linux-kernel - Re: aim7 scalability issue on 4 socket machine

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Date:	Fri, 18 Sep 2009 09:15:02 -0400
From:	Lee Schermerhorn <Lee.Schermerhorn@...com>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	Hugh Dickins <hugh.dickins@...cali.co.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Zhang, Yanmin" <yanmin_zhang@...ux.intel.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	Arjan van de Ven <arjan@...ux.intel.com>
Subject: Re: aim7 scalability issue on 4 socket machine

On Fri, 2009-09-18 at 09:12 +0200, Andi Kleen wrote:
> On Fri, Sep 18, 2009 at 07:53:58AM +0100, Hugh Dickins wrote:
> > On Thu, 17 Sep 2009, Andrew Morton wrote:
> > > On Fri, 18 Sep 2009 10:02:19 +0800 "Zhang, Yanmin" <yanmin_zhang@...ux.intel.com> wrote:
> > > > > 
> > > > > So, Yanmin, please retest with http://lkml.org/lkml/2009/9/13/25
> > > > > and let us know if that works as well for you - thanks.
> > > > I tested Lee's patch and it does fix the issue.
> > 
> > Thanks for checking and reporting back, Yanmin.
> > 
> > > 
> > > Do we think we should cook up something for -stable?
> > 
> > Gosh, I laughed at Lee (sorry!) for suggesting it for -stable:
> > is stable really for getting a better number out of a benchmark?
> 
> When your system is large enough scalability problems (e.g.
> lock contention) can be a serious bug. i.e. when your workload
> is 150% slower than expected that can well be a show stopper.
> 
> Admittedly the workload in this case was a benchmark, but it's
> not that far fetched to expect the same problem in a real application.
> 
> We had a similar problem with the accounting lock some time 
> ago, I think that patch also went in.
> 
> So yes I think simple non intrusive fixes for serious scalability
> problems should be stable candidates.
> 
> > > Either this is a regression or the workload is particularly obscure. 
> > 
> > I've not cross-checked descriptions, but assume Lee was actually
> > testing on exactly the same kind of upcoming Nehalem as Yanmin, and
> > that machine happens to have characteristics which show up badly here.
> 
> AFAIK Lee usually tests on large IA64 boxes.

In this case, it's an x86_64 [DL785] platform--an 8 socket, 4 core
Shanghai in a glueless, "twisted ladder" config.

Lee


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