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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: John Naylor <johncnaylorls@gmail.com>
Cc: Andres Freund <andres@anarazel.de>, Jakub Wartak <jakub.wartak@enterprisedb.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>, David Geier <geidav.pg@gmail.com>
Date: 2026-03-24T07:16:07Z
Lists: pgsql-hackers

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Hi John,

On Mon, Mar 23, 2026 at 12:01 AM John Naylor <johncnaylorls@gmail.com> wrote:
> It's not that bad that the hard-coded indexes are in two places, but
> it's also not necessary. I think "#define EAX 0 ...etc" is a fine,
> straightforward increase in readability compared to what we have now,
> and I don't see any downside. I suppose one argument in favor of the
> struct is that it avoids declaring variables of type
> array-of-4-unsigned in multiple places, but I think the array is fine,
> and adding a new typedef is additional cognitive friction.

Sounds good, lets do it that way - adjusted to use macros instead of a struct.

> Speaking of signedness, why is the array of ints sometimes signed and
> sometimes unsigned?

The signedness is a MSVC-ism - I think its reasonable for us to work
with unsigned integers in our code, and pass them by casting to
__cpuid/__cpuidex (the MSVC variants).

> + * Returns false if the CPUID leaf/subleaf is not supported.
>
> Okay, thanks. I'd suggest using "true if X is supported" or "true if X
> is supported, false otherwise" phrasing.

Yup, agreed, that reads better, adjusted.

See attached v13 with your feedback addressed in 0001 and 0005,
otherwise the same as before.

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