I agree, its confusing for many and that confusion arises from the fact that you usually talk of shared_buffers in MB or GB whereas hugepages have to be configured in units of 2mb. But once they understand they realize its pretty simple.
Don, we have experienced the same not just with postgres but also with oracle. I havent been able to get to the root of it, but what we usually do is, we add another 100-200 pages and that works for us. If the SGA or shared_buffers is high eg 96gb, then we add 250-500 pages. Those few hundred MBs may be wasted (because the moment you configure hugepages, the operating system considers it as used and does not use it any more) but nowadays, servers have 64 or 128 gb RAM easily and wasting that 500mb to 1gb does not hurt really.
HTH
On Wed, Jun 9, 2021 at 9:28 PM Tom Lane <tgl@sss.pgh.pa.us> wrote:
>
> Magnus Hagander <magnus@hagander.net> writes:
> > On Wed, Jun 9, 2021 at 9:15 PM Tom Lane <tgl@sss.pgh.pa.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/