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

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Message-ID: <b040c32a0710171008s4acf8d80y137ace2bfd0a894e@mail.gmail.com>
Date:	Wed, 17 Oct 2007 10:08:00 -0700
From:	"Ken Chen" <kenchen@...gle.com>
To:	"Siddha, Suresh B" <suresh.b.siddha@...el.com>
Cc:	"Ingo Molnar" <mingo@...e.hu>,
	"Nick Piggin" <nickpiggin@...oo.com.au>,
	"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 10/16/07, Siddha, Suresh B <suresh.b.siddha@...el.com> wrote:
> 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?

yes, we observed the bad behavior on quad-core system with separate L2
cache as well.

- Ken
-
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