linux-kernel - Re: jbd2: don't wake kjournald unnecessarily

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Date:	Wed, 23 Jan 2013 10:44:21 +0100
From:	Jan Kara <jack@...e.cz>
To:	Eric Sandeen <sandeen@...hat.com>
Cc:	Jan Kara <jack@...e.cz>, Theodore Ts'o <tytso@....edu>,
	Sedat Dilek <sedat.dilek@...il.com>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	Ext4 Developers List <linux-ext4@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	linux-next <linux-next@...r.kernel.org>, mszeredi@...e.cz
Subject: Re: jbd2: don't wake kjournald unnecessarily

On Tue 22-01-13 19:37:46, Eric Sandeen wrote:
> On 1/22/13 5:50 PM, Jan Kara wrote:
> > 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@...il.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...
> 
> I am still trying to get our perf guys to collect that data, FWIW...
> I will send it when I get it.  I bugged them again today.  :)
> 
> (Just to be sure: I was going to measure the wakeups the old way, and the
> avoided wakeups with the new change; sound ok?)
  Yes, that would be what I'm interested in.

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