Re: Parallel heap vacuum

Hi,

Thanks for the new patches. I've repeated my benchmarking on v8, and I
agree this looks fine - the speedups are reasonable and match what I'd
expect on this hardware. I don't see any suspicious results like with
the earlier patches, where it got much faster thanks to the absence of
SKIP_PAGE_THRESHOLD logic.

Attached is the benchmarking script, CSV with raw results, and then also
two PDF reports comparing visualizing the impact of the patch by
comparing it to current master.

* parallel-vacuum-duration.pdf - Duration of the vacuum, and duration
relative to master (green - faster, read - slower). The patch is clearly
an improvement, with speedup up to ~3x depending on the index count and
a fraction of updated rows.

* parallel-vacuum-reads.pdf - Average read speed, as reported by VACUUM
VERBOSE. With the patch it can reach up to ~3GB/s, which is about the
max value possible on this hardware - so that's nice. I'll try to test
this on a better storage, to see how far it can go.

I haven't done any actual code review on the new patches, I'll try to
find time for that sometime next week.

regards

--
Tomas Vondra