linux-kernel - Re: [PATCH 2/6] mm: vmscan: Scale number of pages reclaimed by reclaim/compaction based on failures

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Date:	Fri, 10 Aug 2012 09:14:28 +0100
From:	Mel Gorman <mgorman@...e.de>
To:	Rik van Riel <riel@...hat.com>
Cc:	Minchan Kim <minchan@...nel.org>, Linux-MM <linux-mm@...ck.org>,
	Jim Schutt <jaschut@...dia.gov>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/6] mm: vmscan: Scale number of pages reclaimed by
 reclaim/compaction based on failures

On Thu, Aug 09, 2012 at 04:29:57PM -0400, Rik van Riel wrote:
> On 08/09/2012 05:20 AM, Mel Gorman wrote:
> 
> >The intention is that an allocation can fail but each subsequent attempt will
> >try harder until there is success. Each allocation request does a portion
> >of the necessary work to spread the cost between multiple requests.
> 
> At some point we need to stop doing that work, though.
> 
> Otherwise we could end up back at the problem where
> way too much memory gets evicted, and we get swap
> storms.

That's the case without this patch as it'll still be running
reclaim/compaction just less aggressively. For it to continually try like
the system must be either continually under load preventing compaction ever
working (which may be undesirable for order-3 and the like) or so badly
fragmented it cannot recover (not aware of a situation where this happened).

You could add a separate patch that checked if
defer_shift == COMPACT_MAX_DEFER_SHIFT and to disable reclaim/compaction in
that case but that will require enough SWAP_CLUSTER_MAX pages to be reclaimed
over time or a large process to exit before compaction succeeds again.

I would expect rates under load to be very low with such a patch
applied.

-- 
Mel Gorman
SUSE Labs
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