linux-kernel - Re: [x86_32] With 4GB installed, in which cases low mem total is less than 896MB?

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Message-ID: <87skx0vhbu.fsf@basil.nowhere.org>
Date:	Fri, 02 May 2008 17:47:17 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	Fabio De Francesco <fabiomdf@...ce.it>
Cc:	Arjan van de Ven <arjan@...radead.org>,
	linux-kernel@...r.kernel.org, Alan Cox <alan@...rguk.ukuu.org.uk>
Subject: Re: [x86_32] With 4GB installed, in which cases low mem total is less than 896MB?

Fabio De Francesco <fabiomdf@...ce.it> writes:
>
> Unfortunetally my poor knowledge of Linux kernel mechanisms (and
> code!) prevents me from understanding what could be the causes for
> that overhead. Do you mean it is a hardware issue? Or is it a kernel
> one? Anyway another machine of mine, that is equipped with 2GB RAM,
> doesn't show any proportional reduction of low memory at all. So
> this makes me think (oh, dear!).

You might want to read http://halobates.de/memorywaste.pdf, in
particular 3.5.2. The numbers arjan quoted and in the paper
are for 64bit kernels, on 32bit the overhead is ~32bytes/page. This
mem_map overhead happens for both highmem and lowmem, but is always
allocated in lowmem.

-Andi
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