Re: Streaming I/O, vectored I/O (WIP)

On Thu, Mar 28, 2024 at 2:02 PM Thomas Munro <thomas.munro@gmail.com> wrote:
> ... In practice on a non-toy system, that's always going to be
> io_combine_limit.  ...

And to be more explicit about that: you're right that we initialise
max_pinned_buffers such that it's usually at least io_combine_limit,
but then if you have a very small buffer pool it gets clobbered back
down again by LimitAdditionalBins() and may finish up as low as 1.
You're not allowed to pin more than 1/Nth of the whole buffer pool,
where N is approximately max connections (well it's not exactly that
but that's the general idea).  So it's a degenerate case, but it can
happen that max_pinned_buffers is lower than io_combine_limit and then
it's important not to set distance higher or you'd exceed the allowed
limits (or more likely the circular data structure would implode).