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

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Date:	Tue, 10 May 2011 12:31:04 +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 10:21:55PM +0800, Jan Kara wrote:
> On Fri 06-05-11 13:29:55, Wu Fengguang wrote:
> > 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()...
> > 
> > Jan, I got some results for ext4. The total dd+tar+sync time is
> > decreased from 177s to 167s. The other numbers are either raised or
> > dropped.
>   Nice, but what I was more curious about was to understand why you saw
> enqueued=1031 all the time.

Maybe some unknown interactions with XFS? Attached is another trace
with both writeback_single_inode and writeback_queue_io.

> BTW, I'd suppose that the better performance
> numbers come from sync using page tagging, right?

No. Many of the tests predate the sync livelock patches.

> Because from the traces
> it seems that not much IO is going on until sync is called. And I expect
> that tagging can bring you some performance because now you sync a file in
> one big sweep instead of 4MB chunks...
 
But ext4 already does 128MB write chunk size..

> > 1902.672610: writeback_queue_io: older=4296543506 age=30000 enqueue=0
> > 1905.209570: writeback_queue_io: older=4296546051 age=30000 enqueue=0
> > 1907.294936: writeback_queue_io: older=4296548143 age=30000 enqueue=0
> > 1909.607301: writeback_queue_io: older=4296550462 age=30000 enqueue=0
> > 1912.290627: writeback_queue_io: older=4296553154 age=30000 enqueue=0
> > 1914.331197: writeback_queue_io: older=4296555201 age=30000 enqueue=0
> > 1927.275838: writeback_queue_io: older=4296568186 age=30000 enqueue=0
> > 1927.277794: writeback_queue_io: older=4296568188 age=30000 enqueue=0
> > 1927.279504: writeback_queue_io: older=4296568189 age=30000 enqueue=0
> > 1927.279923: writeback_queue_io: older=4296568190 age=30000 enqueue=0
> > 1929.981734: writeback_queue_io: older=4296600898 age=2 enqueue=13227
> > 1932.840150: writeback_queue_io: older=4296600898 age=2869 enqueue=0
> > 1932.840781: writeback_queue_io: older=4296603768 age=0 enqueue=0
> > 1932.840787: writeback_queue_io: older=4296573768 age=30000 enqueue=0
> > 1932.991596: writeback_queue_io: older=4296603919 age=0 enqueue=1
> > 1937.975765: writeback_queue_io: older=4296578919 age=30000 enqueue=0
> > 1942.960305: writeback_queue_io: older=4296583919 age=30000 enqueue=0
> > 1947.944925: writeback_queue_io: older=4296588919 age=30000 enqueue=0
> > 1952.929427: writeback_queue_io: older=4296593919 age=30000 enqueue=0
> > 1957.914031: writeback_queue_io: older=4296598919 age=30000 enqueue=0
> > 1962.898507: writeback_queue_io: older=4296603919 age=30000 enqueue=1
> > 1962.898518: writeback_queue_io: older=4296603919 age=30000 enqueue=0
>   OK, so now enqueue numbers look like what I'd expect. I'm relieved :)
> Thanks for running the tests.

You are welcome :)

Thanks,
Fengguang

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