Hi,
On 2024-11-27 16:05:47 -0500, Robert Haas wrote:
> On Wed, Nov 27, 2024 at 3:48 PM Dmitry Dolgov <9erthalion6@gmail.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.
Strictly speaking we don't actually need to map shared buffers to the same
location in each process... We do need that for most other uses of shared
memory, including the buffer mapping table, but not for the buffer data
itself.
Whether it's worth the complexity of dealing with differing locations is
another matter.
Greetings,
Andres Freund
