Re: Changing shared_buffers without restart

Matthias van de Meent <boekewurm+postgres@gmail.com> writes:
> On Thu, 28 Nov 2024 at 18:19, Robert Haas <robertmhaas@gmail.com> wrote:
>> [...] It's unclear to me why
>> operating systems don't offer better primitives for this sort of thing
>> -- in theory there could be a system call that sets aside a pool of
>> address space and then other system calls that let you allocate
>> shared/unshared memory within that space or even at specific
>> addresses, but actually such things don't exist.

> Isn't that more a stdlib/malloc issue? AFAIK, Linux's mmap(2) syscall
> allows you to request memory from the OS at arbitrary addresses - it's
> just that stdlib's malloc doens't expose the 'alloc at this address'
> part of that API.

I think what Robert is concerned about is that there is exactly 0
guarantee that that will succeed, because you have no control over
system-driven allocations of address space (for example, loading
of extensions or JIT code).  In fact, given things like ASLR, there
is pressure on the kernel crew to make that *less* predictable not
more so.  So even if we devise a method that seems to work reliably
today, we could have little faith that it would work with next year's
kernels.

            regards, tom lane