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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: David Geier <geidav.pg@gmail.com>
Cc: John Naylor <johncnaylorls@gmail.com>, 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>
Date: 2026-03-24T06:09:57Z
Lists: pgsql-hackers 
Hi David,

On Mon, Mar 23, 2026 at 1:00 AM David Geier <geidav.pg@gmail.com> wrote:
> > TSC calibration improvements:
> > - Pass TSC frequency to child processes for EXEC_BACKEND, this makes
> > TSC calibration usable on Windows, per off-list suggestion from Andres
> > - To support the EXEC_BACKEND change, refactor tsc_frequency_khz to
> > have a sentinel value of -1 indicating the TSC was not initialized, vs
> > 0 indicating it was initialized but isn't usable, and drop
> > has_usable_tsc variable
>
> Have you tested the latest patch set on Windows? Let me know if you want
> me to do that.

Not outside of CI - so if you could do testing on Windows, that'd be great!

FWIW, CI does now exercise the TSC calibration logic on Windows, which
is part why the last patch set added the handling of EXEC_BACKEND to
pass down the frequency to the client connections, because I otherwise
saw this impacting Windows CI runtimes - they're normal with the
version posted.

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