linux-kernel - Re: [PATCH] fs: sync: fixed performance regression

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Message-ID: <20130712154314.GC22823@quack.suse.cz>
Date:	Fri, 12 Jul 2013 17:43:14 +0200
From:	Jan Kara <jack@...e.cz>
To:	Paul Taysom <taysom@...omium.org>
Cc:	Alexander Viro <viro@...iv.linux.org.uk>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	jack@...e.cz, sonnyrao@...omium.org
Subject: Re: [PATCH] fs: sync: fixed performance regression

On Thu 11-07-13 13:58:32, Jan Kara wrote:
> On Thu 11-07-13 12:53:46, Jan Kara wrote:
> > On Wed 10-07-13 16:12:36, Paul Taysom wrote:
> > > The following commit introduced a 10x regression for
> > > syncing inodes in ext4 with relatime enabled where just
> > > the atime had been modified.
> > > 
> > >     commit 4ea425b63a3dfeb7707fc7cc7161c11a51e871ed
> > >     Author: Jan Kara <jack@...e.cz>
> > >     Date:   Tue Jul 3 16:45:34 2012 +0200
> > >     vfs: Avoid unnecessary WB_SYNC_NONE writeback during sys_sync and reorder sync passes
> > > 
> > >     See also: http://www.kernelhub.org/?msg=93100&p=2
> > > 
> > > Fixed by putting back in the call to writeback_inodes_sb.
> > > 
> > > I'll attach the test in a reply to this e-mail.
> > > 
> > > The test starts by creating 512 files, syncing, reading one byte
> > > from each of those files, syncing, and then deleting each file
> > > and syncing. The time to do each sync is printed. The process
> > > is then repeated for 1024 files and then the next power of
> > > two up to 262144 files.
> > > 
> > > Note, when running the test, the slow down doesn't always happen
> > > but most of the tests will show a slow down.
> > > 
> > > In response to crbug.com/240422
> > > 
> > > Signed-off-by: Paul Taysom <taysom@...omium.org>
> >   Thanks for report. Rather than blindly reverting the change, I'd like to
> > understand why you see so huge regression. As the changelog in the patch
> > says, flusher thread should be doing async writeback equivalent to the
> > removed one because it gets woken via wakeup_flusher_threads(). But my
> > guess is that for some reason we end up doing all the writeback from
> > sync_inodes_one_sb(). I'll try to reproduce your results and investigate...
>   Hum, so it must be something timing sensitive. I wasn't able to reproduce
> the issue on my test machine in 4 runs of your test program. I was able to
> reproduce it on my laptop on every second run of the test program but once
> I've enabled some tracepoints, the issue disappeared and I didn't see it in
> about 10 runs.
> 
> That being said I think that reverting my patch is just papering over the
> problem. We will do the async pass over inodes twice instead of once
> and thus the timing changes enough that you aren't able to observe the
> problem.
> 
> I'm looking into this more...
  So I finally understood what's going on. If the system has no dirty pages
at all wakeup_flusher_threads() will submit work with nr_pages == 0.  So
wb_writeback() will bail out immediately without doing anything and all the
writeback is left for WB_SYNC_ALL pass of sync(1) which is slow. Attached
patch fixes the problem for me.

								Honza
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR

View attachment "0001-writeback-Fix-occasional-slow-sync-1.patch" of type "text/x-patch" (1368 bytes)