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Re: pg_test_timing tool for EXPLAIN ANALYZE overhead

From: Jay Levitt <jay(dot)levitt(at)gmail(dot)com>
To: Greg Smith <greg(at)2ndQuadrant(dot)com>
Cc: 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 16:10:38
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-hackers
Greg Smith wrote:
> Anyway, the patch does now includes several examples and a short primer on
> PC clock hardware, to help guide what good results look like and why they've
> been impossible to obtain in the past.  That's a bit Linux-centric, but the
> hardware described covers almost all systems using Intel or AMD processors.
> Only difference with most other operating systems is how aggressively they
> have adopted newer timer hardware.  At least this gives a way to measure all
> of them.

N.B.: Windows has at least two clock APIs, timeGetTime and 
QueryPerformanceCounters (and probably more, these days). They rely on 
different hardware clocks, and can get out of sync with each other; 
meanwhile, QueryPerformanceCounters can get out of sync with itself on 
(older?) multi-CPU boards.

So if you're doing high-res timing, it's good to make sure you aren't 
relying on two different clocks in different places... I ran into this with 
MIDI drivers years ago; and wrote a doc:

and a clock-testing utility:

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