linux-kernel - Re: [PATCH 19/19] mm: sched: numa: Implement slow start for working set sampling

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Message-ID: <50996B8B.30404@redhat.com>
Date:	Tue, 06 Nov 2012 14:56:59 -0500
From:	Rik van Riel <riel@...hat.com>
To:	Mel Gorman <mgorman@...e.de>
CC:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Ingo Molnar <mingo@...nel.org>,
	Johannes Weiner <hannes@...xchg.org>,
	Hugh Dickins <hughd@...gle.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux-MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 19/19] mm: sched: numa: Implement slow start for working
 set sampling

On 11/06/2012 04:14 AM, Mel Gorman wrote:
> From: Peter Zijlstra <a.p.zijlstra@...llo.nl>
>
> Add a 1 second delay before starting to scan the working set of
> a task and starting to balance it amongst nodes.
>
> [ note that before the constant per task WSS sampling rate patch
>    the initial scan would happen much later still, in effect that
>    patch caused this regression. ]
>
> The theory is that short-run tasks benefit very little from NUMA
> placement: they come and go, and they better stick to the node
> they were started on. As tasks mature and rebalance to other CPUs
> and nodes, so does their NUMA placement have to change and so
> does it start to matter more and more.
>
> In practice this change fixes an observable kbuild regression:
>
>     # [ a perf stat --null --repeat 10 test of ten bzImage builds to /dev/shm ]
>
>     !NUMA:
>     45.291088843 seconds time elapsed                                          ( +-  0.40% )
>     45.154231752 seconds time elapsed                                          ( +-  0.36% )
>
>     +NUMA, no slow start:
>     46.172308123 seconds time elapsed                                          ( +-  0.30% )
>     46.343168745 seconds time elapsed                                          ( +-  0.25% )
>
>     +NUMA, 1 sec slow start:
>     45.224189155 seconds time elapsed                                          ( +-  0.25% )
>     45.160866532 seconds time elapsed                                          ( +-  0.17% )
>
> and it also fixes an observable perf bench (hackbench) regression:
>
>     # perf stat --null --repeat 10 perf bench sched messaging
>
>     -NUMA:
>
>     -NUMA:                  0.246225691 seconds time elapsed                   ( +-  1.31% )
>     +NUMA no slow start:    0.252620063 seconds time elapsed                   ( +-  1.13% )
>
>     +NUMA 1sec delay:       0.248076230 seconds time elapsed                   ( +-  1.35% )
>
> The implementation is simple and straightforward, most of the patch
> deals with adding the /proc/sys/kernel/balance_numa_scan_delay_ms tunable
> knob.
>
> Signed-off-by: Peter Zijlstra <a.p.zijlstra@...llo.nl>
> Cc: Linus Torvalds <torvalds@...ux-foundation.org>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Peter Zijlstra <a.p.zijlstra@...llo.nl>
> Cc: Andrea Arcangeli <aarcange@...hat.com>
> Cc: Rik van Riel <riel@...hat.com>
> [ Wrote the changelog, ran measurements, tuned the default. ]
> Signed-off-by: Ingo Molnar <mingo@...nel.org>
> Signed-off-by: Mel Gorman <mgorman@...e.de>

Reviewed-by: Rik van Riel <riel@...hat.com>

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