Horton

Re: AIO / read stream heuristics adjustments for index prefetching

Melanie Plageman <melanieplageman@gmail.com>

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From: Melanie Plageman <melanieplageman@gmail.com>
To: Andres Freund <andres@anarazel.de>
Cc: Nazir Bilal Yavuz <byavuz81@gmail.com>, pgsql-hackers@postgresql.org, Thomas Munro <thomas.munro@gmail.com>, Peter Geoghegan <pg@bowt.ie>, Tomas Vondra <tv@fuzzy.cz>
Date: 2026-04-03T16:45:50Z
Lists: pgsql-hackers 
On Thu, Apr 2, 2026 at 9:33 AM Andres Freund <andres@anarazel.de> wrote:
>
> > +                /*
> > +                 * XXX: Should we actually reduce this at any time other than
> > +                 * a reset? For now we have to, as this is also a condition
> > +                 * for re-enabling fast_path.
> > +                 */
> > +                if (stream->combine_distance > 1)
> > +                    stream->combine_distance--;
> >
> > I don't think we need to reduce this other than reset.
>
> Hm. I go back and forth on that one :)

Separate from the fast-path enablement, we also probably want to
decrease combine distance when we decrease readahead_distance because
there is a point where we still want to parallelize the IOs even when
the distance is lower and to do that, we need to make smaller IOs. I'm
not sure where this point is, but I wonder if a few 256kB IOs is
faster than 1 1MB IO (could test that with fio actually). I imagine
that there is some size where that is true because of peculiarities in
how drives (and cloud storage) issue/break up IOs after they are a
certain size, etc.

- Melanie

Commits

  1. aio: io_uring: Trigger async processing for large IOs

  2. read stream: Split decision about look ahead for AIO and combining

  3. read_stream: Move logic about IO combining & issuing to helpers

  4. read_stream: Only increase read-ahead distance when waiting for IO

  5. read_stream: Prevent distance from decaying too quickly

  6. read_stream: Issue IO synchronously while in fast path

  7. aio: io_uring: Allow IO methods to check if IO completed in the background

  8. bufmgr: Return whether WaitReadBuffers() needed to wait