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Re: pg_test_timing tool for EXPLAIN ANALYZE overhead

From: Greg Smith <greg(at)2ndQuadrant(dot)com>
To: Marti Raudsepp <marti(at)juffo(dot)org>
Cc: Jay Levitt <jay(dot)levitt(at)gmail(dot)com>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>, ants(dot)aasma(at)eesti(dot)ee
Subject: Re: pg_test_timing tool for EXPLAIN ANALYZE overhead
Date: 2012-02-22 17:36:28
Message-ID: 4F45279C.6080008@2ndQuadrant.com (view raw or flat)
Thread:
Lists: pgsql-hackers
On 02/22/2012 12:25 PM, Marti Raudsepp wrote:
> On Wed, Feb 22, 2012 at 18:44, Greg Smith<greg(at)2ndquadrant(dot)com>  wrote:
>> As far as I've been able to tell, there aren't any issues unique to Windows
>> there.  Multiple cores can have their TSC results get out of sync on Windows
>> for the same reason they do on Linux systems, and there's also the same
>> frequency/temperature issues.
>
> Not on recent Linux kernel versions. Linux automatically detects when
> the TSC is unstable (due to power management or out-of-sync
> cores/sockets) and automatically falls back to the more expensive HPET
> or ACPI methods.

 From the patch:

Newer operating systems may check for the known TSC problems and
switch to a slower, more stable clock source when they are seen.
If your system supports TSC time but doesn't default to that, it
may be disabled for a good reason.

I ran into a case like you're showing here in my longer exploration of 
this at 
http://archives.postgresql.org/message-id/4EDF1871.2010209@2ndQuadrant.com 
  I stopped just short of showing what the TSC error message looked 
like.  I hoped that with the above and some examples showing dmesg | 
grep, that would be enough to lead enough people toward finding this on 
their own.

-- 
Greg Smith   2ndQuadrant US    greg(at)2ndQuadrant(dot)com   Baltimore, MD
PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.com

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