linux-kernel - Re: [Bugme-new] [Bug 15618] New: 2.6.18->2.6.32->2.6.33 huge regression in performance

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Date:	Tue, 23 Mar 2010 19:27:35 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, bugzilla-daemon@...zilla.kernel.org,
	bugme-daemon@...zilla.kernel.org, ant.starikov@...il.com,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [Bugme-new] [Bug 15618] New: 2.6.18->2.6.32->2.6.33 huge
 regression in performance

* Andrew Morton <akpm@...ux-foundation.org> wrote:

> On Tue, 23 Mar 2010 18:34:09 +0100
> Ingo Molnar <mingo@...e.hu> wrote:
> 
> > 
> > It shows a very brutal amount of page fault invoked mmap_sem spinning 
> > overhead.
> > 
> 
> Yes.  Note that we fall off a cliff at nine threads on a 16-way.  As soon as 
> a core gets two threads scheduled onto it?

it's AMD Opterons so no SMT.

My (wild) guess would be that 8 cpus can still do cacheline ping-pong 
reasonably efficiently, but it starts breaking down very seriously with 9 or 
more cores bouncing the same single cache-line.

Breakdowns in scalability are usually very non-linear, for hardware and 
software reasons. '8 threads' sounds like a hw limit to me. From the scheduler 
POV there's no big difference between 8 or 9 CPUs used [this is non-HT] - with 
8 or 7 cores still idle.

	Ingo
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