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Re: Reduce timing overhead of EXPLAIN ANALYZE using rdtsc?

John Naylor <johncnaylorls@gmail.com>

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From: John Naylor <johncnaylorls@gmail.com>
To: Lukas Fittl <lukas@fittl.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: 2026-04-02T12:55:31Z
Lists: pgsql-hackers 
On Thu, Apr 2, 2026 at 3:50 PM Lukas Fittl <lukas@fittl.com> wrote:
>
> On Wed, Apr 1, 2026 at 7:18 PM John Naylor <johncnaylorls@gmail.com> wrote:

> > +AC_CACHE_CHECK([for __cpuidex], [pgac_cv__cpuidex],
> > +[AC_LINK_IFELSE([AC_LANG_PROGRAM([#ifdef _MSC_VER
> > +    #include <intrin.h>
> > +    #else
> > +    #include <cpuid.h>
> > +    #endif],
> > +  [[int exx[4] = {0, 0, 0, 0};
> > +  __cpuidex(exx, 7, 0);
> > +  ]])],
> > +  [pgac_cv__cpuidex="yes"],
> > +  [pgac_cv__cpuidex="no"])])
> > +if test x"$pgac_cv__cpuidex" = x"yes"; then
> > +  AC_DEFINE(HAVE__CPUIDEX, 1, [Define to 1 if you have __cpuidex.])
> >  fi
> >
> > MSVC doesn't use autoconf, so we can leave out the intrin.h and keep
> > using unsigned int.
>
> I think that might be a problem for Clang on Windows, which has MSVC
> compatibility headers, and will set _MSC_VER, see [0] and [1].
>
> It probably works in practice, but it seems better to match the
> autoconf checks to the actual code?

Hmm, the previous coding had <intrin.h>, and this is just adding
<cpuid.h> as well, so I left it alone.

The only change I made was to make the #include stanza in pg_cpu_x86.c
match its neighbors.

I thought to add a note to the commit message about the casts to
signed int, but only after pushing did I notice that the explanation
looks out of place since no new code uses __cpuidex yet, and warnings
don't matter for configure checks. Hope it's not too confusing.

--
John Naylor
Amazon Web Services

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