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Re: Reduce timing overhead of EXPLAIN ANALYZE using rdtsc?

John Naylor <johncnaylorls@gmail.com>

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From: John Naylor <johncnaylorls@gmail.com>
To: Lukas Fittl <lukas@fittl.com>
Cc: Andres Freund <andres@anarazel.de>, Jakub Wartak <jakub.wartak@enterprisedb.com>, Hannu Krosing <hannuk@google.com>, Robert Haas <robertmhaas@gmail.com>, Pavel Stehule <pavel.stehule@gmail.com>, vignesh C <vignesh21@gmail.com>, Michael Paquier <michael@paquier.xyz>, Ibrar Ahmed <ibrar.ahmad@gmail.com>, Maciek Sakrejda <m.sakrejda@gmail.com>, pgsql-hackers <pgsql-hackers@postgresql.org>, David Geier <geidav.pg@gmail.com>
Date: 2026-04-07T04:56:04Z
Lists: pgsql-hackers 
On Tue, Apr 7, 2026 at 10:42 AM Lukas Fittl <lukas@fittl.com> wrote:
>
> On Mon, Apr 6, 2026 at 5:40 PM Andres Freund <andres@anarazel.de> wrote:

> > I wonder if the cpuid tests should be a bit further abstracted into
> > pg_cpu_x86.c.
> >
> > E.g. instead of tsc_detect_frequency() checking for PG_RDTSCP,
> > PG_TSC_INVARIANT, PG_TSC_ADJUST we could have
> >
> > PG_TSC_AVAILABLE /* RDTSCP & INVARIANT */
> > PG_TSC_KNOWN_RELIABLE /* PG_TSC_AVAILABLE && PG_TSC_ADJUST */
> > PG_TSC_FREQUENCY_KNOWN /* x86_tsc_frequency_khz works */
> >
> > and always run all of that during set_x86_features().
>
> I think that could work, but I kept the flags in features closer to
> being direct mappings to CPUID bits since that seemed to be intent of
> how John designed the facility originally.
>
> John, do you have thoughts on this? (I've not changed it for now)

The intent is as how you described, but it's not because that way was
the best of all possible ways, more that it seemed to fit well
underneath the existing runtime checks. It's also already not quite a
direct mapping, since if OSXSAVE is not available, we don't bother
checking for/setting AVX at all.

--
John Naylor
Amazon Web Services

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