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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: Tom Lane <tgl@sss.pgh.pa.us>, Haibo Yan <tristan.yim@gmail.com>, pgsql-hackers <pgsql-hackers@postgresql.org>, David Geier <geidav.pg@gmail.com>
Date: 2026-04-12T19:58:36Z
Lists: pgsql-hackers 
On Sun, Apr 12, 2026 at 11:30 AM Andres Freund <andres@anarazel.de> wrote:
>
> On 2026-04-12 14:20:31 -0400, Tom Lane wrote:
> > 166           uint64          loop_count;
> > 167
> > 168           loop_count = test_timing(test_duration, TIMING_CLOCK_SOURCE_SYSTEM, false);
> > >>>     CID 1691465:         Incorrect expression  (DIVIDE_BY_ZERO)
> > >>>     In function call "output", division by expression "loop_count" which may be zero has undefined behavior.
> > 169           output(loop_count);
> >
> > AFAICS it's correct to complain.  test_timing() visibly can return zero,
> > but of the three places where test_timing() is followed by output()
> > only one has a defense against that.
>
> I think it should be unreachable as-is (but we should fix it anyway): If the
> system clock source doesn't work, we have much bigger issues. If rdtscp works,
> rdtsc should better work as well...

Yeah, agreed, in practice this won't be reached, but good to fix.

> Maybe it's enough to add an Assert() to clarify this?  But I guess just
> printing a message in that unreachable case would also work.

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;
}

That'd align with the proposed patch to warn about calibrated TSC
frequency diverging more than 10%.

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