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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: Zsolt Parragi <zsolt.parragi@percona.com>
Cc: Andres Freund <andres@anarazel.de>, John Naylor <johncnaylorls@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>, David Geier <geidav.pg@gmail.com>
Date: 2026-04-07T08:13:17Z
Lists: pgsql-hackers 
On Tue, Apr 7, 2026 at 12:32 AM Zsolt Parragi <zsolt.parragi@percona.com> wrote:
>
> > Its intentionally uint64, per this comment above it:
> >
> > * Note we utilize unsigned integers even though ticks are stored as a signed
> > * value to encourage compilers to generate better assembly, since we can be
> > * sure these values are not negative.
> >
> > In my earlier Compiler Explorer tests that did actually make a
> > difference for the generated assembly.
>
> Isn't that comment more about ticks_per_ns_scaled?
>
> For max_ticks_no_overflow the only use is with a cast to int64, so I
> didn't expect much assembly difference. Now I actually checked
> locally/godbolt, and I don't see any actual differences. Making
> max_ticks_no_overflow int64 and removing that cast generates exactly
> the same code.
>
> For ticks_per_ns_scaled, gcc 9-10 actually generates +1 mov
> instruction with int64, but that's not present in more recent
> versions.
>
> Recent compiler versions only have an idiv/div and shr/sar difference.
> Idiv is slower than div on intel, so that is a point for keeping
> ticks_per_ns_scaled unsigned.
>
> For arm I see the same lsr/asr and udiv/sdiv difference.
>
> https://godbolt.org/z/4r5GTbrs3
>
> (the main gcc vs clang difference seems to be clang's 32 bit division
> optimization)

Thanks for re-checking, and I think you're correct in your assessment
that max_ticks_no_overflow could be signed. But I also don't think it
does any harm for it to be unsigned, since we know it will never be
negative, and we're correctly using PG_INT64_MAX when initializing it
(i.e. we use the max that's valid for ticks, which is int64).

I don't feel strongly about this. I'll let Andres make the call
whether its worth changing.

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