linux-kernel - Re: [PATCH 12/14] vmscan: Do not writeback pages in direct reclaim

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Date:	Wed, 7 Jul 2010 10:50:10 +0100
From:	Mel Gorman <mel@....ul.ie>
To:	Wu Fengguang <fengguang.wu@...el.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	linux-mm@...ck.org, Dave Chinner <david@...morbit.com>,
	Chris Mason <chris.mason@...cle.com>,
	Nick Piggin <npiggin@...e.de>, Rik van Riel <riel@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Christoph Hellwig <hch@...radead.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Andrea Arcangeli <aarcange@...hat.com>, Jan Kara <jack@...e.cz>
Subject: Re: [PATCH 12/14] vmscan: Do not writeback pages in direct reclaim

On Wed, Jul 07, 2010 at 01:03:38PM +0800, Wu Fengguang wrote:
> Hi Mel,
> 
> > Second, using systemtap, I was able to see that file-backed dirty
> > pages have a tendency to be near the end of the LRU even though they
> > are a small percentage of the overall pages in the LRU. I'm hoping
> > to figure out why this is as it would make avoiding writeback a lot
> > less controversial.
> 
> Your intuitions are correct -- the current background writeback logic
> fails to write elder inodes first. Under heavy loads the background
> writeback job may run for ever, totally ignoring the time order of
> inode->dirtied_when. This is probably why you see lots of dirty pages
> near the end of LRU.
> 

Possible. In a mail to Christoph, I asserted that writeback of elder inodes
was happening first but I obviously could be mistaken.

> Here is an old patch for fixing this. Sorry for being late. I'll
> pick up and refresh the patch series ASAP.  (I made a mistake last
> year to post too many patches at one time. I'll break them up into
> more manageable pieces.)
> 
> [PATCH 31/45] writeback: sync old inodes first in background writeback
> <https://kerneltrap.org/mailarchive/linux-fsdevel/2009/10/7/6476313>
> 

I'll check it out as an alternative to forward-flushing based on the
amount of dirty pages encountered during scanning. Thanks.

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