Re: index prefetching

On Wed, Jul 16, 2025 at 9:58 AM Tomas Vondra <tomas(at)vondra(dot)me> wrote:
> > The "simple" patch has _bt_readpage reset the read stream. That
> > doesn't make any sense to me. Though it does explain why the "complex"
> > patch does so many more fadvise calls.
> >
>
> Why it doesn't make sense? The reset_stream_reset() restarts the stream
> after it got "terminated" on the preceding leaf page (by returning
> InvalidBlockNumber).

Resetting the prefetch distance at the end of _bt_readpage doesn't
make any sense to me. Why there? It makes about as much sense as doing
so every 7th index tuple. Reaching the end of _bt_readpage isn't
meaningful -- since it in no way signifies that the scan has been
terminated (it might have been, but you're not checking that at all).

> It'd be better to "pause" the stream somehow, but
> there's nothing like that yet. We have to terminate it and start again.

I don't follow.

> Te pattern of fadvise+pread for the same block seems a bit silly. And
> this is not just about "sync" method, the other methods will have a
> similar issue with no starting the I/O earlier. The fadvise is just
> easier to trace/inspect.

It's not at all surprising that you're seeing duplicate prefetch
requests. I have no reason to believe that it's important to suppress
those ourselves, rather than leaving it up to the OS (though I also
have no reason to believe that the opposite is true).

--
Peter Geoghegan