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

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Message-ID: <20100726113245.GC6284@localhost>
Date:	Mon, 26 Jul 2010 19:32:45 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	Mel Gorman <mel@....ul.ie>
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-fsdevel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>
Subject: Re: [PATCH 2/6] writeback: the kupdate expire timestamp should be
 a moving target

On Mon, Jul 26, 2010 at 06:52:00PM +0800, Mel Gorman wrote:
> 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

Hah forgot to enable spell checking.

> s/writeback_control.older_than_this used/writeback_control.older_than_this is used/

It's "used to", my god.

> but other than that.
> 
> Acked-by: Mel Gorman <mel@....ul.ie>

Thanks,
Fengguang
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