Re: AIO v2.5

Hi,

On 2025-03-14 15:43:15 -0400, Andres Freund wrote:
> Open items:
>
> - The upstream BAS_BULKREAD is so small that throughput is substantially worse
> once a table reaches 1/4 shared_buffers. That patch in the patchset as-is is
> probably not good enough, although I am not sure about that
>
>
> - The set_max_safe_fds() issue for io_uring
>
>
> - Right now effective_io_concurrency cannot be set > 0 on Windows and other
> platforms that lack posix_fadvise. But with AIO we can read ahead without
> posix_fadvise().
>
> It'd not really make anything worse than today to not remove the limit, but
> it'd be pretty weird to prevent windows etc from benefiting from AIO. Need
> to look around and see whether it would require anything other than doc
> changes.

A fourth, smaller, question:

- Should the docs for debug_io_direct be rephrased and if so, how?

Without read-stream-AIO debug_io_direct=data has completely unusable
performance if there's ever any data IO - and if there's no IO there's no
point in using the option.

Now there is a certain set of workloads where performance with
debug_io_direct=data can be better than master, sometimes substantially
so. But at the same time, without support for at least:

- AIO writes for at least checkpointer, bgwriter

doing one synchronous IO for each buffer is ... slow.

- read-streamified index vacuuming

And probably also:
- AIO-ified writes for writes executed by backends, e.g. due to strategies

Doing one synchronous IO for each buffer is ... slow. And e.g. with COPY
we do a *lot* of those. OTOH, it could be fine if most modifications are
done via INSERTs instead of COPY.

- prefetching for non-BHS index accesses

Without prefetching, a well correlated index-range scan will be orders of
magnitude slower with DIO.

- Anything bypassing shared_buffers, like RelationCopyStorage() or
bulk_write.c will be extremely slow

The only saving grace is that these aren't all *that* common.

Due to those constraints I think it's pretty clear we can't remove the debug_
prefix at this time.

Perhaps it's worth going from

<para>
Currently this feature reduces performance, and is intended for
developer testing only.
</para>
to
<para>
Currently this feature reduces performance in many workloads, and is
intended for testing only.
</para>

I.e. qualify the downside with "many workloads" and widen the audience ever so
slightly?

Greetings,

Andres Freund