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

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Date:	Fri, 20 Mar 2015 10:02:23 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Dave Chinner <david@...morbit.com>
Cc:	Mel Gorman <mgorman@...e.de>, Ingo Molnar <mingo@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Aneesh Kumar <aneesh.kumar@...ux.vnet.ibm.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linux-MM <linux-mm@...ck.org>, xfs@....sgi.com,
	ppc-dev <linuxppc-dev@...ts.ozlabs.org>
Subject: Re: [PATCH 4/4] mm: numa: Slow PTE scan rate if migration failures occur

On Thu, Mar 19, 2015 at 9:13 PM, Dave Chinner <david@...morbit.com> 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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