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Re: Reduce timing overhead of EXPLAIN ANALYZE using rdtsc?

Lukas Fittl <lukas@fittl.com>

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From: Lukas Fittl <lukas@fittl.com>
To: Andres Freund <andres@anarazel.de>
Cc: Haibo Yan <tristan.yim@gmail.com>, pgsql-hackers <pgsql-hackers@postgresql.org>, David Geier <geidav.pg@gmail.com>, Tom Lane <tgl@sss.pgh.pa.us>
Date: 2026-04-24T06:16:07Z
Lists: pgsql-hackers

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On Sun, Apr 12, 2026 at 1:24 PM Tom Lane <tgl@sss.pgh.pa.us> wrote:
>
> Lukas Fittl <lukas@fittl.com> writes:
> > I think either is fine. If we did it with a message, how about this at
> > the beginning of the output function?
>
> > if (loop_count == 0)
> > {
> >   printf(_("WARNING: No timing measurements collected. Report this as a bug to <%s>.\n"), PACKAGE_BUGREPORT);
> >   return;
> > }
>
> WFM.

Thanks for confirming!

I've checked with Andres off-list whether he wants to fix this
separately, or together with the earlier proposed patch to show TSC
debug info in pg_test_timing. Andres proposed we take care of this in
the same commit.

Attached v29 patch that contains those earlier changes, plus this
Coverity fix. I've also adjusted x86_tsc_frequency_khz slightly to
clarify the new arguments, and wordsmithed the commit message a bit
for clarity plus explain why we're making this change now.

FWIW, I don't think the TSC debug info change violates any feature
freeze guidelines, and it would have helped investigate the issue on
drongo / have it fail independently of the tablesample tests being
affected by the issue. I think any TSC frequency related bug reports
during beta period will also be greatly aided by having this.

Thanks,
Lukas

-- 
Lukas Fittl

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: Show additional TSC clock source debug info

  2. instrumentation: Avoid CPUID 0x15/0x16 for Hypervisor TSC frequency

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

  4. Allow retrieving x86 TSC frequency/flags from CPUID

  5. instrumentation: Standardize ticks to nanosecond conversion method

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

  7. Check for __cpuidex and __get_cpuid_count separately

  8. pg_test_timing: Reduce per-loop overhead

  9. Refactor handling of x86 CPUID instructions

  10. instrumentation: Drop INSTR_TIME_SET_CURRENT_LAZY macro

  11. Rename pg_crc32c_sse42_choose.c for general purpose

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

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

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