netdev - Re: big picture UDP/IP performance question re 2.6.18 -> 2.6.32

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Date:	Tue, 4 Oct 2011 14:16:49 -0500 (CDT)
From:	Christoph Lameter <cl@...two.org>
To:	starlight@...nacle.cx
cc:	Eric Dumazet <eric.dumazet@...il.com>,
	linux-kernel@...r.kernel.org, netdev <netdev@...r.kernel.org>,
	Willy Tarreau <w@....eu>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Stephen Hemminger <stephen.hemminger@...tta.com>
Subject: Re: big picture UDP/IP performance question re 2.6.18  -> 2.6.32

On Mon, 3 Oct 2011, starlight@...nacle.cx wrote:

> I've come to the conclusion that Eric is right
> and the primary issue is an increase in the
> cost of scheduler context switches.  Have
> been watching this number and it has held
> pretty close to 200k/sec under all scenarios
> and kernel versions, so it has to be
> a longer code-path, bigger cache pressure
> or both in the scheduler.  Sadly this makes
> newer kernels a no-go for us.

We had similar experiences. Basically latency constantly gets screwed up
by the new fancy features being added to the scheduler and network
subsystem (most notorious is the new "fair" scheduler, 2.6.23 made a big
step down). The kernel has a fairly constant regression in terms of
latency release after release. Only the new and more efficient processors
periodically provide some compensation (and some isolated patches to
actually improve things get in but these are usually watered down one or
two releases after those improvements have been made).
View attachment "dmesg_c6_263904.txt" of type "TEXT/PLAIN" (66457 bytes)