Re: hyperthreaded cpu still an issue in 8.4?

On Tue, 28 Jul 2009, Scott Marlowe wrote:

> Just FYI, I ran the same basic test but with -c 10 since -c shouldn't
> really be greater than -s

That's only true if you're running the TPC-B-like or other write tests,
where access to the small branches table becomes a serious hotspot for
contention.  The select-only test has no such specific restriction as it
only operations on the big accounts table.  Often peak throughput is
closer to a very small multiple on the number of cores though, and
possibly even clients=cores, presumably because it's more efficient to
approximately peg one backend per core rather than switch among more than
one on each--reduced L1 cache contention etc.  That's the behavior you
measured when your test showed better results with c=10 than c=16 on a 8
core system, rather than suffering less from the "c must be < s"
contention limitation.

Sadly I don't have or expect to have a W5580 in the near future though,
the X5550 @ 2.67GHz is the bang for the buck sweet spot right now and
accordingly that's what I have in the lab at Truviso.  As Merlin points
out, that's still plenty to spank any select-only pgbench results I've
ever seen.  The multi-threaded pgbench batch submitted by Itagaki Takahiro
recently is here just in time to really exercise these new processors
properly.

--
* Greg Smith gsmith@gregsmith.com http://www.gregsmith.com Baltimore, MD