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Re: Reduce timing overhead of EXPLAIN ANALYZE using rdtsc?

Jakub Wartak <jakub.wartak@enterprisedb.com>

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From: Jakub Wartak <jakub.wartak@enterprisedb.com>
To: Lukas Fittl <lukas@fittl.com>
Cc: Andres Freund <andres@anarazel.de>, 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: 2026-02-25T11:57:19Z
Lists: pgsql-hackers 
On Wed, Feb 25, 2026 at 11:01 AM Lukas Fittl <lukas@fittl.com> wrote:
[..]
> See attached v9, rebased, with feedback addressed, unless otherwise noted.

FWIW, I've tried this on FreeBSD and I couldn't understand why I
couldn't enable TSC
(atlought) the cpuid tools reported RDTSCP being available. I have found out
that set_tsc_frequency_khz() returns false and tsc_frequency_khz==0 before even
inquiry for is_rdtscp_available() is done. As this was on virtualbox VM (so not
suppoprted by current code), I think this is pretty clear we'll need some way of
informing users why he cannot SET timing_clock_source to TSC. Possibly errhint()
could differentiate the reason? E.g.
  "failed to auto-detect TSC frequency (potential hypervisor issue)" or
  "CPU RDTSCP instructions are not available"

-J.

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