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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: Andres Freund <andres@anarazel.de>
Cc: Lukas Fittl <lukas@fittl.com>, 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: 2026-04-02T02:17:49Z
Lists: pgsql-hackers 
On Thu, Apr 2, 2026 at 7:54 AM Andres Freund <andres@anarazel.de> wrote:
>
> On 2026-03-25 18:17:44 -0700, Lukas Fittl wrote:
> > From 7cc7e230e05947892b2aa479eba45144beede48e Mon Sep 17 00:00:00 2001
> > From: Lukas Fittl <lukas@fittl.com>
> > Date: Sat, 31 Jan 2026 08:49:46 -0800
> > Subject: [PATCH v14 1/6] Check for HAVE__CPUIDEX and HAVE__GET_CPUID_COUNT
> >  separately
>
> John, do you want to take this one, or should I take it?

I can. I took another look just now:

-# Check for __get_cpuid_count() and __cpuidex() in a similar fashion.
+# Check for __get_cpuid_count() and __cpuidex() separately, since we sometimes
+# need __cpuidex() even if __get_cpuid_count() is available.

I'm not sure we need to document here that we need them separately. It
seems more important to do that for the __cpuidex helper that's in a
later patch. I don't feel strongly, though.

I'd probably just do

# Check for __get_cpuid_count()
...

# Check for __cpuidex()
...

Either way, we do need a space between separate tests (ditto Meson).

+if cc.links('''
+    #ifdef _MSC_VER
     #include <intrin.h>
+    #else
+    #include <cpuid.h>
+    #endif
     int main(int arg, char **argv)
     {
-        unsigned int exx[4] = {0, 0, 0, 0};
+        int exx[4] = {0, 0, 0, 0};
         __cpuidex(exx, 7, 0);
     }

Hmm, maybe we can declare unsigned and cast to signed for MSVC as we
do where the code is used.

+# __cpuidex()
+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.

-#if defined(HAVE__GET_CPUID) || defined(HAVE__GET_CPUID_COUNT)
-#include <cpuid.h>
-#endif
-
-#if defined(HAVE__CPUID) || defined(HAVE__CPUIDEX)
+#if defined(HAVE__CPUID) || defined(HAVE__GET_CPUID) ||
defined(HAVE__GET_CPUID_COUNT) || defined(HAVE__CPUIDEX)
+#if defined(_MSC_VER)
 #include <intrin.h>
+#else
+#include <cpuid.h>
+#endif /* defined(_MSC_VER) */
 #endif

Now I'm thinking at least one of these will be defined, otherwise we'd
have an #error, so we can just gate solely on _MSC_VER.

-- 
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