Re: BitmapHeapScan streaming read user and prelim refactoring

io_combine_limit 1, effective_io_concurrency 16, read ahead kb 16

On Fri, Feb 14, 2025 at 12:18 PM Tomas Vondra <tomas(at)vondra(dot)me> wrote:
>
> Based on off-list discussion with Melanie, I ran a modified version of
> the benchmark, with these changes:

Thanks! It looks like the worst offender is io_combine_limit 1 (128
kB), effective_io_concurrency 16, read_ahead_kb 16. This is a
combination that people shouldn't run in practice I think -- an
io_combine_limit larger than read ahead.

But we do still see regressions with io_combine_limit 1,
effective_io_concurrency 16 at other read_ahead_kb values. Therefore
I'd be interested to see that subset (ioc 1, eic 16, diff read ahead
values) with the attached patch.

> FWIW this does not change anything in the detection of sequential access
> patterns, discussed nearby, because the benchmarks started before Andres
> looked into that. If needed, I can easily rerun these tests, I just need
> a patch to apply.

Attached a patch that has all the commits squashed + removes
sequential detection.

- Melanie