Re: Changing shared_buffers without restart

On Wed, Nov 27, 2024 at 3:48 PM Dmitry Dolgov <9erthalion6(at)gmail(dot)com> wrote:
> My understanding is that clashing of mappings (either at creation time
> or when resizing) could happen only withing the process address space,
> and the assumption is that by the time we prepare the mapping layout all
> the rest of mappings for the process are already done.

I don't think that's correct at all. First, the user could type LOAD
'whatever' at any time. But second, even if they don't or you prohibit
them from doing so, the process could allocate memory for any of a
million different things, and that could require mapping a new region
of memory, and the OS could choose to place that just after an
existing mapping, or at least close enough that we can't expand the
object size as much as desired.

If we had an upper bound on the size of shared_buffers and could
reserve that amount of address space at startup time but only actually
map a portion of it, then we could later remap and expand into the
reserved space. Without that, I think there's absolutely no guarantee
that the amount of address space that we need is available when we
want to extend a mapping.

--
Robert Haas
EDB: http://www.enterprisedb.com