Some of what I will write is speculative. I've read a lot, but never
used PAE. I am an AMD fan boy, and we've had 64bit systems covered
since 2003.
Your findings which suggest that you cannot get 4GB used w/o PAE are
probably correct. PCI and ACPI BIOS live at the top of memory. Expect
to lose on the order of 64-128M of memory there... I don't know about
700M.
* Bruce Miller <subscribe [ at ] brmiller [ dot ] ca> [071229 13:14]:
> Q. Some sources state that enabling PAE in the kernel leaves a
> significant performance hit. All these sources are two years old
> or more. Is this problem inherent or has it been worked around?
> How significant is the hit? Since the default Ubuntu kernel on
> the LiveCD acknowledges ~3.3 GB of RAM, should I consider
> "leaving alone"?
PAE is designed to give you really fast swapping. The RAM that is
beyond what the CPU can normally use by directly accessing physical
memory locations will be used as swap. Memory will be copied back and
forth between accessible pages and PAE pages.
> Q. A posting by Falko Timme on howtoforge.com from mid-December
> 2007 states that there is a bug in the GRUB implementation on
> Debian Etch which prevents the system from recognizing more than
> ~3.3 GB RAM. (The exact figure on his 6GB system was a little
> higher than on my 4GB system.) He offers a patch and recommends
> recompiling GRUB. Does anyone know if the version of GRUB in
> Ku/Ubuntu 7.10 Gutsy Gibbon contains the same bug?
I am not surprised. I bet the issue comes from the BIOS. The BIOS
service routine that tells grub how much ram is available was probably
designed in the 90s.
Fortunately building debian/ubuntu packages is pretty easy...
sudo apt-get install devscripts
sudo apt-get build-dep grub
apt-get source grub
cd grub*
debuild -b
sudo debi
You can also look in grub* directory to figure out if the patch in
question is already applied.
> P.S. Since posting the above first on ubuntuforums.org, I have
> learned that the so-called "amd64" 64but versions work perfectly
> well on my Intel Core 2 Duo E2160 processor. My recall is that
> there now exist satisfactory solutions to longstanding problems
> with Flash, the nVidia proprietary blob, etc. Off to do more Googling.
I don't use proprietary drivers so I have not ran into this. I use
flash in a 32bit chroot. This is a pretty common hacky solution to it.
You can also run a 64bit kernel and 32bit userspace. Best of both
worlds. Although, I don't know how nVidia drivers play here.
-Bart
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