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Subject: Re: [RFC][PATCH 00/26] sched/numa
From: Peter Zijlstra <a.p.zijlstra@chello.nl>
To: Avi Kivity <avi@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
        Andrew Morton <akpm@linux-foundation.org>,
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        Bharata B Rao <bharata.rao@gmail.com>,
        Lee Schermerhorn <Lee.Schermerhorn@hp.com>,
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        Johannes Weiner <hannes@cmpxchg.org>, linux-kernel@vger.kernel.org,
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Date: Mon, 19 Mar 2012 13:20:57 +0100
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On Mon, 2012-03-19 at 13:42 +0200, Avi Kivity wrote:
> It's the standard space/time tradeoff.  Once solution wants more
> storage, the other wants more faults.
> 
> Note scanners can use A/D bits which are cheaper than faults.

I'm not convinced.. the scanner will still consume time even if the
system is perfectly balanced -- it has to in order to determine this.

So sure, A/D/other page table magic can make scanners faster than faults
however you only need faults when you're actually going to migrate a
task. Whereas you always need to scan, even in the stable state.

So while the per-instance times might be in favour of scanning, I'm
thinking the accumulated time is in favour of faults.