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

[Andi Kleen]

Fabio De Francesco <fabiomdf@xxxxxxxx> 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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