Re: Reduce timing overhead of EXPLAIN ANALYZE using rdtsc?

Hi,

On 2025-09-01 12:36:24 +0200, David Geier wrote:
> > Open questions I have:
> > - Could we rely on checking whether the TSC timesource is invariant (via
> > CPUID), instead of relying on Linux choosing it as a clocksource?
>
> Why do you want to do that? Are you concerned that Linux might pick a
> different clock source even though invariant TSC is available?

Not sure about Lukas, but I'm slightly concerned about making this a linux
specific mechanism unnecessarily.

> We could code our own check but looking at the Linux kernel code, this
> is a bit more involved if we want to do it completely right. They check
> e.g. if the TSC is also synchronized across different CPUs, which is not
> the case if they're on different chassis (see unsynchronized_tsc() ->
> apic_is_clustered_box()).

I think Linux has higher fidelity requirements than our instrumentation usage
- with linux an inaccurate clock would lead to broken timers, wrong wall clock
etc, whereas for us it's just a skewed instrumentation result.

Greetings,

Andres Freund