Re: Reduce timing overhead of EXPLAIN ANALYZE using rdtsc?

David Geier <geidav.pg@gmail.com>

From: David Geier <geidav.pg@gmail.com>

To: Pavel Stehule <pavel.stehule@gmail.com>, Tomas Vondra <tomas.vondra@enterprisedb.com>

Cc: vignesh C <vignesh21@gmail.com>, Lukas Fittl <lukas@fittl.com>, Michael Paquier <michael@paquier.xyz>, Ibrar Ahmed <ibrar.ahmad@gmail.com>, Maciek Sakrejda <m.sakrejda@gmail.com>, Andres Freund <andres@anarazel.de>, pgsql-hackers <pgsql-hackers@postgresql.org>

Date: 2023-01-18T12:52:05Z

Lists: pgsql-hackers

Commits

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pg_test_timing: Also test RDTSC[P] timing, report time source, TSC frequency
- 16fca4825483 19 (unreleased) landed
Allow retrieving x86 TSC frequency/flags from CPUID
- bcb2cf41f964 19 (unreleased) landed
instrumentation: Standardize ticks to nanosecond conversion method
- 0022622c93d9 19 (unreleased) landed
instrumentation: Use Time-Stamp Counter on x86-64 to lower overhead
- 294520c44487 19 (unreleased) landed
Zero initialize uses of instr_time about to trigger compiler warnings
- 25b2aba0c3a5 16.0 landed
instr_time: Represent time as an int64 on all platforms
- 03023a2664f8 16.0 landed
Add 250c8ee07ed to git-blame-ignore-revs
- ff23b592ad66 16.0 cited

On 1/16/23 21:39, Pavel Stehule wrote:
>
> po 16. 1. 2023 v 21:34 odesílatel Tomas Vondra 
> <tomas.vondra@enterprisedb.com> napsal:
>
>     Hi,
>
>     there's minor bitrot in the Mkvcbuild.pm change, making cfbot unhappy.
>
>     As for the patch, I don't have much comments. I'm wondering if it'd be
>     useful to indicate which timing source was actually used for EXPLAIN
>     ANALYZE, say something like:
>
>      Planning time: 0.197 ms
>      Execution time: 0.225 ms
>      Timing source: clock_gettime (or tsc)
>
>     There has been a proposal to expose this as a GUC (or perhaps as
>     explain
>     option), to allow users to pick what timing source to use. I
>     wouldn't go
>     that far - AFAICS is this is meant to be universally better when
>     available. But knowing which source was used seems useful.
>
>
> +1

Thanks for looking at the patch.

I'll fix the merge conflict.

I like the idea of exposing the timing source in the EXPLAIN ANALYZE output.
It's a good tradeoff between inspectability and effort, given that RDTSC 
should always be better to use.
If there are no objections I go this way.

-- 
David Geier
(ServiceNow)