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Subject: Re: [RFC PATCH v3 0/3] sched: simplify the select_task_rq_fair()
From: Peter Zijlstra <a.p.zijlstra@chello.nl>
To: Michael Wang <wangyun@linux.vnet.ibm.com>
Cc: Mike Galbraith <efault@gmx.de>, Ingo Molnar <mingo@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>, Paul Turner <pjt@google.com>,
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        Ram Pai <linuxram@us.ibm.com>,
        "Nikunj A. Dadhania" <nikunj@linux.vnet.ibm.com>,
        Namhyung Kim <namhyung@kernel.org>
Date: Fri, 22 Feb 2013 10:39:22 +0100
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On Fri, 2013-02-22 at 17:10 +0800, Michael Wang wrote:
> On 02/22/2013 04:21 PM, Peter Zijlstra wrote:
> > On Fri, 2013-02-22 at 10:36 +0800, Michael Wang wrote:
> >> According to my understanding, in the old world, wake_affine() will
> >> only
> >> be used if curr_cpu and prev_cpu share cache, which means they are in
> >> one package, whatever search in llc sd of curr_cpu or prev_cpu, we
> >> won't
> >> have the chance to spread the task out of that package.
> > 
> > Nah, look at where SD_WAKE_AFFINE is set. Only 'remote/big' NUMA domains
> > don't have it set, but 'small' NUMA systems will have it set over the
> > entire domain tree.
> 
> Oh, I missed that point...
> 
> But I don't get the reason to make NUMA level affine, cpus in different
> nodes share cache? doesn't make sense...

Contrary, it makes more sense, the more expensive it is to run 'remote'
the better it is to pull 'related' tasks together.