linux-kernel - Re: [PATCH 0/3] Removal of lumpy reclaim V2

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Message-ID: <4F85BC8E.3020400@redhat.com>
Date:	Wed, 11 Apr 2012 13:17:02 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Mel Gorman <mgorman@...e.de>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	Konstantin Khlebnikov <khlebnikov@...nvz.org>,
	Hugh Dickins <hughd@...gle.com>, Ying Han <yinghan@...gle.com>,
	Linux-MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/3] Removal of lumpy reclaim V2

On 04/11/2012 12:38 PM, Mel Gorman wrote:

> Success rates are completely hosed for 3.4-rc2 which is almost certainly
> due to [fe2c2a10: vmscan: reclaim at order 0 when compaction is enabled]. I
> expected this would happen for kswapd and impair allocation success rates
> (https://lkml.org/lkml/2012/1/25/166) but I did not anticipate this much
> a difference: 80% less scanning, 37% less reclaim by kswapd

Also, no gratuitous pageouts of anonymous memory.
That was what really made a difference on a somewhat
heavily loaded desktop + kvm workload.

> In comparison, reclaim/compaction is not aggressive and gives up easily
> which is the intended behaviour. hugetlbfs uses __GFP_REPEAT and would be
> much more aggressive about reclaim/compaction than THP allocations are. The
> stress test above is allocating like neither THP or hugetlbfs but is much
> closer to THP.

Next step: get rid of __GFP_NO_KSWAPD for THP, first
in the -mm kernel

> Mainline is now impaired in terms of high order allocation under heavy load
> although I do not know to what degree as I did not test with __GFP_REPEAT.
> Keep this in mind for bugs related to hugepage pool resizing, THP allocation
> and high order atomic allocation failures from network devices.

This might be due to smaller allocations not bumping
the compaction deferring code, when we have deferred
compaction for a higher order allocation.

I wonder if the compaction deferring code is simply
too defer-happy, now that we ignore compaction at
lower orders than where compaction failed?
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