Re: AIO / read stream heuristics adjustments for index prefetching - Mailing list pgsql-hackers

On Tue, Mar 31, 2026 at 12:02 PM Andres Freund <andres@anarazel.de> wrote:
>
> 0008: WIP: read stream: Split decision about look ahead for AIO and combining
>
>     Until now read stream has used a single look-ahead distance to control
>     lookahead for both IO combining and read-ahead. That's sub-optimal, as we
>     want to do IO combining even when we don't need to do any readahead, as
>     avoiding the syscall overhead is important to reduce CPU overhead when
>     data is in the kernel page cache.
>
>     This is a prototype for what it could look like to split those
>     decisions. Thereby fixing the regression mentioned in 0006.

I wonder if we need to keep the combine_limit member in the read
stream. Could we just use io_combine_limit without ramping up and
down? This is mainly for code complexity reasons. Perhaps to allow
fast path reentry, we could use distance_decay_holdoff == 0 and
ios_in_progress == 0 instead of combine_distance == 0.

- Melanie