Re: Changing shared_buffers without restart

Matthias van de Meent <boekewurm+postgres(at)gmail(dot)com> writes:
> I mean, we can do the following to get a nice contiguous empty address
> space no other mmap(NULL)s will get put into:

> /* reserve size bytes of memory */
> base = mmap(NULL, size, PROT_NONE, ...flags, ...);
> /* use the first small_size bytes of that reservation */
> allocated_in_reserved = mmap(base, small_size, PROT_READ |
> PROT_WRITE, MAP_FIXED, ...);

> With the PROT_NONE protection option the OS doesn't actually allocate
> any backing memory, but guarantees no other mmap(NULL, ...) will get
> placed in that area such that it overlaps with that allocation until
> the area is munmap-ed, thus allowing us to reserve a chunk of address
> space without actually using (much) memory.

Well, that's all great if it works portably. But I don't see one word
in either POSIX or the Linux mmap(2) man page that promises those
semantics for PROT_NONE. I also wonder how well a giant chunk of
"unbacked" address space will interoperate with the OOM killer,
top(1)'s display of used memory, and other things that have caused us
headaches with large shared-memory arenas.

Maybe those issues are all in the past and this'll work great.
I'm not holding my breath though.

regards, tom lane