Re: Use CLOCK_MONOTONIC_COARSE for instr_time when available

Hi,

On 2025-03-26 23:09:42 -0400, Tom Lane wrote:
> =?UTF-8?B?5p2o5rGf5Y2O?= <yjhjstz(at)gmail(dot)com> writes:
> > This patch modifies the instr_time.h header to prefer CLOCK_MONOTONIC_COARSE
> > when available.
>
> As far as I know, our usage of instr_time really needs the highest
> resolution available, because we are usually trying to measure pretty
> short intervals. You say that this patch reduces execution time,
> and I imagine that's true ... but I wonder if it doesn't do so at
> the cost of totally destroying the reliability of the output numbers.

The reason, on x86, the timestamp querying has a somewhat high overhead is
that the "accurate" "read the tsc" instruction serves as a barrier for
out-of-order execution. With modern highly out-of-order execution that means
we'll wait for all scheduled instructions to finish before determining the
current time, multiple times for each tuple. That of course slows things down
substantially.

There's a patch to use the version of rdtsc that does *not* have barrier
semantics:
https://postgr.es/m/CAP53PkzO2KpscD-tgFW_V-4WS%2BvkniH4-B00eM-e0bsBF-xUxg%40mail.gmail.com

Greetings,

Andres Freund