Re: Mainline kernel OLTP performance update

[Matthew Wilcox <matthew@wil.cx>]

On Wed, Jan 14, 2009 at 04:35:57PM -0800, Andrew Morton wrote:
> On Tue, 13 Jan 2009 15:44:17 -0700
> "Wilcox, Matthew R" <matthew.r.wilcox@intel.com> wrote:
> >
> 
> (top-posting repaired.  That @intel.com address is a bad influence ;))

Alas, that email address goes to an Outlook client.  Not much to be done
about that.

> (cc linux-scsi)
> 
> > > This is latest 2.6.29-rc1 kernel OLTP performance result. Compare to
> > > 2.6.24.2 the regression is around 3.5%.
> > > 
> > > Linux OLTP Performance summary
> > > Kernel#            Speedup(x)   Intr/s  CtxSw/s us%  sys%   idle%  iowait%
> > > 2.6.24.2                1.000   21969   43425   76   24     0      0
> > > 2.6.27.2                0.973   30402   43523   74   25     0      1
> > > 2.6.29-rc1              0.965   30331   41970   74   26     0      0

> But the interrupt rate went through the roof.

Yes.  I forget why that was; I'll have to dig through my archives for
that.

> A 3.5% slowdown in this workload is considered pretty serious, isn't it?

Yes.  Anything above 0.3% is statistically significant.  1% is a big
deal.  The fact that we've lost 3.5% in the last year doesn't make
people happy.  There's a few things we've identified that have a big
effect:

 - Per-partition statistics.  Putting in a sysctl to stop doing them gets
   some of that back, but not as much as taking them out (even when
   the sysctl'd variable is in a __read_mostly section).  We tried a
   patch from Jens to speed up the search for a new partition, but it
   had no effect.

 - The RT scheduler changes.  They're better for some RT tasks, but not
   the database benchmark workload.  Chinang has posted about
   this before, but the thread didn't really go anywhere.
   http://marc.info/?t=122903815000001&r=1&w=2

SLUB would have had a huge negative effect if we were using it -- on the
order of 7% iirc.  SLQB is at least performance-neutral with SLAB.

-- 
Matthew Wilcox				Intel Open Source Technology Centre
"Bill, look, we understand that you're interested in selling us this
operating system, but compare it to ours.  We can't possibly take such
a retrograde step."