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Re: Reduce timing overhead of EXPLAIN ANALYZE using rdtsc?

Andres Freund <andres@anarazel.de>

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From: Andres Freund <andres@anarazel.de>
To: Jakub Wartak <jakub.wartak@enterprisedb.com>
Cc: 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: 2026-02-04T14:22:54Z
Lists: pgsql-hackers 
Hi,

On 2026-02-03 12:44:21 -0500, Andres Freund wrote:
> Hm. I think it'd make sense to use eventually use this clock source for other
> timing tasks too, not just explain.  E.g. pg_stat_statements could benefit
> quite a bit from reducing the timing overhead.
> 
> Whereas it'll not make sense for anything that needs wall clock times - which
> imo makes a "clock_source" GUC misnamed.  Maybe "clock_source_timing" or such?

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.

Greetings,

Andres Freund

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