Re: AIO / read stream heuristics adjustments for index prefetching

Hi,

Forgot to send this one earlier:

On 2026-04-03 11:46:59 -0400, Melanie Plageman wrote:
> On Thu, Apr 2, 2026 at 11:47 AM Andres Freund <andres(at)anarazel(dot)de> wrote:
> >
> > What do you think about the updated patch to achieve that that I posted?
>
> here is some review on 0005 and 0006 earlier posted
> concrete things:
> -------
> - I’d reorder stream→distance == 0 in read_stream_look_ahead() (and
> issue), I found myself asking why it wasn’t first

It'd not be correct in should_issue_now(). We can't move it before

/* there is no pending IO that could be issued */
if (pending_read_nblocks == 0)
return false;

because then we'd trigger a call to read_stream_start_pending_read(), without
there being a pending read.

We can't move it before

/* never start more IOs than our cap */
if (stream->ios_in_progress >= stream->max_ios)
return false;

because that could lead us to exceeding max_ios.

Greetings,

Andres Freund