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SubjectRe: jbd2: don't wake kjournald unnecessarily
On Mon 21-01-13 18:11:30, Ted Tso wrote:
> On Tue, Jan 22, 2013 at 12:04:32AM +0100, Sedat Dilek wrote:
> >
> > Beyond the FUSE/LOOP fun, will you apply this patch to your linux-next GIT tree?
> >
> > Feel free to add...
> >
> > Tested-by: Sedat Dilek <sedat.dilek@gmail.com>
> >
> > A similiar patch for JBD went through your tree into mainline (see [1] and [2]).
>
> I'm not at all convinced that this patch has anything to do with your
> problem. I don't see how it could affect things, and I believe you
> mentioned that you saw the problem even with this patch applied? (I'm
> not sure; some of your messages which you sent were hard to
> understand, and you mentioned something about trying to send messages
> when low on sleep :-).
>
> In any case, the reason why I haven't pulled this patch into the ext4
> tree is because I was waiting for Eric and some of the performance
> team folks at Red Hat to supply some additional information about why
> this commit was making a difference in performance for a particular
> proprietary, closed source benchmark.
Just a small correction - it was aim7 AFAIK which isn't closed source
(anymore). You can download it from SourceForge
(http://sourceforge.net/projects/aimbench/files/aim-suite7/Initial%20release/).
Now I have some reservations about what the benchmark does but historically
it has found quite a few issues for us as well.

> I'm very suspicious about applying patches under the "cargo cult"
> school of programming. ("We don't understand why it makes a
> difference, but it seems to be good, so bombs away!" :-)
Well, neither am I ;) But it is obvious the patch speeds up
log_start_commit() by 'a bit' (taking spinlock, disabling irqs, ...). And
apparently 'a bit' is noticeable for particular workload on a particular
machine - commit statistics Eric provided showed that clearly. I'd still be
happier if Eric also told us how much log_start_commit() calls there were
so that one could verify that 'a bit' could indeed multiply to a measurable
difference. But given how simple the patch is, I gave away after a while
and just merged it...

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


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