linux-kernel - Re: [PATCH 2/6] mm: compaction: Acquire the zone->lru

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Message-ID: <505B6658.1080706@redhat.com>
Date:	Thu, 20 Sep 2012 14:54:16 -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 2/6] mm: compaction: Acquire the zone->lru_lock as late
 as possible

On 09/20/2012 10:04 AM, Mel Gorman wrote:
> Compactions migrate scanner acquires the zone->lru_lock when scanning a range
> of pages looking for LRU pages to acquire. It does this even if there are
> no LRU pages in the range. If multiple processes are compacting then this
> can cause severe locking contention. To make matters worse commit b2eef8c0
> (mm: compaction: minimise the time IRQs are disabled while isolating pages
> for migration) releases the lru_lock every SWAP_CLUSTER_MAX pages that are
> scanned.
>
> This patch makes two changes to how the migrate scanner acquires the LRU
> lock. First, it only releases the LRU lock every SWAP_CLUSTER_MAX pages if
> the lock is contended. This reduces the number of times it unnecessarily
> disables and re-enables IRQs. The second is that it defers acquiring the
> LRU lock for as long as possible. If there are no LRU pages or the only
> LRU pages are transhuge then the LRU lock will not be acquired at all
> which reduces contention on zone->lru_lock.
>
> Signed-off-by: Mel Gorman <mgorman@...e.de>

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

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