linux-kernel - Re: [PATCH 8/9] mm: compaction: Cache if a pageblock was scanned and no pages were isolated

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Message-ID: <20120921175337.GH6665@optiplex.redhat.com>
Date:	Fri, 21 Sep 2012 14:53:38 -0300
From:	Rafael Aquini <aquini@...hat.com>
To:	Mel Gorman <mgorman@...e.de>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Richard Davies <richard@...chsys.com>,
	Shaohua Li <shli@...nel.org>, Rik van Riel <riel@...hat.com>,
	Avi Kivity <avi@...hat.com>,
	QEMU-devel <qemu-devel@...gnu.org>, KVM <kvm@...r.kernel.org>,
	Linux-MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 8/9] mm: compaction: Cache if a pageblock was scanned and
 no pages were isolated

On Fri, Sep 21, 2012 at 11:46:22AM +0100, Mel Gorman wrote:
> When compaction was implemented it was known that scanning could potentially
> be excessive. The ideal was that a counter be maintained for each pageblock
> but maintaining this information would incur a severe penalty due to a
> shared writable cache line. It has reached the point where the scanning
> costs are an serious problem, particularly on long-lived systems where a
> large process starts and allocates a large number of THPs at the same time.
> 
> Instead of using a shared counter, this patch adds another bit to the
> pageblock flags called PG_migrate_skip. If a pageblock is scanned by
> either migrate or free scanner and 0 pages were isolated, the pageblock
> is marked to be skipped in the future. When scanning, this bit is checked
> before any scanning takes place and the block skipped if set.
> 
> The main difficulty with a patch like this is "when to ignore the cached
> information?" If it's ignored too often, the scanning rates will still
> be excessive. If the information is too stale then allocations will fail
> that might have otherwise succeeded. In this patch
> 
> o CMA always ignores the information
> o If the migrate and free scanner meet then the cached information will
>   be discarded if it's at least 5 seconds since the last time the cache
>   was discarded
> o If there are a large number of allocation failures, discard the cache.
> 
> The time-based heuristic is very clumsy but there are few choices for a
> better event. Depending solely on multiple allocation failures still allows
> excessive scanning when THP allocations are failing in quick succession
> due to memory pressure. Waiting until memory pressure is relieved would
> cause compaction to continually fail instead of using reclaim/compaction
> to try allocate the page. The time-based mechanism is clumsy but a better
> option is not obvious.
> 
> Signed-off-by: Mel Gorman <mgorman@...e.de>
> Acked-by: Rik van Riel <riel@...hat.com>
> ---

Acked-by: Rafael Aquini <aquini@...hat.com>

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