Re: [RFC] Should "SHOW huge_pages" display the effective value "off" when the huge page is unavailable?

Fujii Masao <masao(dot)fujii(at)gmail(dot)com> writes:
> On Mon, Feb 6, 2017 at 4:01 PM, Tsunakawa, Takayuki
> <tsunakawa(dot)takay(at)jp(dot)fujitsu(dot)com> wrote:
>> I don't have a strong opinion on that, but I think a bit that it would be better to reflect the effective setting, i.e. SHOW displays huge_pages as off, not try.

> Not sure if this is best way to do that, but I agree that it's helpful if
> we can see whether the server actually uses huge page or not in
> huge_page=try case.

If the proposal is to actually change the stored value of huge_pages,
I would say "absolutely not". Suppose that you change "try" to "on",
and there's a backend crash and restart so that the postmaster needs
to reallocate shared memory, and this time it's unable to obtain
huge pages for some reason. Taking the database down would be entirely
the wrong thing. Also, how would you handle postgresql.conf reload
situations?

If the proposal is to have SHOW report something other than the setting
of the variable, that's not a great plan either. It's generally important
that the output of SHOW be something that's acceptable to SET, as not
having that equivalence will break assorted client-side code.

I think this desire would be better addressed by some kind of specialized
inquiry function, which would also be able to return more information than
just a naked "on/off" bit. People might for instance wish to know what
hugepage size is in use.

regards, tom lane