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Re: [patch 3/5] mm: try to distribute dirty pages fairly across zones

To: Johannes Weiner <jweiner@xxxxxxxxxx>
Subject: Re: [patch 3/5] mm: try to distribute dirty pages fairly across zones
From: Pekka Enberg <penberg@xxxxxxxxxxxxxx>
Date: Fri, 30 Sep 2011 10:35:25 +0300
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>, Mel Gorman <mgorman@xxxxxxx>, Christoph Hellwig <hch@xxxxxxxxxxxxx>, Dave Chinner <david@xxxxxxxxxxxxx>, Wu Fengguang <fengguang.wu@xxxxxxxxx>, Jan Kara <jack@xxxxxxx>, Rik van Riel <riel@xxxxxxxxxx>, Minchan Kim <minchan.kim@xxxxxxxxx>, Chris Mason <chris.mason@xxxxxxxxxx>, "Theodore Ts'o" <tytso@xxxxxxx>, Andreas Dilger <adilger.kernel@xxxxxxxxx>, Shaohua Li <shaohua.li@xxxxxxxxx>, xfs@xxxxxxxxxxx, linux-btrfs@xxxxxxxxxxxxxxx, linux-ext4@xxxxxxxxxxxxxxx, linux-mm@xxxxxxxxx, linux-fsdevel@xxxxxxxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx
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Hi Johannes!

On Fri, Sep 30, 2011 at 10:17 AM, Johannes Weiner <jweiner@xxxxxxxxxx> wrote:
> But there is a flaw in that we have a zoned page allocator which does
> not care about the global state but rather the state of individual
> memory zones.  And right now there is nothing that prevents one zone
> from filling up with dirty pages while other zones are spared, which
> frequently leads to situations where kswapd, in order to restore the
> watermark of free pages, does indeed have to write pages from that
> zone's LRU list.  This can interfere so badly with IO from the flusher
> threads that major filesystems (btrfs, xfs, ext4) mostly ignore write
> requests from reclaim already, taking away the VM's only possibility
> to keep such a zone balanced, aside from hoping the flushers will soon
> clean pages from that zone.

The obvious question is: how did you test this? Can you share the results?

                        Pekka

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