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From: Magnus Hagander <magnus@hagander.net>
Date: Wed, 9 Jun 2021 21:30:52 +0200
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Subject: Re: Estimating HugePages Requirements?
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Julien Rouhaud <rjuju123@gmail.com>, Don Seiler <don@seiler.us>,
	pgsql-admin <pgsql-admin@postgresql.org>
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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/