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From: Rik van Riel <riel@redhat.com>
To: Ingo Molnar <mingo@kernel.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>,
	linux-kernel@vger.kernel.org, linux-mm@kvack.org,
	Paul Turner <pjt@google.com>,
	Lee Schermerhorn <Lee.Schermerhorn@hp.com>,
	Christoph Lameter <cl@linux.com>, Mel Gorman <mgorman@suse.de>,
	Andrew Morton <akpm@linux-foundation.org>,
	Andrea Arcangeli <aarcange@redhat.com>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [PATCH 5/8] sched, numa, mm: Add adaptive NUMA affinity support
Date: Fri, 16 Nov 2012 13:23:01 -0500	[thread overview]
Message-ID: <50A68485.7030502@redhat.com> (raw)
In-Reply-To: <20121116181433.GA4763@gmail.com>

On 11/16/2012 01:14 PM, Ingo Molnar wrote:
>
> * Rik van Riel <riel@redhat.com> wrote:
>
>> On 11/12/2012 11:04 AM, Peter Zijlstra wrote:
>>
>>> We change the load-balancer to prefer moving tasks in order of:
>>>
>>>    1) !numa tasks and numa tasks in the direction of more faults
>>>    2) allow !ideal tasks getting worse in the direction of faults
>>>    3) allow private tasks to get worse
>>>    4) allow shared tasks to get worse
>>>
>>> This order ensures we prefer increasing memory locality but when
>>> we do have to make hard decisions we prefer spreading private
>>> over shared, because spreading shared tasks significantly
>>> increases the interconnect bandwidth since not all memory can
>>> follow.
>>
>> Combined with the fact that we only turn a certain amount of
>> memory into NUMA ptes each second, could this result in a
>> program being classified as a private task one second, and a
>> shared task a few seconds later?
>
> It's a statistical method, like most of scheduling.
>
> It's as prone to oscillation as tasks are already prone to being
> moved spuriously by the load balancer today, due to the per CPU
> load average being statistical and them being slightly above or
> below a critical load average value.
>
> Higher freq oscillation should not happen normally though, we
> dampen these metrics and have per CPU hysteresis.
>
> ( We can also add explicit hysteresis if anyone demonstrates
>    real oscillation with a real workload - wanted to keep it
>    simple first and change it only as-needed. )

This heuristic is by no means simple, and there still is no
explanation for the serious performance degradations that
were seen on a 4 node system running specjbb in 4 node-sized
JVMs.

I asked a number of questions on this patch yesterday, and
am hoping to get explanations at some point :)

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  reply	other threads:[~2012-11-16 18:23 UTC|newest]

Thread overview: 37+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2012-11-12 16:04 [PATCH 0/8] Announcement: Enhanced NUMA scheduling with adaptive affinity Peter Zijlstra
2012-11-12 16:04 ` [PATCH 1/8] sched, numa, mm: Introduce sched_feat_numa() Peter Zijlstra
2012-11-12 16:04 ` [PATCH 2/8] sched, numa, mm: Implement THP migration Peter Zijlstra
2012-11-12 16:04 ` [PATCH 3/8] sched, numa, mm: Add credits for NUMA placement Peter Zijlstra
2012-11-12 16:04 ` [PATCH 4/8] sched, numa, mm: Add last_cpu to page flags Peter Zijlstra
2012-11-13 11:55   ` Ingo Molnar
2012-11-13 16:09   ` Rik van Riel
2012-11-12 16:04 ` [PATCH 5/8] sched, numa, mm: Add adaptive NUMA affinity support Peter Zijlstra
2012-11-13  0:02   ` Christoph Lameter
2012-11-13  8:19     ` Ingo Molnar
2012-11-13 22:57   ` Rik van Riel
2012-11-16 18:06   ` Rik van Riel
2012-11-16 18:14     ` Ingo Molnar
2012-11-16 18:23       ` Rik van Riel [this message]
2012-11-29 19:34   ` Andi Kleen
2012-11-12 16:04 ` [PATCH 6/8] sched, numa, mm: Implement constant, per task Working Set Sampling (WSS) rate Peter Zijlstra
2012-11-12 16:04 ` [PATCH 7/8] sched, numa, mm: Count WS scanning against present PTEs, not virtual memory ranges Peter Zijlstra
2012-11-12 16:04 ` [PATCH 8/8] sched, numa, mm: Implement slow start for working set sampling Peter Zijlstra
2012-11-12 18:48 ` Benchmark results: "Enhanced NUMA scheduling with adaptive affinity" Ingo Molnar
2012-11-15 10:08   ` Mel Gorman
2012-11-15 18:52     ` Rik van Riel
2012-11-15 21:27       ` Mel Gorman
2012-11-15 20:32     ` Linus Torvalds
2012-11-15 22:04       ` Rik van Riel
2012-11-16 14:14         ` Mel Gorman
2012-11-16 19:50           ` Andrea Arcangeli
2012-11-16 20:05             ` Mel Gorman
2012-11-16 16:16       ` Ingo Molnar
2012-11-16 15:56     ` Ingo Molnar
2012-11-16 16:25       ` Mel Gorman
2012-11-16 17:49         ` Ingo Molnar
2012-11-16 19:04           ` Mel Gorman
2012-11-12 23:43 ` [PATCH 0/8] Announcement: Enhanced NUMA scheduling with adaptive affinity Christoph Lameter
2012-11-13  7:24   ` Ingo Molnar
2012-11-15 14:26     ` Christoph Lameter
2012-11-16 15:59       ` Ingo Molnar
2012-11-16 20:57         ` Christoph Lameter

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