On Wed 02-08-17 20:52:51, Andrew Perepechko wrote:
> > On Tue 01-08-17 15:02:42, Jan Kara wrote:
> > > When several processes race updating the same dquot, they very often all
> > > end up updating dquot on disk even though another process has already
> > > written dquot for them while they were waiting for dqio_sem - in my test
> > > above the ratio of commit_dqblk / dquot_commit calls was 59%. What we
> > > could
> > > do is that dquot_mark_dquot_dirty() would return "current sequence of
> > > dquot", dquot_commit() would then get sequence that is required to be
> > > written and if that is already written (we would also store in dquot
> > > latest
> > > written sequence), it would bail out doing nothing. This should cut down
> > > dqio_mutex hold times and thus wait times but I need to experiment and
> > > measure that...
> >
> > I've been experimenting with this today but this idea didn't bring any
> > benefit in my testing. Was your setup with multiple users or a single user?
> > Could you give some testing to my patches to see whether they bring some
> > benefit to you?
> >
> > Honza
>
> Hi Jan!
>
> My setup was with a single user. Unfortunately, it may take some time before
> I can try a patched kernel other than RHEL6 or RHEL7 with the same test,
> we have a lot of dependencies on these kernels.
>
> The actual test we ran was mdtest.
>
> By the way, we had 15+% performance improvement in creates from the
> change that was discussed earlier in this thread:
>
> EXT4_SB(dquot->dq_sb)->s_qf_names[GRPQUOTA]) {
> + if (test_bit(DQ_MOD_B, &dquot->dq_flags))
> + return 0;
> dquot_mark_dquot_dirty(dquot);
> return ext4_write_dquot(dquot);
>
> The idea was that if we know that some thread is somewhere between
> mark_dirty and clear_dirty, then we can avoid blocking on dqio_mutex,
> since that thread will update the ondisk dquot for us.
>
> I think, you also mentioned that some mark_dquot_dirty callers, such
> as do_set_dqblk, may not be running with an open transaction handle,
> so we cannot assume this optimization is atomic. However, we don't
> use do_set_dqblk and seem safe wrt journalling.
OK, thanks for info. I'm reluctant to make ext4_mark_dquot_dirty() return
before quota data is actually copied to the transaction which is what
probably brings you the benefit. I'll think about it some more.
Honza
--
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR
