Date: Mon, 9 Aug 2021 20:38:32 -0700
From: Andres Freund <andres@anarazel.de>
To: Justin Pryzby <pryzby@telsasoft.com>
Cc: Don Seiler <don@seiler.us>, P C <puravc@gmail.com>,
	Magnus Hagander <magnus@hagander.net>,
	Julien Rouhaud <rjuju123@gmail.com>, Tom Lane <tgl@sss.pgh.pa.us>,
	pgsql-hackers@postgresql.org
Subject: Re: Estimating HugePages Requirements?
Message-ID: <20210810033832.ueaicvx34ehh7q2p@alap3.anarazel.de>
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Hi,

On 2021-08-09 18:58:53 -0500, Justin Pryzby wrote:
> Define shared_buffers as the exact size to be allocated/requested from the OS
> (regardless of whether they're huge pages or not), and have postgres compute
> everything else based on that.  So shared_buffers=2GB would end up being 1950MB
> (or so) of buffer cache.  We'd have to check that after the other allocations,
> there's still at least 128kB left for the buffer cache.  Maybe we'd have to
> bump the minimum value of shared_buffers.

I don't like that. How much "other" shared memory we're going to need is
very hard to predict and depends on extensions, configuration options
like max_locks_per_transaction, max_connections to a significant
degree. This way the user ends up needing to guess at least as much as
before to get to a sensible shared_buffers.

Greetings,

Andres Freund