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Re: Reduce timing overhead of EXPLAIN ANALYZE using rdtsc?

Ants Aasma <ants.aasma@cybertec.at>

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From: Ants Aasma <ants.aasma@cybertec.at>
To: Andres Freund <andres@anarazel.de>
Cc: Jakub Wartak <jakub.wartak@enterprisedb.com>, Lukas Fittl <lukas@fittl.com>, David Geier <geidav.pg@gmail.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: 2026-02-05T09:23:15Z
Lists: pgsql-hackers 
On Wed, 4 Feb 2026 at 16:23, Andres Freund <andres@anarazel.de> wrote:
> I forgot another important one we really should use the fast timing for:
> track_io_timing/track_wal_io_timing. The overhead of IO timing can be
> noticeable on busy systems and it's hard to believe that the inaccuracy
> matters.  This is probably particularly relevant when measuring IO intensive
> workloads where the data resides in the kernel page cache, but doesn't fit
> into s_b.

I have been wanting for those two to be turned on by default with a
"fast enough" clock source. But determining this seemed complicated. I
think if the clock source for them is rdtsc we know that it is
definitely fast enough to be on by default.

--
Ants Aasma

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