Re: Reduce timing overhead of EXPLAIN ANALYZE using rdtsc?

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Andres Freund <andres(at)anarazel(dot)de>
Cc: Robert Haas <robertmhaas(at)gmail(dot)com>, David Geier <geidav(dot)pg(at)gmail(dot)com>, vignesh C <vignesh21(at)gmail(dot)com>, Lukas Fittl <lukas(at)fittl(dot)com>, Michael Paquier <michael(at)paquier(dot)xyz>, Ibrar Ahmed <ibrar(dot)ahmad(at)gmail(dot)com>, Maciek Sakrejda <m(dot)sakrejda(at)gmail(dot)com>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Reduce timing overhead of EXPLAIN ANALYZE using rdtsc?
Date: 2023-01-17 17:26:57
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Andres Freund <andres(at)anarazel(dot)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.

> 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

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