linux-kernel - Re: [PATCH] numa,sched: only consider less busy nodes as numa balancing destination

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Message-ID: <1431342675.1418.148.camel@sauron.fi.intel.com>
Date:	Mon, 11 May 2015 14:11:15 +0300
From:	Artem Bityutskiy <dedekind1@...il.com>
To:	Rik van Riel <riel@...hat.com>
Cc:	linux-kernel@...r.kernel.org, mgorman@...e.de,
	peterz@...radead.org, jhladky@...hat.com
Subject: Re: [PATCH] numa,sched: only consider less busy nodes as numa
 balancing destination

On Fri, 2015-05-08 at 16:03 -0400, Rik van Riel wrote:
> This works well when dealing with tasks that are constantly
> running, but fails catastrophically when dealing with tasks
> that go to sleep, wake back up, go back to sleep, wake back
> up, and generally mess up the load statistics that the NUMA
> balancing code use in a random way.

Sleeping is what happens a lot I believe in this workload: processes do
a lot of network I/O, file I/O too, and a lot of IPC.

Would you please expand on this a bit more - why would this scenario
"mess up load statistics" ?

> If the normal scheduler load balancer is moving tasks the
> other way the NUMA balancer is moving them, things will
> not converge, and tasks will have worse memory locality
> than not doing NUMA balancing at all.

Are the regular and NUMA balancers independent?

Are there mechanisms to detect ping-pong situations? I'd like to verify
your theory, and these kinds of mechanisms would be helpful.

> Currently the load balancer has a preference for moving
> tasks to their preferred nodes (NUMA_FAVOUR_HIGHER, true),
> but there is no resistance to moving tasks away from their
> preferred nodes (NUMA_RESIST_LOWER, false).  That setting
> was arrived at after a fair amount of experimenting, and
> is probably correct.

I guess I can try making NUMA_RESIST_LOWER to be true and see what
happens. But probably first I need to confirm that your theory
(balancers playing ping-pong) is correct, any hints on how would I do
this?

Thanks!

Artem.


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