linux-kernel - Re: idle issues running sembench on 128 cpus

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Date:	Thu, 5 May 2011 01:47:44 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Andi Kleen <andi@...stfloor.org>
cc:	Dave Kleikamp <dkleikamp@...il.com>,
	Chris Mason <chris.mason@...cle.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Tim Chen <tim.c.chen@...ux.intel.com>,
	linux-kernel@...r.kernel.org, lenb@...nel.org, paulmck@...ibm.com
Subject: Re: idle issues running sembench on 128 cpus

On Thu, 5 May 2011, Andi Kleen wrote:
> On Thu, May 05, 2011 at 01:29:49AM +0200, Thomas Gleixner wrote:
> > That makes sense, but merging the timeouts race free will be a real
> > PITA.
> 
> For this case one could actually use a spinlock between the siblings.
> That shouldn't be a problem as long as it's not a global spinlock.

Care to give it a try ?

> > > Also if it's HPET you could actually use multiple independent HPET channels.
> > > I remember us discussing this a long time ago... Not sure if it's worth
> > > it, but it may be a small relief.
> > 
> > Multiple broadcast devices. That sounds still horrible :)
> 
> 
> It would cut contention in half or more at least. Not great,
> but sometimes you take everything you can get.

To a certain degree. If the code pain is larger than the benefit ...
 
> Here's a new patch without the raw. Boots on my Westmere.

> +	cpu = raw_smp_processor_id();

Hmm. quilt refresh perhaps ? I know that feeling :)

Thanks,

	tglx
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