Re: Reduce timing overhead of EXPLAIN ANALYZE using rdtsc?

On Fri, Apr 3, 2026 at 2:06 AM John Naylor <johncnaylorls(at)gmail(dot)com> wrote:
>
> On Fri, Apr 3, 2026 at 6:16 AM Lukas Fittl <lukas(at)fittl(dot)com> wrote:
> > v16
>
> Just some minor quibbles on 0002:

Thanks for the review!

>
> --- a/src/include/port/pg_cpu.h
> +++ b/src/include/port/pg_cpu.h
> @@ -23,6 +23,12 @@ typedef enum X86FeatureId
> /* scalar registers and 128-bit XMM registers */
> PG_SSE4_2,
> PG_POPCNT,
> + PG_HYPERVISOR,
>
> The hypervisor flag doesn't really belong with an instruction family.
> Maybe a separate category like "identification"?

Yeah, I've pondered different names here, but "identification" seems
good for now - I agree it doesn't belong with the earlier flags.

>
> + /* TSC flags */
> + PG_RDTSCP,
> + PG_TSC_INVARIANT,
> + PG_TSC_ADJUST,
>
> Maybe spell out time stamp counter in the comment, since this will be
> the first time the reader encounters that in this file.

Makes sense, done.

FWIW, TSC can be spelled out either as "Time Stamp Counter" or
"Time-Stamp Counter". I've gone with the latter for now since that's
what was done elsewhere, and is how the Intel manuals reference it as
well.

>
> + * For some other Hypervisors that have an invariant TSC, e.g. HyperV, we would
> + * need to access an MSR to get the frequency (which is typically not available
>
> Maybe spell out MSR too, because I for one don't know what that is.

Done. I've also removed the note re: TSC calibration in that same
comment, since that's in a later commit and I don't think its
necessary to talk about it in that comment anyway.

>
> + X86Features[PG_HYPERVISOR] = reg[ECX] >> 31 & 1;
> + have_osxsave = reg[ECX] & (1 << 27);
> +
> + pg_cpuid_subleaf(0x07, 0, reg);
> +
> + X86Features[PG_TSC_ADJUST] = (reg[EBX] & (1 << 1)) != 0;
>
> Some inconsistency in shift style.

Fixed.

See attached v17.

Thanks,
Lukas

--
Lukas Fittl