Re: PGC_SIGHUP shared_buffers?

On Fri, 16 Feb 2024 at 21:24, Heikki Linnakangas <hlinnaka(at)iki(dot)fi> wrote:
>
> On 16/02/2024 06:28, Robert Haas wrote:
> > 3. Reserve lots of address space and then only use some of it. I hear
> > rumors that some forks of PG have implemented something like this. The
> > idea is that you convince the OS to give you a whole bunch of address
> > space, but you try to avoid having all of it be backed by physical
> > memory. If you later want to increase shared_buffers, you then get the
> > OS to back more of it by physical memory, and if you later want to
> > decrease shared_buffers, you hopefully have some way of giving the OS
> > the memory back. As compared with the previous two approaches, this
> > seems less likely to be noticeable to most PG code. Problems include
> > (1) you have to somehow figure out how much address space to reserve,
> > and that forms an upper bound on how big shared_buffers can grow at
> > runtime and (2) you have to figure out ways to reserve address space
> > and back more or less of it with physical memory that will work on all
> > of the platforms that we currently support or might want to support in
> > the future.
>
> A variant of this approach:
>
> 5. Re-map the shared_buffers when needed.
>
> Between transactions, a backend should not hold any buffer pins. When
> there are no pins, you can munmap() the shared_buffers and mmap() it at
> a different address.

This can quite realistically fail to find an unused memory region of
sufficient size when the heap is sufficiently fragmented, e.g. through
ASLR, which would make it difficult to use this dynamic
single-allocation shared_buffers in security-hardened environments.

Kind regards,

Matthias van de Meent
Neon (https://neon.tech)