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Re: Hyperthreading (was: Two identical systems, radically different performance)

From: Shaun Thomas <sthomas(at)optionshouse(dot)com>
To: Craig James <cjames(at)emolecules(dot)com>
Cc: <pgsql-performance(at)postgresql(dot)org>
Subject: Re: Hyperthreading (was: Two identical systems, radically different performance)
Date: 2012-10-10 12:52:37
Message-ID: 50756F95.7040006@optionshouse.com (view raw or flat)
Thread:
Lists: pgsql-performance
On 10/09/2012 06:30 PM, Craig James wrote:

>                ra:8192 walb:1M   ra:256 walb:1M    ra:256 walb:256kB
>                ----------------  ----------------  -----------------
> -c  -t        Run1  Run2  Run3  Run4  Run5  Run6  Run7  Run8  Run9
> 40  2500      4261  3722  4243  9286  9240  5712  9310  8530  8872
> 50  2000      4138  4399  3865  9213  9351  9578  8011  7651  8362

I think I speak for more than a few people here when I say: wat.

About the only thing I can ask, is: did you make these tests fair? And 
by fair, I mean:

echo 3 > /proc/sys/vm/drop_caches
pg_ctl -D /your/pg/dir restart

Between every test to make sure shared buffers and the OS inode cache 
was empty before the start of each test? If you're using that bash-style 
for-loop you attached earlier, probably not. Still though, I don't think 
that would account for this much variance between having read-ahead at 
8M as opposed to 256kb.

My head hurts.

-- 
Shaun Thomas
OptionsHouse | 141 W. Jackson Blvd. | Suite 500 | Chicago IL, 60604
312-444-8534
sthomas(at)optionshouse(dot)com

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