linux-kernel - Re: [patch] sched: fix improper load balance across sched domain

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Message-ID: <20071017072003.GA18044@elte.hu>
Date:	Wed, 17 Oct 2007 09:20:03 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Ken Chen <kenchen@...gle.com>
Cc:	Nick Piggin <nickpiggin@...oo.com.au>,
	"Siddha, Suresh B" <suresh.b.siddha@...el.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [patch] sched: fix improper load balance across sched domain


* Ken Chen <kenchen@...gle.com> wrote:

> We recently discovered a nasty performance bug in the kernel CPU load 
> balancer where we were hit by 50% performance regression.
> 
> When tasks are assigned to a subset of CPUs that span across 
> sched_domains (either ccNUMA node or the new multi-core domain) via 
> cpu affinity, kernel fails to perform proper load balance at these 
> domains, due to several logic in find_busiest_group() miss identified 
> busiest sched group within a given domain. This leads to inadequate 
> load balance and causes 50% performance hit.
[...]
> So proposing the following fix: add addition logic in 
> find_busiest_group to detect intrinsic imbalance within the busiest 
> group.  When such condition is detected, load balance goes into spread 
> mode instead of default grouping mode.

thanks - i've added your fix to the scheduler queue, and i'll check it 
with a few workloads too. (Right now the scheduler queue is blocked by a 
showstopper crasher bug in group scheduling and we are trying to fix 
that first, before doing any other change.)

	Ingo
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