[Gllug] 4G Memory Restriction

[Daniel P. Berrange]

On Mon, Apr 10, 2006 at 11:38:20AM +0100, Steve Nelson wrote:
> On 4/10/06, Tet <tethys at gmail.com> wrote:
> 
> > Of course, these days, AMD have now made the decision simple -- just
> > go straight to 64 bits. Do not pass go, do not collect 200 pounds.
> > Anything else just doesn't make sense in today's world.
> 
> Right - so supposing I am running on 64 bit architecture (for example
> I've just built some rhel 3 and 4 machines on DL585s which use
> Opterons).  I've got 32 bit kernel and userland on this machine.  As
> per Anthony, the width of the bus is the restricting factor, but
> because  I don't have 64 bit tools am I still in the same situation of
> only being able to address half of that?

I'm not saying you should use a 32-bit kernel on 64-bit CPUs, but if (due
to 3rd-party app / kernel module support requirements) you did need to
use a 32-bit kernel, then go with the HugeMem one. Ordinarily Hugemem
would be a significant performance hit, but the AMD-64 CPUs have some 
magic which means they can avoid (all) of the performance hit associated
with the page table split required by the 4g/4g split. So you maximise
your 32-bit address space, giving both kernel & userspace a full 4 GB to
play with. You still really really want to use 64-bit kernel & OS if at
all possible though.   The RHEL 64-bit OS does contain compatability
libraries to allow common 32-bit appplications to run unmodified.

Dan.
-- 
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|=-   berrange at redhat.com  -  Daniel Berrange  -  dan at berrange.com    -=|
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