From: | Andres Freund <andres(at)anarazel(dot)de> |
---|---|
To: | David Geier <geidav(dot)pg(at)gmail(dot)com> |
Cc: | Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com>, Tomas Vondra <tomas(dot)vondra(at)enterprisedb(dot)com>, vignesh C <vignesh21(at)gmail(dot)com>, Lukas Fittl <lukas(at)fittl(dot)com>, Michael Paquier <michael(at)paquier(dot)xyz>, Ibrar Ahmed <ibrar(dot)ahmad(at)gmail(dot)com>, Maciek Sakrejda <m(dot)sakrejda(at)gmail(dot)com>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org> |
Subject: | Re: Reduce timing overhead of EXPLAIN ANALYZE using rdtsc? |
Date: | 2023-01-21 04:14:39 |
Message-ID: | 20230121041439.zraxavau2wqf2ys3@awork3.anarazel.de |
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Lists: | pgsql-hackers |
Hi,
On 2023-01-20 07:43:00 +0100, David Geier wrote:
> On 1/18/23 13:52, David Geier wrote:
> > On 1/16/23 21:39, Pavel Stehule wrote:
> > >
> > > po 16. 1. 2023 v 21:34 odesílatel Tomas Vondra
> > > <tomas(dot)vondra(at)enterprisedb(dot)com> napsal:
> > >
> > > Hi,
> > >
> > > there's minor bitrot in the Mkvcbuild.pm change, making cfbot
> > > unhappy.
> > >
> > > As for the patch, I don't have much comments. I'm wondering if
> > > it'd be
> > > useful to indicate which timing source was actually used for EXPLAIN
> > > ANALYZE, say something like:
> > >
> > > Planning time: 0.197 ms
> > > Execution time: 0.225 ms
> > > Timing source: clock_gettime (or tsc)
> > >
> > > +1
> >
> > I like the idea of exposing the timing source in the EXPLAIN ANALYZE
> > output.
> > It's a good tradeoff between inspectability and effort, given that RDTSC
> > should always be better to use.
> > If there are no objections I go this way.
> Thinking about this a little more made me realize that this will cause
> different pg_regress output depending on the platform. So if we go this
> route we would at least need an option for EXPLAIN ANALYZE to disable it. Or
> rather have it disabled by default and allow for enabling it. Thoughts?
The elapsed time is already inherently unstable, so we shouldn't have any test
output showing the time.
But I doubt showing it in every explain is a good idea - we use instr_time in
plenty of other places. Why show it in explain, but not in all those other
places?
Greetings,
Andres Freund
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