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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: David Geier <geidav.pg@gmail.com>, 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>
Date: 2026-02-24T02:01:51Z
Lists: pgsql-hackers 
On Tue, Feb 24, 2026 at 5:28 AM Andres Freund <andres@anarazel.de> wrote:
> On 2026-02-23 16:24:57 +0100, David Geier wrote:
> > The code wasn't compiling properly on Windows because __x86_64__ is not
> > defined in Visual C++. I've changed the code to use
> >
> >   #if defined(__x86_64__) || defined(_M_X64)
>
> Independently of this patchset I wonder if it'd be worth introducing a
> PG_ARCH_X64 or such, to avoid this kind of thing.

+1

I've already borrowed USE_SSE2 for this meaning in commit b9278871f,
but that's conflating two different things and I'd actually prefer the
above, plus one that includes 32-bit as well.

+static bool
+is_rdtscp_available()
+{
+ uint32 r[4] = {0, 0, 0, 0};
+
+#if defined(HAVE__GET_CPUID)
+ if (!__get_cpuid(0x80000001, &r[0], &r[1], &r[2], &r[3]))
+ return false;
+#elif defined(HAVE__CPUID)
+ __cpuid(r, 0x80000001);
+#else
+#error cpuid instruction not available
+#endif
+
+ return (r[3] & (1 << 27)) != 0;
+}

I'm hoping to centralize CPU-specific checks like the above:

https://www.postgresql.org/message-id/CANWCAZbKQ2im1r4ztcyfqNh_6gaJzyewabExYrkACNvMNyqxog%40mail.gmail.com

is_rdtscp_available() is an easy thing to delegate to my patch, but I
agree it would be easier if that was abstracted a bit more so that a
different leaf can be passed each time. The latter could also be used
to simplify the frequency and hypervisor stuff as well.

--
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