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Re: hyperthreaded cpu still an issue in 8.4?

From: Stefan Kaltenbrunner <stefan(at)kaltenbrunner(dot)cc>
To: Greg Smith <gsmith(at)gregsmith(dot)com>
Cc: Scott Marlowe <scott(dot)marlowe(at)gmail(dot)com>, 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: 2009-07-29 04:31:09
Message-ID: 4A6FD08D.9020807@kaltenbrunner.cc (view raw or flat)
Thread:
Lists: pgsql-performance
Greg Smith wrote:
> 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.

Well the real problem is that pgbench itself does not scale too well to 
lots of concurrent connections and/or to high transaction rates so it 
seriously skews the result. If you look 
http://www.kaltenbrunner.cc/blog/index.php?/archives/26-Benchmarking-8.4-Chapter-1Read-Only-workloads.html.
It is pretty clear that 90k(or the 83k I got due to the slower E5530) 
tps is actually a pgench limit and that the backend really can do almost 
twice as fast (I only demonstrated ~140k tps using sysbench there but I 
later managed to do ~160k tps with queries that are closer to what 
pgbench does in the lab)


Stefan

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