Date:	Mon, 07 Oct 2013 14:45:17 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Mel Gorman <mgorman@...e.de>
CC:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...nel.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Linux-MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 29/63] sched: Set preferred NUMA node based on number
 of private faults

On 10/07/2013 06:29 AM, Mel Gorman wrote:
> Ideally it would be possible to distinguish between NUMA hinting faults that
> are private to a task and those that are shared. If treated identically
> there is a risk that shared pages bounce between nodes depending on
> the order they are referenced by tasks. Ultimately what is desirable is
> that task private pages remain local to the task while shared pages are
> interleaved between sharing tasks running on different nodes to give good
> average performance. This is further complicated by THP as even
> applications that partition their data may not be partitioning on a huge
> page boundary.
> 
> To start with, this patch assumes that multi-threaded or multi-process
> applications partition their data and that in general the private accesses
> are more important for cpu->memory locality in the general case. Also,
> no new infrastructure is required to treat private pages properly but
> interleaving for shared pages requires additional infrastructure.
> 
> To detect private accesses the pid of the last accessing task is required
> but the storage requirements are a high. This patch borrows heavily from
> Ingo Molnar's patch "numa, mm, sched: Implement last-CPU+PID hash tracking"
> to encode some bits from the last accessing task in the page flags as
> well as the node information. Collisions will occur but it is better than
> just depending on the node information. Node information is then used to
> determine if a page needs to migrate. The PID information is used to detect
> private/shared accesses. The preferred NUMA node is selected based on where
> the maximum number of approximately private faults were measured. Shared
> faults are not taken into consideration for a few reasons.
> 
> First, if there are many tasks sharing the page then they'll all move
> towards the same node. The node will be compute overloaded and then
> scheduled away later only to bounce back again. Alternatively the shared
> tasks would just bounce around nodes because the fault information is
> effectively noise. Either way accounting for shared faults the same as
> private faults can result in lower performance overall.
> 
> The second reason is based on a hypothetical workload that has a small
> number of very important, heavily accessed private pages but a large shared
> array. The shared array would dominate the number of faults and be selected
> as a preferred node even though it's the wrong decision.
> 
> The third reason is that multiple threads in a process will race each
> other to fault the shared page making the fault information unreliable.
> 
> [riel@...hat.com: Fix complication error when !NUMA_BALANCING]
> Signed-off-by: Mel Gorman <mgorman@...e.de>

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

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