Re: [PATCH] fs: sync: fixed performance regression

[Paul Taysom]

`On Fri, Jul 12, 2013 at 8:43 AM, Jan Kara <jack@xxxxxxx> wrote:
> 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@xxxxxxx>
>> > >     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@xxxxxxxxxxxx>
>> >   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@xxxxxxx>
> SUSE Labs, CR

Jan,
Your fix is a clear win! Not only did it fix the sync after read
problem but it made the sync after create faster too.
Thanks,
Paul
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