Message-ID: <53F65E62.3020909@agliodbs.com>
Date: Thu, 21 Aug 2014 14:02:26 -0700
From: Josh Berkus <josh@agliodbs.com>
Organization: PostgreSQL Experts Inc.
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To: Bruce Momjian <bruce@momjian.us>
CC: pgsql-performance@postgresql.org,
 Mark Kirkwood <mark.kirkwood@catalyst.net.nz>,
 "Graeme B. Bell" <grb@skogoglandskap.no>
Subject: Re: Turn off Hyperthreading! WAS: 60 core performance with
 9.3
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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 am also
> unclear exactly what you tested, as I didn't see it mentioned in the
> email --- CPU type, CPU count, and operating system would be the minimal
> information required.

Ooops!  I thought I'd posted that earlier, but I didn't.

The processors in question is the Intel(R) Xeon(R) CPU E7- 4850, with 4
of them for a total of 40 cores or 80 HT cores.

OS is RHEL with 2.6.32-431.3.1.el6.x86_64.

I've emailed a kernel hacker who works at Intel for comment; for one
thing, I'm wondering if the older kernel version is a problem for a
system like this.

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


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