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From: Josh Berkus <josh@agliodbs.com>
To: Bruce Momjian <bruce@momjian.us>
Cc: pgsql-performance@postgresql.org, Mark Kirkwood <mark.kirkwood@catalyst.net.nz>
Cc: Graeme B. Bell <grb@skogoglandskap.no>
Cc: Greg Smith <gregsmithpgsql@gmail.com>
Subject: Re: Turn off Hyperthreading! WAS: 60 core performance with 9.3
Date: Thu, 21 Aug 2014 14:17:13 -0700
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On 08/21/2014 02:11 PM, Bruce Momjian wrote:
> On Thu, Aug 21, 2014 at 02:02:26PM -0700, Josh Berkus wrote:
>> On 08/20/2014 07:40 PM, Bruce Momjian wrote:
>>> On Wed, Aug 20, 2014 at 12:13:50PM -0700, Josh Berkus wrote:
>>>> On a read-write test, it's 10% faster with HT off as well.
>>>>
>>>> Further, from their production machine we've seen that having HT on
>>>> causes the machine to slow down by 5X whenever you get more than 40
>>>> cores (as in 100% of real cores or 50% of HT cores) worth of activity.
>>>>
>>>> So we're definitely back to "If you're using PostgreSQL, turn off
>>>> Hyperthreading".
>>>
>>> Not sure how you can make such a blanket statement when so many people
>>> have tested and shown the benefits of hyper-threading.  
>>
>> 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.

-- 
Josh Berkus
PostgreSQL Experts Inc.
http://pgexperts.com


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