Re: Spinlocks, yet again: analysis and proposed patches

On Thu, 20 Oct 2005 23:03:47 +0100
Simon Riggs <simon(at)2ndquadrant(dot)com> wrote:

> On Wed, 2005-10-19 at 14:07 -0700, Mark Wong wrote:
> > >
> > > This isn't exactly elegant coding, but it provides a useful improvement
> > > on an 8-way SMP box when run on 8.0 base. OK, lets be brutal: this looks
> > > pretty darn stupid. But it does follow the CPU optimization handbook
> > > advice and I did see a noticeable improvement in performance and a
> > > reduction in context switching.
>
> > > I'm not in a position to try this again now on 8.1beta, but I'd welcome
> > > a performance test result from anybody that is. I'll supply a patch
> > > against 8.1beta for anyone wanting to test this.
> >
> > Ok, I've produce a few results on a 4 way (8 core) POWER 5 system, which
> > I've just set up and probably needs a bit of tuning. I don't see much
> > difference but I'm wondering if the cacheline sizes are dramatically
> > different from Intel/AMD processors. I still need to take a closer look
> > to make sure I haven't grossly mistuned anything, but I'll let everyone
> > take a look:
>
> Well, the Power 5 architecture probably has the lowest overall memory
> delay you can get currently so in some ways that would negate the
> effects of the patch. (Cacheline is still 128 bytes, AFAICS). But it's
> clear the patch isn't significantly better (like it was with 8.0 when we
> tried this on the 8-way Itanium in Feb).
>
> > cvs 20051013
> > http://www.testing.osdl.org/projects/dbt2dev/results/dev4-014/19/
> > 2501 notpm
> >
> > cvs 20051013 w/ lw.patch
> > http://www.testing.osdl.org/projects/dbt2dev/results/dev4-014/20/
> > 2519 notpm
>
> Could you re-run with wal_buffers = 32 ? (Without patch) Thanks

Ok, sorry for the delay. I've bumped up the wal_buffers to 2048 and
redid the disk layout. Here's where I'm at now:

cvs 20051013
http://www.testing.osdl.org/projects/dbt2dev/results/dev4-014/40/
3257 notpm

cvs 20051013 w/ lw.patch
http://www.testing.osdl.org/projects/dbt2dev/results/dev4-014/42/
3285 notpm

Still not much of a difference with the patch. A quick glance over the
iostat data suggests I'm still not i/o bound, but the i/o wait is rather
high according to vmstat. Will try to see if there's anything else
obvious to get the load up higher.

Mark