Re: [PATCH 0/11] Per-bdi writeback flusher threads v9

Frederic Weisbecker
2009-06-04T22:35:07Z

On Thu, Jun 04, 2009 at 10:10:12PM +0200, Jens Axboe wrote:
> On Thu, Jun 04 2009, Jens Axboe wrote:
> > On Thu, Jun 04 2009, Frederic Weisbecker wrote:
> > > On Thu, Jun 04, 2009 at 12:07:26PM -0700, Andrew Morton wrote:
> > > > On Thu, 4 Jun 2009 17:20:44 +0200 Frederic Weisbecker <fweisbec@gmail.com> wrote:
> > > >
> > > > > I've just tested it on UP in a single disk.
> > > >
> > > > I must say, I'm stunned at the amount of testing which people are
> > > > performing on this patchset. Normally when someone sends out a
> > > > patchset it just sort of lands with a dull thud.
> > > >
> > > > I'm not sure what Jens did right to make all this happen, but thanks!
> > >
> > >
> > > I don't know how he did either. I was reading theses patches and *something*
> > > pushed me to my testbox, and then I tested...
> > >
> > > Jens, how do you do that?
> >
> > Heh, not sure :-)
> >
> > But indeed, thanks for the testing. It looks quite interesting. I'm
> > guessing it probably has to do with who ends up doing the balancing and
> > that the flusher threads block, it may change the picture a bit. So it
> > may just be that it'll require a few vm tweaks. I'll definitely look
> > into it and try and reproduce your results.
> >
> > Did you run it a 2nd time on each drive and check if the results were
> > (approximately) consistent on the two drives?
>
> each partition... What IO scheduler did you use on hda?

CFQ.

> The main difference with this test case is that before we had two super
> blocks, each with lists of dirty inodes. pdflush would attack those. Now
> we have both the inodes from the two supers on a single set of lists on
> the bdi. So either we have some ordering issue there (which is causing
> the unfairness), or something else is.

Yeah.
But although these flushers are per-bdi, with a single list (well, three)
of dirty inodes, it looks like the writeback is still performed per
superblock, I mean the bdi work gives the concerned superblock
and the bdi list is iterated in generic_sync_wb_inodes() which
only processes the inodes for the given superblock. So there is
a bit of a per superblock serialization there and....

(Note, the above is just written for myself in the secret hope I could
understand better these patches by writing my brainstorming...)

> So perhaps you can try with noop on hda to see if that changes the
> picture?

The result with noop is even more impressive.

See: http://kernel.org/pub/linux/kernel/people/frederic/dbench-noop.pdf

Also a comparison, noop with pdflush against noop with bdi writeback:

http://kernel.org/pub/linux/kernel/people/frederic/dbench-noop-cmp.pdf

Frederic.