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Re: [PATCH 4/4] mm: numa: Slow PTE scan rate if migration failures occur

To: Dave Chinner <david@xxxxxxxxxxxxx>
Subject: Re: [PATCH 4/4] mm: numa: Slow PTE scan rate if migration failures occur
From: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Date: Fri, 20 Mar 2015 10:02:23 -0700
Cc: Mel Gorman <mgorman@xxxxxxx>, Ingo Molnar <mingo@xxxxxxxxxx>, Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>, Aneesh Kumar <aneesh.kumar@xxxxxxxxxxxxxxxxxx>, Linux Kernel Mailing List <linux-kernel@xxxxxxxxxxxxxxx>, Linux-MM <linux-mm@xxxxxxxxx>, xfs@xxxxxxxxxxx, ppc-dev <linuxppc-dev@xxxxxxxxxxxxxxxx>
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Sender: linus971@xxxxxxxxx
On Thu, Mar 19, 2015 at 9:13 PM, Dave Chinner <david@xxxxxxxxxxxxx> wrote:
>
> Testing now. It's a bit faster - three runs gave 7m35s, 7m20s and
> 7m36s. IOWs's a bit better, but not significantly. page migrations
> are pretty much unchanged, too:
>
>            558,632      migrate:mm_migrate_pages ( +-  6.38% )

Ok. That was kind of the expected thing.

I don't really know the NUMA fault rate limiting code, but one thing
that strikes me is that if it tries to balance the NUMA faults against
the *regular* faults, then maybe just the fact that we end up taking
more COW faults after a NUMA fault then means that the NUMA rate
limiting code now gets over-eager (because it sees all those extra
non-numa faults).

Mel, does that sound at all possible? I really have never looked at
the magic automatic rate handling..

                         Linus

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