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Date:	Mon, 08 Sep 2008 23:02:21 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	csnook@...hat.com
Cc:	rick.jones2@...com, netdev@...r.kernel.org
Subject: Re: RFC: Nagle latency tuning

From: Chris Snook <csnook@...hat.com>
Date: Tue, 09 Sep 2008 01:56:12 -0400

[ Please hit enter every 80 columns or so, your emails are
  unreadable until I reformat your text by hand, thanks. ]

> That's not the problem I'm talking about here.  The problem I'm
> seeing is that if your burst of messages is too small to fill the
> MTU, the network stack will just sit there and stare at you for
> precisely 40 ms (an eternity for a financial app) before
> transmitting.  Andi may be correct that it's actually the delayed
> ACK we're seeing, but I can't figure out where that 40 ms magic
> number is coming from.
>
> The easiest way to see the problem is to open a TCP socket to an
> echo daemon on loopback, make a bunch of small writes totaling less
> than your loopback MTU (accounting for overhead), and see how long
> it takes to get your echoes.  You can probably do this with netcat,
> though I haven't tried.  People don't expect loopback to have 40 ms
> latency when the box is lightly loaded, so they'd really like to
> tweak that down when it's hurting them.

That's informative, but please provide a specific test case and
example trace so we can discuss something concrete.

Thank you.
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