Date: Thu, 21 Aug 2014 17:26:17 -0400
From: Bruce Momjian <bruce@momjian.us>
To: Josh Berkus <josh@agliodbs.com>
Cc: pgsql-performance@postgresql.org,
 Mark Kirkwood <mark.kirkwood@catalyst.net.nz>,
 "Graeme B. Bell" <grb@skogoglandskap.no>,
 Greg Smith <gregsmithpgsql@gmail.com>
Subject: Re: Turn off Hyperthreading! WAS: 60 core performance
 with 9.3
Message-ID: <20140821212617.GE26706@momjian.us>
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On Thu, Aug 21, 2014 at 02:17:13PM -0700, Josh Berkus wrote:
> >> Actually, I don't know that anyone has posted the benefits of HT.  Link?
> >>  I want to compare results so that we can figure out what's different
> >> between my case and theirs.  Also, it makes a big difference if there is
> >> an advantage to turning HT on for some workloads.
> > 
> > I had Greg Smith test my system when it was installed, tested it, and
> > recommended hyper-threading.  The system is Debian Squeeze
> > (2.6.32-5-amd64), CPUs are dual Xeon E5620, 8 cores, 16 virtual cores.
> 
> Can you post some numerical results?
> 
> I'm serious.  It's obviously easier for our users if we can blanket
> recommend turning HT off; that's a LOT easier for them than "you might
> want to turn HT off if these conditions ...".  So I want to establish
> that HT is a benefit sometimes if it is.
> 
> I personally have never seen HT be a benefit.  I've seen it be harmless
> (most of the time) but never beneficial.

I know that when hyperthreading was introduced that it was mostly a
negative, but then this was improved, and it might have gotten bad
again.  I am afraid results are based on the type of CPU, so I am not
sure we can know a general answer.

I know I asked Greg Smith, and I assume he would know.

-- 
  Bruce Momjian  <bruce@momjian.us>        http://momjian.us
  EnterpriseDB                             http://enterprisedb.com

  + Everyone has their own god. +


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