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Re: hyperthreaded cpu still an issue in 8.4?

From: Greg Smith <gsmith(at)gregsmith(dot)com>
To: Scott Marlowe <scott(dot)marlowe(at)gmail(dot)com>
Cc: Merlin Moncure <mmoncure(at)gmail(dot)com>, Dave Youatt <dave(at)meteorsolutions(dot)com>, pgsql-performance(at)postgresql(dot)org
Subject: Re: hyperthreaded cpu still an issue in 8.4?
Date: 2009-07-28 23:21:24
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Lists: pgsql-performance
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 

* Greg Smith gsmith(at)gregsmith(dot)com Baltimore, MD

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