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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: pgsql-hackers <pgsql-hackers@postgresql.org>, David Geier <geidav.pg@gmail.com>
Date: 2026-04-08T06:09:23Z
Lists: pgsql-hackers 
Hi,

It looks like this recent failure on buildfarm member drongo might be
related to the timing changes - mainly suspecting it because the
commits are in the (slightly larger) set that changed, and the error
is in a timing related module that uses INSTR_TIME_SET_CURRENT.

>From https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=drongo&dt=2026-04-08%2001%3A57%3A00

# diff --strip-trailing-cr -U3
C:/prog/bf/root/HEAD/pgsql/contrib/tsm_system_time/expected/tsm_system_time.out
C:/prog/bf/root/HEAD/pgsql.build/testrun/tsm_system_time/regress/results/tsm_system_time.out
# --- C:/prog/bf/root/HEAD/pgsql/contrib/tsm_system_time/expected/tsm_system_time.out
   2023-01-23 04:39:00.533642000 +0000
# +++ C:/prog/bf/root/HEAD/pgsql.build/testrun/tsm_system_time/regress/results/tsm_system_time.out
   2026-04-08 04:03:15.248127800 +0000
# @@ -15,7 +15,7 @@
#  SELECT count(*) FROM test_tablesample TABLESAMPLE system_time (100000);
#   count
#  -------
# -    31
# +    16
#  (1 row)
#
#  -- bad parameters should get through planning, but not execution:
# 1 of 1 tests failed.
# The differences that caused some tests to fail can be viewed in the
file "C:/prog/bf/root/HEAD/pgsql.build/testrun/tsm_system_time/regress/regression.diffs".
# A copy of the test summary that you see above is saved in the file
"C:/prog/bf/root/HEAD/pgsql.build/testrun/tsm_system_time/regress/regression.out".

Haven't had a chance to dig through it yet, just noting it to start.


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