Hi,
On 2021-06-14 23:20:47 +0200, Jehan-Guillaume de Rorthais wrote:
> > On 2021-06-14 16:10:32 +0200, Jehan-Guillaume de Rorthais wrote:
> > > In the patch in attachment, I tried to fix this by using kind of an internal
> > > hook for pgstat_report_wait_start and pgstat_report_wait_end. This allows to
> > > "instrument" wait events only when required, on the fly, dynamically.
> >
> > That's *far worse*. You're adding an indirect function call. Which requires
> > loading a global variable and then a far call to a different function. You're
> > changing a path that's ~2 instructions with minimal dependencies (and no
> > branches (i.e. fully out of order executable) to something on the order of ~15
> > instructions with plenty dependencies and at least two branches (call, ret).
>
> Oh, I didn't realized it would affect all queries, even when log_statement_stats
> was off. Thank you for your explanation.
Maybe I just am misunderstanding what you were doing? As far as I can
tell your patch changed pgstat_report_wait_start() to be an indirect
function call - right? Then yes, this adds overhead to everything.
You *could* add a pgstat_report_wait_(start|end)_with_time() or such and
only use that in places that won't have a high frequency. But I just
don't quite see the use-case for that.
Greetings,
Andres Freund