On Fri 12-07-13 09:59:00, Paul Taysom wrote: > `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 for testing! I've sent the patch to Al for inclusion. Honza -- Jan Kara <jack@xxxxxxx> SUSE Labs, CR -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
