linux-kernel - Re: [PATCH 6/6] writeback: refill b

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Date:	Fri, 6 May 2011 00:47:12 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	Jan Kara <jack@...e.cz>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mel@...ux.vnet.ibm.com>,
	Mel Gorman <mel@....ul.ie>, Dave Chinner <david@...morbit.com>,
	Itaru Kitayama <kitayama@...bb4u.ne.jp>,
	Minchan Kim <minchan.kim@...il.com>,
	Linux Memory Management List <linux-mm@...ck.org>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 6/6] writeback: refill b_io iff empty

On Fri, May 06, 2011 at 12:37:08AM +0800, Jan Kara wrote:
> On Wed 04-05-11 15:39:31, Wu Fengguang wrote:
> > To help understand the behavior change, I wrote the writeback_queue_io
> > trace event, and found very different patterns between
> > - vanilla kernel
> > - this patchset plus the sync livelock fixes
> > 
> > Basically the vanilla kernel each time pulls a random number of inodes
> > from b_dirty, while the patched kernel tends to pull a fixed number of
> > inodes (enqueue=1031) from b_dirty. The new behavior is very interesting...
>   This regularity is really strange. Did you have a chance to look more into
> it? I find it highly unlikely that there would be exactly 1031 dirty inodes
> in b_dirty list every time you call move_expired_inodes()...

Yeah that's the weird point. The other things I noticed are more
regular "flusher - dd - flusher - dd - ..." writeout patterns after
the patches.  In vanilla kernel it behaves more randomly and there are
many balance_dirty_pages() IOs from tar.

I'll try to collect more traces in ext4 tomorrow. Sorry it's too late
for me now.

Thanks,
Fengguang
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