From owner-freebsd-net Tue Oct 15 18:30:48 2002 Delivered-To: freebsd-net@freebsd.org Received: from mx1.FreeBSD.org (mx1.freebsd.org [216.136.204.125]) by hub.freebsd.org (Postfix) with ESMTP id ABB1F37B401 for ; Tue, 15 Oct 2002 18:30:46 -0700 (PDT) Received: from carp.icir.org (carp.icir.org [192.150.187.71]) by mx1.FreeBSD.org (Postfix) with ESMTP id 4F0E343E7B for ; Tue, 15 Oct 2002 18:30:46 -0700 (PDT) (envelope-from rizzo@carp.icir.org) Received: from carp.icir.org (localhost [127.0.0.1]) by carp.icir.org (8.12.3/8.12.3) with ESMTP id g9G1UipJ029135; Tue, 15 Oct 2002 18:30:44 -0700 (PDT) (envelope-from rizzo@carp.icir.org) Received: (from rizzo@localhost) by carp.icir.org (8.12.3/8.12.3/Submit) id g9G1Ui2C029134; Tue, 15 Oct 2002 18:30:44 -0700 (PDT) (envelope-from rizzo) Date: Tue, 15 Oct 2002 18:30:44 -0700 From: Luigi Rizzo To: Paul Herman Cc: Lars Eggert , Steve Francis , Kirill Ponomarew , freebsd-net@FreeBSD.ORG Subject: Re: delayed ACK Message-ID: <20021015183044.A29061@carp.icir.org> References: <3DAC8206.1080604@isi.edu> <20021015162359.B8363-100000@mammoth.eat.frenchfries.net> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline User-Agent: Mutt/1.2.5.1i In-Reply-To: <20021015162359.B8363-100000@mammoth.eat.frenchfries.net>; from pherman@frenchfries.net on Tue, Oct 15, 2002 at 05:25:42PM -0700 Sender: owner-freebsd-net@FreeBSD.ORG Precedence: bulk List-ID: List-Archive: (Web Archive) List-Help: (List Instructions) List-Subscribe: List-Unsubscribe: X-Loop: FreeBSD.org 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