Re: index prefetching

Hi,

On 2025-07-16 16:20:25 +0200, Tomas Vondra wrote:
> On 7/16/25 16:07, Peter Geoghegan wrote:
> >> 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).
> > 
> 
> True, but in practice those duplicate calls are fairly expensive. Even
> just calling fadvise() on data you already have in page cache costs
> something (not much, but it's clearly visible for cached queries).

This imo isn't something worth optimizing for - if you use an io_method that
actually can execute IO asynchronously this issue does not exist, as the start
of the IO will already have populated the buffer entry (without BM_VALID set,
of course). Thus we won't start another IO for that block.

Greetings,

Andres Freund