Re: RHEL4 x86 kernel with more than 4 GB of RAM

[Arjan van de Ven <arjan infradead org>]

On Fri, 2005-08-05 at 21:02 +1000, jason andrade wrote:
> On Fri, 5 Aug 2005, Arjan van de Ven wrote:
> 
> >
> >> I thought that a 32bit kernel could address up to 4GB of memory (2^32
> >> bytes = 4GB) and that special addressing was needed (e.g. PAE) for
> >> more memory, with negative effects on performances and compatibility.
> >> Did I miss anything here?
> >
> > correct; you need PAE to address more. But PAE is enabled for the -smp
> 
> some other questions to explore this>
> 
> if you're using emt64 or amd64 systems is it correct that you won't be
> using PAE to address > 4G of memory ?  

that is a question which is more tricky than it sounds. The 64 bit
pagetable format is *identical* to the PAE pagetable format. 

The big difference is that the kernel is able to address above the 4Gb
directly, so no weird EMS.sys-resembling tricks needed anymore.

So on the one hand you don't have some of the nasties of PAE, on the
other hand you are effectively using PAE anyway. (it really makes sense
from a chip pov; it's just the same logic anyway now)


> but you need to run RHEL X x86_64
> to do this ?

yeah

> 
> if you have exactly 4G of memory does PAE on a 32bit install still come
> into play or does it have to be > 4G ?

PAE is enabled on all -smp kernels regardless of the amount of memory
you have.

also at exactly 4Gb you have some border effects; the bios will reserve
a bit of memory for the PCI mmio space just under 4Gb. Some chipsets
will move the ram that was there to above 4Gb and make it available to
you (and thus you would need PAE to address it), while others just make
the memory hidden and unavailable, resulting in no need for PAE to
address all ram (the kernel-smp kernel uses PAE anyway though)