Horton

Re: Reduce timing overhead of EXPLAIN ANALYZE using rdtsc?

Tom Lane <tgl@sss.pgh.pa.us>

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From: Tom Lane <tgl@sss.pgh.pa.us>
To: Andres Freund <andres@anarazel.de>
Cc: Robert Haas <robertmhaas@gmail.com>, David Geier <geidav.pg@gmail.com>, 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>, pgsql-hackers <pgsql-hackers@postgresql.org>
Date: 2023-01-17T17:26:57Z
Lists: pgsql-hackers

Commits

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  1. pg_test_timing: Also test RDTSC[P] timing, report time source, TSC frequency

  2. Allow retrieving x86 TSC frequency/flags from CPUID

  3. instrumentation: Standardize ticks to nanosecond conversion method

  4. instrumentation: Use Time-Stamp Counter on x86-64 to lower overhead

  5. Zero initialize uses of instr_time about to trigger compiler warnings

  6. instr_time: Represent time as an int64 on all platforms

  7. Add 250c8ee07ed to git-blame-ignore-revs

Andres Freund <andres@anarazel.de> writes:
> Here's an updated version of the move to representing instr_time as
> nanoseconds. It's now split into a few patches:

I took a quick look through this.

> 0001) Add INSTR_TIME_SET_ZERO() calls where otherwise 0002 causes gcc to
>       warn
>       Alternatively we can decide to deprecate INSTR_TIME_SET_ZERO() and
>       just allow to assign 0.

I think it's probably wise to keep the macro.  If we ever rethink this
again, we'll be glad we kept it.  Similarly, IS_ZERO is a good idea
even if it would work with just compare-to-zero.  I'm almost tempted
to suggest you define instr_time as a struct with a uint64 field,
just to help keep us honest about that.

> 0003) Add INSTR_TIME_SET_SECOND()
>       This is used in 0004. Just allows setting an instr_time to a time in
>       seconds, allowing for a cheaper loop exit condition in 0004.

Code and comments are inconsistent about whether it's SET_SECOND or
SET_SECONDS.  I think I prefer the latter, but don't care that much.

> 0004) report nanoseconds in pg_test_timing

Didn't examine 0004 in any detail, but the others look good to go
other than these nits.

			regards, tom lane