Re: AIO / read stream heuristics adjustments for index prefetching

On Tue, Mar 31, 2026 at 12:02 PM Andres Freund <andres(at)anarazel(dot)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