Re: PGC_SIGHUP shared_buffers?

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.

--
Heikki Linnakangas
Neon (https://neon.tech)