Re: Parallel heap vacuum

On Mon, Feb 17, 2025 at 1:11 PM Masahiko Sawada <sawada.mshk@gmail.com> wrote:
>
> On Fri, Feb 14, 2025 at 2:21 PM Melanie Plageman
> <melanieplageman@gmail.com> wrote:
> >
> > Since the failure rate is defined as a percent, couldn't we just have
> > parallel workers set eager_scan_remaining_fails when they get their
> > chunk assignment (as a percentage of their chunk size)? (I haven't
> > looked at the code, so maybe this doesn't make sense).
>
> IIUC since the chunk size eventually becomes 1, we cannot simply just
> have parallel workers set the failure rate to its assigned chunk.

Yep. The ranges are too big (1-8192). The behavior would be too
different from serial.

> > Also, if you start with only doing parallelism for the third phase of
> > heap vacuuming (second pass over the heap), this wouldn't be a problem
> > because eager scanning only impacts the first phase.
>
> Right. I'm inclined to support only the second heap pass as the first
> step. If we support parallelism only for the second pass, it cannot
> help speed up freezing the entire table in emergency situations, but
> it would be beneficial for cases where a big table have a large amount
> of spread garbage.
>
> At least, I'm going to reorganize the patch set to support parallelism
> for the second pass first and then the first heap pass.

Makes sense.

- Melanie