Re: Sub-millisecond [autovacuum_]vacuum_cost_delay broken

On Fri, Mar 10, 2023 at 11:02 AM 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 10:26 AM Melanie Plageman
> > <melanieplageman(at)gmail(dot)com> wrote:
> >> I think that 4753ef37e0ed undid the work caf626b2c did to support
> >> sub-millisecond delays for vacuum and autovacuum.
>
> > Given that some of the clunkier underlying kernel primitives have
> > milliseconds in their interface, I don't think it would be possible to
> > make a usec-based variant of WaitEventSetWait() that works everywhere.
> > Could it possibly make sense to do something that accumulates the
> > error, so if you're using 0.5 then every second vacuum_delay_point()
> > waits for 1ms?
>
> Yeah ... using float math there was cute, but it'd only get us so far.
> 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.

> Can we fix this by making VacuumCostBalance carry the extra fractional
> delay, or would a separate variable be better?

I was wondering the same thing, but not being too familiar with that
code, no opinion on that yet.