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Re: Reduce timing overhead of EXPLAIN ANALYZE using rdtsc?

David Geier <geidav.pg@gmail.com>

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From: David Geier <geidav.pg@gmail.com>
To: Lukas Fittl <lukas@fittl.com>, Jakub Wartak <jakub.wartak@enterprisedb.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>
Date: 2026-02-04T10:06:50Z
Lists: pgsql-hackers 
> I do think its reasonable for us to check this directly, since we're
> already working with cpuid information anyway, and we also need the
> TSC frequency data in the Hypervisor specific leafs (i.e. its not just
> about getting the VM vendor name itself). For example, I don't think
> HyperV provides TSC frequency in 0x40000010 - it was something VMware
> added initially, that KVM subsequently added.

And then we have a hard dependency on systemd, where at the moment the
user can try to use RDTSC also on systems without.

>> Also it would be cool if the patch would provide some way of reporting back
>> what clock_source was really used in case of FAST_CLOCK_SOURCE_AUTO.
>> Something like huge_pages_status or some elog(DEBUG).
> 
> Agreed, I think that would be useful.

+1

--
David Geier

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