linux-kernel - lmbench ctxsw regression with CFS

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Message-ID: <20070802021525.GC15595@wotan.suse.de>
Date:	Thu, 2 Aug 2007 04:15:25 +0200
From:	Nick Piggin <npiggin@...e.de>
To:	Ingo Molnar <mingo@...e.hu>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: lmbench ctxsw regression with CFS

Hi,

I didn't follow all of the scheduler debates and flamewars, so apologies
if this was already covered. Anyway.

lmbench 3 lat_ctx context switching time with 2 processes bound to a
single core increases by between 25%-35% on my Core2 system (didn't do
enough runs to get more significance, but it is around 30%). The problem
bisected to the main CFS commit.

I was really hoping that a smaller runqueue data structure could actually
increase performance with the common case of small numbers of tasks :(

I assume this was a known issue before CFS was merged. Do you know what is
causing the slowdown? Any plans to fix it?

Thanks,
Nick
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