From: Ingo Molnar <mingo@elte.hu>
To: Nick Piggin <piggin@cyberone.com.au>
Cc: colpatch@us.ibm.com, linux-kernel <linux-kernel@vger.kernel.org>,
Michael Hohnbaum <hohnbaum@us.ibm.com>
Subject: Re: [rfc][patch] kernel/sched.c oddness?
Date: Thu, 3 Oct 2002 08:54:17 +0200 (CEST) [thread overview]
Message-ID: <Pine.LNX.4.44.0210030840110.4477-100000@localhost.localdomain> (raw)
In-Reply-To: <3D9B89FD.1060400@cyberone.com.au>
> [...] However, I noticed on my 2xSMP system that quite unbalanced loads
> weren't getting even CPU time best example - 3 processes in busywait
> loops - one would get 100% of one cpu while two would get 50% each of
> the other.
this was done intentionally, and this scenario (1+2 tasks) is the very
worst scenario. The problem is that by trying to balance all 3 tasks we
now have 3 tasks that trash their cache going from one CPU to another.
(this is what happens with your patch - even with another approach we'd
have to trash at least one task)
By keeping 2 tasks on one CPU and 1 task on the other CPU we avoid
cross-CPU migration of threads. Think about the 2+3 or 4+5 tasks case
rather, do we want absolutely perfect balancing, or good SMP affinity and
good combined performance?
Ingo
next prev parent reply other threads:[~2002-10-03 6:38 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2002-10-02 18:41 [rfc][patch] kernel/sched.c oddness? Matthew Dobson
2002-10-03 0:06 ` Nick Piggin
2002-10-03 0:30 ` Matthew Dobson
2002-10-03 6:54 ` Ingo Molnar [this message]
2002-10-03 8:32 ` Nick Piggin
2002-10-03 21:15 ` Matthew Dobson
2002-10-04 7:46 ` Ingo Molnar
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