linux-kernel - Re: [PATCH 3/3] sched,numa: do not set preferred_node on migration to a second choice node

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Date:	Fri, 25 Apr 2014 10:09:59 +0100
From:	Mel Gorman <mgorman@...e.de>
To:	riel@...hat.com
Cc:	linux-kernel@...r.kernel.org, mingo@...nel.org,
	peterz@...radead.org, chegu_vinod@...com
Subject: Re: [PATCH 3/3] sched,numa: do not set preferred_node on migration
 to a second choice node

On Fri, Apr 11, 2014 at 01:00:29PM -0400, riel@...hat.com wrote:
> From: Rik van Riel <riel@...hat.com>
> 
> Setting the numa_preferred_node for a task in task_numa_migrate
> does nothing on a 2-node system. Either we migrate to the node
> that already was our preferred node, or we stay where we were.
> 
> On a 4-node system, it can slightly decrease overhead, by not
> calling the NUMA code as much. Since every node tends to be
> directly connected to every other node, running on the wrong
> node for a while does not do much damage.
> 

Guess what size of machine I do the vast bulk of testing of automatic
NUMA balancing on!

> However, on an 8 node system, there are far more bad nodes
> than there are good ones, and pretending that a second choice
> is actually the preferred node can greatly delay, or even
> prevent, a workload from converging.
> 
> The only time we can safely pretend that a second choice
> node is the preferred node is when the task is part of a
> workload that spans multiple NUMA nodes.
> 
> Signed-off-by: Rik van Riel <riel@...hat.com>
> Tested-by: Vinod Chegu <chegu_vinod@...com>

Acked-by: Mel Gorman <mgorman@...e.de>

-- 
Mel Gorman
SUSE Labs
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