4 Gig in new Prec 490 showing as 3 Gig?

[jarod at wilsonet.com (Jarod Wilson)]

On Thursday 03 May 2007 11:46:37 V. Alex Brennen wrote:
> On Thu, 2007-05-03 at 11:10 -0400, Derek Atkins wrote:
> > Jerry Feldman <gaf at blu.org> writes:
> > > As mentioned previously, to see more than 3GB you need to have PAE
> > > enabled. Linux will be able to access the 4GB (or in this case 3.5GB),
> > > but on a 32-bit system your applications are limited to 3GB.
> > > Here is a pretty good detailed explanation:
> > > http://kerneltrap.org/node/2450
> >
> > Okay... So how does one enable PAE?  (I've got 4GB in my new t60
> > but it seems to only see 3GB of it)
>
> The Linux Kernel will enable it automatically if it is necessary.

iff your kernel is built with PAE support, which in my world, is not the case 
for the base kernel. Red Hat's stock i686 kernel is non-PAE, as there are 
i686 systems that don't support PAE, and will fail to boot. Red Hat also 
ships an i686 kernel-PAE package for those that need PAE. But so far as I 
know, PAE is really only relevant/needed if you have *more* than 4GB of RAM 
on a 32-bit system. That seems to fall in line with the rest of Alex's 
comments below.


> To access your 4GB of RAM, you'll just need to compile your kernel with
> CONFIG_HIGHMEM which is under the "menuconfig":
>
>   Processor type and features  ---> High Memory Support
>
> My understanding of PAE is that it isn't necessisary for 4GB and your
> system will not enable it.  A Linux Kernel with the CONFIG_HIGHMEM
> option enabled will simply create an additional layer of abstraction
> between the primary kernel memory map and the physical memory addresses.
> With PAE, a third layer of abstraction is used, rather than the two
> layers (High Mem and Normal Memory), because the 64GB map is not able to
> easily fit in the High Mem allocation.
>
> The help page for the High Memory Support option in "menuconfig" or the
> kernel's text file documentation explains all of this in great detail.
> I have a number of intel based servers with 4GB of RAM and a few with
> 12GB.  I had to spend some time reading through all that info a few
> months ago. Because, I initially incorrectly assumed that I would need
> CONFIG_HIGHMEM only for the 12GB systems and not for the 4GB installs.

-- 
Jarod Wilson
jarod at wilsonet.com

-- 
This message has been scanned for viruses and
dangerous content by MailScanner, and is
believed to be clean.