linux-ext4 - Re: [patch 3/5] mm: try to distribute dirty pages fairly across zones

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Date:	Fri, 30 Sep 2011 10:35:25 +0300
From:	Pekka Enberg <penberg@...helsinki.fi>
To:	Johannes Weiner <jweiner@...hat.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mgorman@...e.de>,
	Christoph Hellwig <hch@...radead.org>,
	Dave Chinner <david@...morbit.com>,
	Wu Fengguang <fengguang.wu@...el.com>, Jan Kara <jack@...e.cz>,
	Rik van Riel <riel@...hat.com>,
	Minchan Kim <minchan.kim@...il.com>,
	Chris Mason <chris.mason@...cle.com>,
	"Theodore Ts'o" <tytso@....edu>,
	Andreas Dilger <adilger.kernel@...ger.ca>,
	Shaohua Li <shaohua.li@...el.com>, xfs@....sgi.com,
	linux-btrfs@...r.kernel.org, linux-ext4@...r.kernel.org,
	linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [patch 3/5] mm: try to distribute dirty pages fairly across zones

Hi Johannes!

On Fri, Sep 30, 2011 at 10:17 AM, Johannes Weiner <jweiner@...hat.com> 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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