Re: Parallel heap vacuum

On Tue, Feb 18, 2025 at 4:43 PM Melanie Plageman
<melanieplageman(at)gmail(dot)com> wrote:
>
> On Mon, Feb 17, 2025 at 1:11 PM Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
> >
> > On Fri, Feb 14, 2025 at 2:21 PM Melanie Plageman
> > <melanieplageman(at)gmail(dot)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.

I've attached the updated patches. In this version, I focused on
parallelizing only the second pass over the heap. It's more
straightforward than supporting the first pass, it still requires many
preliminary changes though.

Regards,

--
Masahiko Sawada
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