Re: Estimating HugePages Requirements?

On Wed, Jun 9, 2021 at 9:28 PM Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>
> Magnus Hagander <magnus(at)hagander(dot)net> writes:
> > On Wed, Jun 9, 2021 at 9:15 PM Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> >> Just try to start the server and see if it complains.
>
> > Well, I have to *stop* the existing one first, most likely, otherwise
> > there won't be enough huge pages (or indeed memory) available.
>
> I'm not following. If you have a production server running, its
> pg_shmem_allocations total should already be a pretty good guide
> to what you need to configure HugePages for. You need to know to
> round that up, of course --- but if you aren't building a lot of
> slop into the HugePages configuration anyway, you'll get burned
> down the road.

I'm talking about the case when you want to *change* the value for
shared_buffers (or other parameters that would change the amount of
required huge pages), on a system where you're using huge pages.
pg_shmem_allocations will tell you what you need with the current
value, not what you need with the new value.

But yes, you can do some math around it and make a well educated
guess. But it would be very convenient to have the system able to do
that for you.

--
Magnus Hagander
Me: https://www.hagander.net/
Work: https://www.redpill-linpro.com/