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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>, David Geier <geidav.pg@gmail.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: 2026-02-25T11:15:49Z
Lists: pgsql-hackers

Attachments

On Tue, Feb 24, 2026 at 2:22 PM Lukas Fittl <lukas@fittl.com> wrote:
>
> On Mon, Feb 23, 2026 at 6:02 PM John Naylor <johncnaylorls@gmail.com> wrote:
> >
> > 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.
>
> +1, would be good to have a consistent definition for this, I hadn't
> realized this differs between platforms. John, do you want to take
> care of adding that since you recently added USE_SSE2?

In the attached I tried like this:

/*
 * compiler-independent macros for CPU architecture
 */
#if defined(__x86_64__) || defined(_M_X64)
#define PG_ARCH_X64
#elif defined(__i386__) || defined(_M_IX86)
#define PG_ARCH_X32
#elif defined(__aarch64__) || defined(_M_ARM64)
#define PG_ARCH_ARM64
#elif defined(__arm__) || defined(__arm) || defined(_M_ARM)
#define PG_ARCH_ARM32
#endif

and adjusted a couple places to suit. The Arm ones aren't used yet so
I could leave them out for now.

> > 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.
>
> Yeah, that makes sense, agreed it'd be nice to centralize the CPU
> architecture specific code that utilizes cpuid/etc.
>
> Looking at your v5/0002 over there, that should work well. As you
> note, is_rdtscp_available is an easy delegation to your logic - I
> think we can probably always fetch the 0x80000001 leaf to check for
> RDTSCP presence in the proposed set_x86_features?

I think that would work. Your pg_cpuid() looks like the abstraction we
need. I think that would also allow my v5/0002 to avoid worrying about
ordering dependencies. (It resets to check leaf 7, but some AVX
features that we don't use need leaf 1)

--
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