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

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Message-ID: <20110419070247.GE23985@dastard>
Date:	Tue, 19 Apr 2011 17:02:47 +1000
From:	Dave Chinner <david@...morbit.com>
To:	Wu Fengguang <fengguang.wu@...el.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>, Jan Kara <jack@...e.cz>,
	Mel Gorman <mel@...ux.vnet.ibm.com>,
	Mel Gorman <mel@....ul.ie>,
	Itaru Kitayama <kitayama@...bb4u.ne.jp>,
	Trond Myklebust <Trond.Myklebust@...app.com>,
	Minchan Kim <minchan.kim@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	linux-fsdevel@...r.kernel.org,
	Linux Memory Management List <linux-mm@...ck.org>
Subject: Re: [PATCH 2/6] writeback: the kupdate expire timestamp should be
 a moving target

On Tue, Apr 19, 2011 at 11:00:05AM +0800, Wu Fengguang wrote:
> Dynamically compute the dirty expire timestamp at queue_io() time.
> 
> 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 evil pageout(). Nevertheless this patch merely addresses part
>   of the problem.

When wb_writeback() is called with for_kupdate set, it initialises
wbc->older_than_this appropriately outside the writeback loop.
queue_io() is called once per writeback_inodes_wb() call, which is
once per loop in wb_writeback. All your change does is re-initialise
older_than_this once per loop in wb_writeback, jus tin a different
and very non-obvious place.

So why didn't you just re-initialise it inside the loop in
wb_writeback() and leave all the other code alone?

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com
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