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

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Message-ID: <20071017022303.GA27457@linux-os.sc.intel.com>
Date:	Tue, 16 Oct 2007 19:23:03 -0700
From:	"Siddha, Suresh B" <suresh.b.siddha@...el.com>
To:	Ken Chen <kenchen@...gle.com>
Cc:	Ingo Molnar <mingo@...e.hu>, Nick Piggin <nickpiggin@...oo.com.au>,
	"Siddha, Suresh B" <suresh.b.siddha@...el.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [patch] sched: fix improper load balance across sched domain

On Tue, Oct 16, 2007 at 12:07:06PM -0700, Ken Chen 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.
> 
> To give you a concrete example, on a dual-core, 2 socket numa system,
> there are 4 logical cpu, organized as:

oops, this issue can easily happen when cores are not sharing caches. I
think this is what happening on your setup, right?

thanks,
suresh
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