linux-kernel - Re: [PATCH 4/6] writeback: sync expired inodes first in background writeback

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Message-ID: <20100726123141.GA13146@localhost>
Date:	Mon, 26 Jul 2010 20:31:41 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	Jan Kara <jack@...e.cz>
Cc:	Mel Gorman <mel@....ul.ie>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Dave Chinner <david@...morbit.com>,
	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 4/6] writeback: sync expired inodes first in background
 writeback

On Mon, Jul 26, 2010 at 08:20:54PM +0800, Jan Kara wrote:
> On Mon 26-07-10 20:00:11, Wu Fengguang wrote:
> > On Mon, Jul 26, 2010 at 06:57:37PM +0800, Mel Gorman wrote:
> > > On Thu, Jul 22, 2010 at 01:09:32PM +0800, Wu Fengguang wrote:
> > > > A background flush work may run for ever. So it's reasonable for it to
> > > > mimic the kupdate behavior of syncing old/expired inodes first.
> > > > 
> > > > The policy is
> > > > - enqueue all newly expired inodes at each queue_io() time
> > > > - retry with halfed expire interval until get some inodes to sync
> > > > 
> > > > CC: Jan Kara <jack@...e.cz>
> > > > Signed-off-by: Wu Fengguang <fengguang.wu@...el.com>
> > > 
> > > Ok, intuitively this would appear to tie into pageout where we want
> > > older inodes to be cleaned first by background flushers to limit the
> > > number of dirty pages encountered by page reclaim. If this is accurate,
> > > it should be detailed in the changelog.
> > 
> > Good suggestion. I'll add these lines:
> > 
> > This is to help reduce the number of dirty pages encountered by page
> > reclaim, eg. the pageout() calls. Normally older inodes contain older
> > dirty pages, which are more close to the end of the LRU lists. So
>   Well, this kind of implicitely assumes that once page is written, it
> doesn't get accessed anymore, right?

No, this patch is not evicting the page :)

> Which I imagine is often true but
> not for all workloads... Anyway I think this behavior is a good start
> also because it is kind of natural to users to see "old" files written
> first.

Thanks,
Fengguang

> > syncing older inodes first helps reducing the dirty pages reached by
> > the page reclaim code.
> 
> 								Honza  
> -- 
> Jan Kara <jack@...e.cz>
> SUSE Labs, CR
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