Re: Parallel Seq Scan vs kernel read ahead

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.

--
Robert Haas
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