linux-kernel - Re: [PATCH 2/6] writeback: the kupdate expire timestamp should be a moving target

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Message-ID: <20100726105200.GK5300@csn.ul.ie>
Date:	Mon, 26 Jul 2010 11:52:00 +0100
From:	Mel Gorman <mel@....ul.ie>
To:	Wu Fengguang <fengguang.wu@...el.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Dave Chinner <david@...morbit.com>, Jan Kara <jack@...e.cz>,
	Christoph Hellwig <hch@...radead.org>,
	Chris Mason <chris.mason@...cle.com>,
	Jens Axboe <jens.axboe@...cle.com>,
	LKML <linux-kernel@...r.kernel.org>,
	linux-fsdevel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH 2/6] writeback: the kupdate expire timestamp should be
	a moving target

On Thu, Jul 22, 2010 at 01:09:30PM +0800, Wu Fengguang wrote:
> Dynamicly compute the dirty expire timestamp at queue_io() time.
> Also remove writeback_control.older_than_this which is no longer used.
> 
> writeback_control.older_than_this used to be determined at entrance to
> the kupdate writeback work. This _static_ timestamp may go stale if the
> kupdate work runs on and on. The flusher may then stuck with some old
> busy inodes, never considering newly expired inodes thereafter.
> 
> This has two possible problems:
> 
> - It is unfair for a large dirty inode to delay (for a long time) the
>   writeback of small dirty inodes.
> 
> - As time goes by, the large and busy dirty inode may contain only
>   _freshly_ dirtied pages. Ignoring newly expired dirty inodes risks
>   delaying the expired dirty pages to the end of LRU lists, triggering
>   the very bad pageout(). Neverthless this patch merely addresses part
>   of the problem.
> 
> CC: Jan Kara <jack@...e.cz>
> Signed-off-by: Wu Fengguang <fengguang.wu@...el.com>

Again, makes sense and I can't see a problem. There are some worth
smithing issues in the changelog such as Dynamicly -> Dynamically and
s/writeback_control.older_than_this used/writeback_control.older_than_this is used/
but other than that.

Acked-by: Mel Gorman <mel@....ul.ie>

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab
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