Date: Wed, 25 Jun 2025 06:11:21 +0000
From: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
To: Christoph Berg <myon@debian.org>
Cc: Tomas Vondra <tomas@vondra.me>, Andres Freund <andres@anarazel.de>,
	Tomas Vondra <tomas.vondra@postgresql.org>,
	pgsql-hackers@lists.postgresql.org
Subject: Re: pgsql: Introduce pg_shmem_allocations_numa view
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Hi,

On Tue, Jun 24, 2025 at 05:30:02PM +0200, Christoph Berg wrote:
> Re: Tomas Vondra
> > If it's a reliable fix, then I guess we can do it like this. But won't
> > that be a performance penalty on everyone? Or does the system split the
> > array into 16-element chunks anyway, so this makes no difference?
> 
> There's still the overhead of the syscall itself. But no idea how
> costly it is to have this 16-step loop in user or kernel space.
> 
> We could claim that on 32-bit systems, shared_buffers would be smaller
> anyway, so there the overhead isn't that big. And the step size should
> be larger (if at all) on 64-bit.

Right, and we already mention in the doc that using those views is "very slow"
or "can take a noticeable amount of time".

> > Anyway, maybe we should start by reporting this to the kernel people. Do
> > you want me to do that, or shall one of you take care of that? I suppose
> > that'd be better, as you already wrote a fix / know the code better.
> 
> Submitted: https://marc.info/?l=linux-mm&m=175077821909222&w=2

Thanks! I had in mind to look at how to report such a bug and provide a patch
but you beat me to it.

Regards,

-- 
Bertrand Drouvot
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com