linux-kernel - linux thread scheduling

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Message-ID: <CALKntY2tXfF3UTfL=kvuotDbdfvp2V_WBhs3hx9CaSPEcbfrdg@mail.gmail.com>
Date:	Wed, 8 Feb 2012 21:45:55 -0500
From:	Xin Tong <xerox.time.tech@...il.com>
To:	linux-kernel <linux-kernel@...r.kernel.org>
Subject: linux thread scheduling

I have a program that spawns 3 threads and there is a great deal of
sharing among the three threads. they all read/update a couple of
shared variables.

I run this on nehalem (8 cores, 4 core in each physical package),  I
find that running the 3 threads on core 0, 1, 2 (using
pthread_setaffinitiy) gives much better results, this is because the
cache coherency protocol performs much better on the same package.
However, if  I leave Linux (Suse Enterprise) to schedule it. it gives
much worse performance, i suspect that linux is scheduling the 3
threads across the physical packages ( 2 on one package, 1 on
another). Is this possible ? why does linux do this ?

Thanks

Xin
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