Date: Mon, 25 Apr 2022 09:15:28 +0900
From: Michael Paquier <michael@paquier.xyz>
To: Magnus Hagander <magnus@hagander.net>
Cc: Nathan Bossart <nathandbossart@gmail.com>,
	"Bossart, Nathan" <bossartn@amazon.com>,
	Fujii Masao <masao.fujii@oss.nttdata.com>,
	Justin Pryzby <pryzby@telsasoft.com>,
	Andres Freund <andres@anarazel.de>,
	Mark Dilger <mark.dilger@enterprisedb.com>,
	Don Seiler <don@seiler.us>,
	PostgreSQL-development <pgsql-hackers@postgresql.org>
Subject: Re: Estimating HugePages Requirements?
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On Fri, Apr 22, 2022 at 09:49:34AM +0200, Magnus Hagander wrote:
> I agree that thats a very narrow use case. And I'm not sure the use case of
> a running server is even that important here - it's really the offline one
> that's important. Or rather, the really compelling one is when there is a
> server running but I want to check the value offline because it will
> change. SHOW doesn't help there because it shows the value based on the
> currently running configuration, not the new one after a restart.

You mean the case of a server where one would directly change
postgresql.conf on a running server, and use postgres -C to see how
much the kernel settings need to be changed before the restart?

> Hmm. So what's the solution on windows? I guess maybe it's not as important
> there because there is no limit on huge pages, but generally getting the
> expected shared memory usage might be useful? Just significantly less
> important.

Contrary to Linux, we don't need to care about the number of large
pages that are necessary because there is no equivalent of
vm.nr_hugepages on Windows (see [1]), do we?  If that were the case,
we'd have a use case for huge_page_size, additionally.

That's the case where shared_memory_size_in_huge_pages comes in
handy, as much as does huge_page_size, and note that
shared_memory_size works on WIN32.

[1]: https://docs.microsoft.com/en-us/windows/win32/memory/large-page-support
--
Michael

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