linux-kernel - Re: [PATCH 5/6] mm: compaction: Cache if a pageblock was scanned and no pages were isolated

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Date:	Thu, 20 Sep 2012 14:55:22 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Mel Gorman <mgorman@...e.de>
CC:	Richard Davies <richard@...chsys.com>,
	Shaohua Li <shli@...nel.org>, 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 5/6] mm: compaction: Cache if a pageblock was scanned
 and no pages were isolated

On 09/20/2012 10:04 AM, 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

Big hammer, but I guess it is effective...

Acked-by: Rik van Riel <riel@...hat.com>

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