Re: [REGRESSION] 2.6.19/2.6.20-rc3 buffered write slowdown

[Nick Piggin]

David Chinner wrote:
> On Thu, Jan 11, 2007 at 10:13:55AM +1100, Nick Piggin wrote:
>
> > David Chinner wrote:
> >
> > > On Wed, Jan 10, 2007 at 03:04:15PM -0800, Christoph Lameter wrote:
> > >
> > >
> > > > On Thu, 11 Jan 2007, David Chinner wrote:
> > > >
> > > >
> > > >
> > > > > The performance and smoothness is fully restored on 2.6.20-rc3
> > > > > by setting dirty_ratio down to 10 (from the default 40), so
> > > > > something in the VM is not working as well as it used to....
> > > >
> > > > dirty_background_ratio is left as is at 10?
> > >
> > >
> > > Yes.
> > >
> > >
> > >
> > > > So you gain performance by switching off background writes via pdflush?
> > >
> > >
> > > Well, pdflush appears to be doing very little on both 2.6.18 and
> > > 2.6.20-rc3. In both cases kswapd is consuming 10-20% of a CPU and
> > > all of the pdflush threads combined (I've seen up to 7 active at
> > > once) use maybe 1-2% of cpu time. This occurs regardless of the
> > > dirty_ratio setting.
> >
> > Hi David,
> >
> > Could you get /proc/vmstat deltas for each kernel, to start with?
>
>
> Sure, but that doesn't really show the how erratic the per-filesystem
> throughput is because the test I'm running is PCI-X bus limited in
> it's throughput at about 750MB/s. Each dm device is capable of about
> 340MB/s write, so when one slows down, the others will typically
> speed up.

But you do also get aggregate throughput drops? (ie. 2.6.20-rc3-worse)

> So, what I've attached is three files which have both
> 'vmstat 5' output and 'iostat 5 |grep dm-' output in them.

Ahh, sorry to be unclear, I meant:

  cat /proc/vmstat > pre
  run_test
  cat /proc/vmstat > post

It might just give us a hint what is changing (however vmstat doesn't
give much interesting in the way of pdflush stats, so it might not
show anything up).

Thanks,
Nick

--
SUSE Labs, Novell Inc.
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