Re: Sub-millisecond [autovacuum_]vacuum_cost_delay broken

On Thu, Mar 9, 2023 at 5:10 PM Thomas Munro <thomas(dot)munro(at)gmail(dot)com> wrote:
>
> 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.

Well, that is reset to zero in vacuum() at the top -- which is called for
each table for autovacuum, so it would get reset to zero between
autovacuuming tables. I dunno how you feel about that...

- Melanie