Re: Parallel heap vacuum

On Tue, Mar 11, 2025 at 5:51 AM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>
> On Tue, Mar 11, 2025 at 5:00 AM Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
> >
> > On Sun, Mar 9, 2025 at 11:28 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> > >
> > >
> > > Does phase 3 also use parallelism? If so, can we try to divide the
> > > ring buffers among workers or at least try vacuum with an increased
> > > number of ring buffers. This would be good to do for both the phases,
> > > if they both use parallelism.
> >
> > No, only phase 1 was parallelized in this test. In parallel vacuum,
> > since it uses (ring_buffer_size * parallel_degree) memory, more pages
> > are loaded during phase 1, increasing cache hits during phase 3.
> >
>
> Shouldn't we ideally try with a vacuum without parallelism with
> ring_buffer_size * parallel_degree to make the comparison better?

Right. I'll share the benchmark test results with such configuration.

> Also, what could be the reason for the variation in data of phase-I?
> Do you restart the system after each run to ensure there is nothing in
> the memory? If not, then shouldn't we try at least a few runs by
> restarting the system before each run to ensure there is nothing
> leftover in memory?

I dropped all page caches by executing 'echo 3 >
/proc/sys/vm/drop_caches' before each run and these results are the
median of 3 runs. I'll investigate it further.

Regards,

--
Masahiko Sawada
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