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Re: [PATCH 6/7] mm: vmscan: Throttle reclaim if encountering too many di

To: Mel Gorman <mgorman@xxxxxxx>
Subject: Re: [PATCH 6/7] mm: vmscan: Throttle reclaim if encountering too many dirty pages under writeback
From: Rik van Riel <riel@xxxxxxxxxx>
Date: Thu, 11 Aug 2011 22:47:33 -0400
Cc: Linux-MM <linux-mm@xxxxxxxxx>, LKML <linux-kernel@xxxxxxxxxxxxxxx>, XFS <xfs@xxxxxxxxxxx>, Dave Chinner <david@xxxxxxxxxxxxx>, Christoph Hellwig <hch@xxxxxxxxxxxxx>, Johannes Weiner <jweiner@xxxxxxxxxx>, Wu Fengguang <fengguang.wu@xxxxxxxxx>, Jan Kara <jack@xxxxxxx>, Minchan Kim <minchan.kim@xxxxxxxxx>
In-reply-to: <1312973240-32576-7-git-send-email-mgorman@xxxxxxx>
References: <1312973240-32576-1-git-send-email-mgorman@xxxxxxx> <1312973240-32576-7-git-send-email-mgorman@xxxxxxx>
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:5.0) Gecko/20110707 Thunderbird/5.0
On 08/10/2011 06:47 AM, Mel Gorman wrote:
Workloads that are allocating frequently and writing files place a
large number of dirty pages on the LRU. With use-once logic, it is
possible for them to reach the end of the LRU quickly requiring the
reclaimer to scan more to find clean pages. Ordinarily, processes that
are dirtying memory will get throttled by dirty balancing but this
is a global heuristic and does not take into account that LRUs are
maintained on a per-zone basis. This can lead to a situation whereby
reclaim is scanning heavily, skipping over a large number of pages
under writeback and recycling them around the LRU consuming CPU.

This patch checks how many of the number of pages isolated from the
LRU were dirty and under writeback. If a percentage of them under
writeback, the process will be throttled if a backing device or the
zone is congested. Note that this applies whether it is anonymous or
file-backed pages that are under writeback meaning that swapping is
potentially throttled. This is intentional due to the fact if the
swap device is congested, scanning more pages and dispatching more
IO is not going to help matters.

The percentage that must be in writeback depends on the priority. At
default priority, all of them must be dirty. At DEF_PRIORITY-1, 50%
of them must be, DEF_PRIORITY-2, 25% etc. i.e. as pressure increases
the greater the likelihood the process will get throttled to allow
the flusher threads to make some progress.

Signed-off-by: Mel Gorman<mgorman@xxxxxxx>
Reviewed-by: Minchan Kim<minchan.kim@xxxxxxxxx>
Acked-by: Johannes Weiner<jweiner@xxxxxxxxxx>

Acked-by: Rik van Riel <riel@xxxxxxxxxx>

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