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Re: [RFC PATCH 0/8] Reduce filesystem writeback from page reclaim v2

To: Mel Gorman <mgorman@xxxxxxx>
Subject: Re: [RFC PATCH 0/8] Reduce filesystem writeback from page reclaim v2
From: Dave Chinner <david@xxxxxxxxxxxxx>
Date: Tue, 26 Jul 2011 21:20:55 +1000
Cc: Linux-MM <linux-mm@xxxxxxxxx>, LKML <linux-kernel@xxxxxxxxxxxxxxx>, XFS <xfs@xxxxxxxxxxx>, Christoph Hellwig <hch@xxxxxxxxxxxxx>, Johannes Weiner <jweiner@xxxxxxxxxx>, Wu Fengguang <fengguang.wu@xxxxxxxxx>, Jan Kara <jack@xxxxxxx>, Rik van Riel <riel@xxxxxxxxxx>, Minchan Kim <minchan.kim@xxxxxxxxx>
In-reply-to: <1311265730-5324-1-git-send-email-mgorman@xxxxxxx>
References: <1311265730-5324-1-git-send-email-mgorman@xxxxxxx>
User-agent: Mutt/1.5.21 (2010-09-15)
On Thu, Jul 21, 2011 at 05:28:42PM +0100, Mel Gorman wrote:
> Warning: Long post with lots of figures. If you normally drink coffee
> and you don't have a cup, get one or you may end up with a case of
> keyboard face.


> Overall, having kswapd avoiding writes does improve performance
> which is not a surprise. Dave asked "do we even need IO at all from
> reclaim?". On NUMA machines, the answer is "yes" unless the VM can
> wake the flusher thread to clean a specific node.

Great answer, Mel. ;)

> When kswapd never
> writes, processes can stall for significant periods of time waiting on
> flushers to clean the correct pages. If all writing is to be deferred
> to flushers, it must ensure that many writes on one node would not
> starve requests for cleaning pages on another node.

Ok, so that's a direction we need to work towards, then.

> I'm currently of the opinion that we should consider merging patches
> 1-7 and discuss what is required before merging. It can be tackled
> later how the flushers can prioritise writing of pages belonging to
> a particular zone before disabling all writes from reclaim.

Sounds reasonable to me.


Dave Chinner

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