On Fri, Aug 15, 2025 at 1:23 PM Andres Freund <andres@anarazel.de> wrote:
> Somewhat random note about I/O waits:
>
> Unfortunately the I/O wait time we measure often massively *over* estimate the
> actual I/O time. If I execute the above query with the patch applied, we
> actually barely ever wait for I/O to complete, it's all completed by the time
> we have to wait for the I/O. What we are measuring is the CPU cost of
> *initiating* the I/O.
I do get that.
This was really obvious when I temporarily switched the prefetch patch
over from using READ_STREAM_DEFAULT to using READ_STREAM_USE_BATCHING
(this is probably buggy, but still seems likely to be representative
of what's possible with some care). I noticed that that change reduced
the reported "shared read" time by 10x -- which had exactly zero impact on
query execution time (at least for the queries I looked at). Since, as
you say, the backend didn't have to wait for I/O to complete either
way.
--
Peter Geoghegan
