ACK Message-ID: <20021015183044.A29061@carp.icir.org> In-Reply-To: <20021015162359.B8363-100000@mammoth.eat.frenchfries.net>; from pherman@frenchfries.net on Tue, Oct 15, 2002 at 05:25:42PM -0700 References: <3DAC8206.1080604@isi.edu> <20021015162359.B8363-100000@mammoth.eat.frenchfries.net>
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this smells a lot as a bad interaction between default window size and mtu -- loopback has 16k default, maybe tar uses a smallish window (32k is default now for net.inet.tcp.sendspace, but used to be 16k at the time), which means only 1 or 2 packets in flight at once, meaning that many times you get the 200ms delay and your throughput goes way down. cheers luigi On Tue, Oct 15, 2002 at 05:25:42PM -0700, Paul Herman wrote: ... > Aha! Another victim who is willing to take a look at this! :-) > > It's an issue that was left unresolved in kern/24645. Bruce Evans > brought this to my attention back during the unrelated "I have > delayed ACK problems" thread on -net in January of 2001 and I then > passed it on to jlemon. If you need a packet trace, let me know, > but you should be able to reproduce it yourself. Even today on my > 4.7-PRERELEASE I still get: > > mammoth# sysctl net.inet.tcp.delayed_ack=0 > net.inet.tcp.delayed_ack: 1 -> 0 > mammoth# time tar cf 127.0.0.1:/tmp/foo /kernel > 0.000u 0.041s 0:00.33 12.1% 350+300k 0+0io 0pf+0w > > mammoth# sysctl net.inet.tcp.delayed_ack=1 > net.inet.tcp.delayed_ack: 0 -> 1 > mammoth# time tar cf 127.0.0.1:/tmp/foo /kernel > 0.014u 0.033s 0:45.90 0.0% 700+600k 0+0io 0pf+0w > ^^^^^^^ > > It seems that lowering lo0 mtu to 1500 makes this particular > problem go away. The magic mtu size is 2100. This makes me think > that this is a big problem across GigE using 8K jumbo frames, not > sure. Also, taring over the IPv6 lo0 interface seems to work OK. > > No idea what causes this. > > -Paul. > > > To Unsubscribe: send mail to majordomo@FreeBSD.org > with "unsubscribe freebsd-net" in the body of the message To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-net" in the body of the message
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