Re: Sub-millisecond [autovacuum_]vacuum_cost_delay broken

On Thu, Mar 9, 2023 at 5:27 PM Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>
> Thomas Munro <thomas(dot)munro(at)gmail(dot)com> writes:
> > On Fri, Mar 10, 2023 at 11:02 AM Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> >> The caf626b2c code would only work well on platforms that have
> >> microsecond-based sleep primitives, so it was already not too portable.
>
> > Also, the previous coding was already b0rked, because pg_usleep()
> > rounds up to milliseconds on Windows (with a surprising formula for
> > rounding), and also the whole concept seems to assume things about
> > schedulers that aren't really universally true. If we actually cared
> > about high res times maybe we should be using nanosleep and tracking
> > the drift? And spreading it out a bit. But I don't know.
>
> Yeah, I was wondering about trying to make it a closed-loop control,
> but I think that'd be huge overkill considering what the mechanism is
> trying to accomplish.
>
> A minimalistic fix could be as attached. I'm not sure if it's worth
> making the state variable global so that it can be reset to zero in
> the places where we zero out VacuumCostBalance etc. Also note that
> this is ignoring the VacuumSharedCostBalance stuff, so you'd possibly
> have the extra delay accumulating in unexpected places when there are
> multiple workers. But I really doubt it's worth worrying about that.

What if someone resets the delay guc and there is still a large residual?