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Re: Reduce timing overhead of EXPLAIN ANALYZE using rdtsc?

Tom Lane <tgl@sss.pgh.pa.us>

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From: Tom Lane <tgl@sss.pgh.pa.us>
To: Lukas Fittl <lukas@fittl.com>
Cc: Haibo Yan <tristan.yim@gmail.com>, Andres Freund <andres@anarazel.de>, pgsql-hackers <pgsql-hackers@postgresql.org>, David Geier <geidav.pg@gmail.com>
Date: 2026-04-12T18:20:31Z
Lists: pgsql-hackers 
Lukas Fittl <lukas@fittl.com> writes:
> FWIW, for archive's sake, drongo is green again now, thanks to commit
> 7fc36c5db550 (Avoid CPUID 0x15/0x16 for Hypervisor TSC frequency).

drongo may be happy, but Coverity is not:

166     	uint64		loop_count;
167     
168     	loop_count = test_timing(test_duration, TIMING_CLOCK_SOURCE_SYSTEM, false);
>>>     CID 1691465:         Incorrect expression  (DIVIDE_BY_ZERO)
>>>     In function call "output", division by expression "loop_count" which may be zero has undefined behavior.
169     	output(loop_count);

AFAICS it's correct to complain.  test_timing() visibly can return zero,
but of the three places where test_timing() is followed by output()
only one has a defense against that.

			regards, tom lane

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