Re: Parallel Seq Scan vs kernel read ahead

On Fri, Jun 26, 2020 at 3:33 AM Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> On Tue, Jun 23, 2020 at 11:53 PM David Rowley <dgrowleyml(at)gmail(dot)com> wrote:
> > In summary, based on these tests, I don't think we're making anything
> > worse in regards to synchronize_seqscans if we cap the maximum number
> > of blocks to allocate to each worker at once to 8192. Perhaps there's
> > some argument for using something smaller than that for servers with
> > very little RAM, but I don't personally think so as it still depends
> > on the table size and It's hard to imagine tables in the hundreds of
> > GBs on servers that struggle with chunk allocations of 16MB. The
> > table needs to be at least ~70GB to get a 8192 chunk size with the
> > current v2 patch settings.
>
> Nice research. That makes me happy. I had a feeling the maximum useful
> chunk size ought to be more in this range than the larger values we
> were discussing before, but I didn't even think about the effect on
> synchronized scans.

+1. This seems about right to me. We can always reopen the
discussion if someone shows up with evidence in favour of a tweak to
the formula, but this seems to address the basic problem pretty well,
and also fits nicely with future plans for AIO and DIO.