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SubjectRe: [PATCH] Fix congestion_wait() sync/async vs read/write confusion
On Tue 14-07-09 09:12:15, Chris Mason wrote:
> On Tue, Jul 14, 2009 at 01:44:19PM +0200, Jan Kara wrote:
> > On Wed 08-07-09 15:12:38, Chris Mason wrote:
> > > On Wed, Jul 08, 2009 at 08:47:03PM +0200, Jens Axboe wrote:
> > > > Hi,
> > > >
> > > > This one isn't great, we currently have broken congestion wait logic in
> > > > the kernel. 2.6.30 is impacted as well, so this patch should go to
> > > > stable too once it's in -git. I'll let this one simmer until tomorrow,
> > > > then ask Linus to pull it. The offending commit breaking this is
> > > > 1faa16d22877f4839bd433547d770c676d1d964c.
> > > >
> > > > Meanwhile, it could potentially cause buffered writeout slowdowns in the
> > > > kernel. Perhaps the 2.6.30 regression in that area is caused by this?
> > > > Would be interesting if the submitter could test. I can't find the list,
> > > > CC'ing Rafael.
> > >
> > > Even if this does slow down some workloads, the bug is not in using the
> > > correct flag ;) So, I'd ack this one.
> > >
> > > Jan Kara was able to reproduce the tiobench 2.6.30 regression, so I've
> > > cc'd him and kept the patch below.
> > Thanks for the patch Chris. I've remeasured tiobench with the 2.6.30 +
> > the fix but it didn't help (which is not too surprising as what I observe
> > is most likely CFQ related as there's no regression with NOOP scheduler).
> > Just to recall:
> > 2.6.29 (CFQ) Avg StdDev
> > 8 38.01 40.26 39.69 -> 39.32 0.955092
> > 16 40.09 38.18 40.05 -> 39.44 0.891104
> >
> > 2.6.30-rc8 (CFQ)
> > 8 36.67 36.81 38.20 -> 37.23 0.69062
> > 16 37.45 36.47 37.46 -> 37.13 0.464351
> >
> > 2.6.30-rc8+fix (CFQ)
> > 8 37.56 37.38 37.98 -> 37.64 0.251396
> > 16 38.11 36.71 37.18 -> 37.33 0.581741
> >
> > So with the fix there's no statistically significant difference and we
> > are still below 2.6.29 results. I'm now going to retest with the WRITE_SYNC
> > changes reverted.
>
> Well, its good the patch didn't make things worse ;) I didn't have the
> highest hopes that it would resolve the regression, but thanks for
> testing!
I've now tried to revert everything which looked WRITE_SYNC related but
it didn't help either. Now, I'm trying to basically bisect CFQ changes and
I'll see whether it goes somewhere...

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


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