Horton

Re: Reduce timing overhead of EXPLAIN ANALYZE using rdtsc?

Andres Freund <andres@anarazel.de>

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From: Andres Freund <andres@anarazel.de>
To: David Geier <geidav.pg@gmail.com>
Cc: Pavel Stehule <pavel.stehule@gmail.com>, Tomas Vondra <tomas.vondra@enterprisedb.com>, vignesh C <vignesh21@gmail.com>, Lukas Fittl <lukas@fittl.com>, Michael Paquier <michael@paquier.xyz>, Ibrar Ahmed <ibrar.ahmad@gmail.com>, Maciek Sakrejda <m.sakrejda@gmail.com>, pgsql-hackers <pgsql-hackers@postgresql.org>
Date: 2023-01-21T04:14:39Z
Lists: pgsql-hackers

Commits

Same data as JSON: GET /api/v1/messages/:b64id/commits the thread's linked commits as JSON, with link sources. API reference →

  1. pg_test_timing: Also test RDTSC[P] timing, report time source, TSC frequency

  2. Allow retrieving x86 TSC frequency/flags from CPUID

  3. instrumentation: Standardize ticks to nanosecond conversion method

  4. instrumentation: Use Time-Stamp Counter on x86-64 to lower overhead

  5. Zero initialize uses of instr_time about to trigger compiler warnings

  6. instr_time: Represent time as an int64 on all platforms

  7. Add 250c8ee07ed to git-blame-ignore-revs

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.vondra@enterprisedb.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