Re: Reduce timing overhead of EXPLAIN ANALYZE using rdtsc?

Hi,

On 2026-02-02 10:22:37 +0100, Jakub Wartak wrote:
> When trying to understand this code I was thinking how it could be
> made smaller or less dependent on such low-level intrinsics, the only
> thing that came to my mind was launching systemd-detect-virt(1) via
> fork+execve, as after all we do have USE_SYSTEMD (for sd_notify(2) already
> consumed in backend/postmaster/postmaster.c) anyway.
>
> Sadly this path for checking VM-types seems like opening can of worms
> - they evolved lots of code to cover various other products,
> see e.g. in detect_vm() and that thing is not exported.
>
> Another way would be probably inquiring their D-Bus API, something like
> below command seems to work:
> busctl get-property org.freedesktop.systemd1
> /org/freedesktop/systemd1 org.freedesktop.systemd1.Manager
> Virtualization
>
> (that seems to be sd_bus_get_property_string(3)).
>
> It's not that I'm recommending usage of any of those (which is linked
> to us most of the time?) or fan of D-Bus (I'm not). I've just thought
> it might be less code to use it for autodetection of VM type, but
> apparently not (?) See their detect_vm_cpuid() with that vm_table[]
> and memcmp() seems to be a more elegant way of writing this.

Yea, I doubt this is the right path... Particularly because I really think we
ought to eventually use this not just on linux but other OSs as well.

> BTW, -1 to fast_clock_source, +1 to clock_source or maybe
> explain_clock_source(?)

Hm. I think it'd make sense to use eventually use this clock source for other
timing tasks too, not just explain. E.g. pg_stat_statements could benefit
quite a bit from reducing the timing overhead.

Whereas it'll not make sense for anything that needs wall clock times - which
imo makes a "clock_source" GUC misnamed. Maybe "clock_source_timing" or such?

Greetings,

Andres Freund