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Old 08-21-2009, 08:52 PM   #1
congogr
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Only see 2.9 our of 4Gigs of RAM on -64bit- installations of Jaunty and Mint 7


Right. This is what keeps me up at nights :

On installations of both Ubuntu 9.04 (64bit) and Mint 7 (64bit), I cannot fully utilize my RAM. Is a change in the kernel needed EVEN on 64bit installations?

==> My system is a ThinkPad z61m; specs "specify" max 3GB or RAM, but I think one of the BIOS updates allowed for support of more. In any case:
==> The system monitor gives me "2.9GiB" of Memory
==> Issuing $ lshw (without sudo) gives:
...
description: Computer
width: 64 bits
capabilities: vsyscall64 vsyscall32
*-core
description: Motherboard
physical id: 0
*-memory
description: System memory
physical id: 0
size: 3070MiB
...

==> Issuing $ sudo lshw gives:
...
*-memory
description: System Memory
physical id: 29
slot: System board or motherboard
size: 4GiB
*-bank:0
description: SODIMM Synchronous
physical id: 0
slot: DIMM 1
size: 2GiB
width: 64 bits
*-bank:1
description: SODIMM Synchronous
physical id: 1
slot: DIMM 2
size: 2GiB
width: 64 bits
...

==> $ free gives:
total used free shared buffers cached
Mem: 3054592 2376104 678488 0 144424 1373612
-/+ buffers/cache: 858068 2196524
Swap: 7839680 0 7839680


To my knowledge, the systems 'sees' the extra RAM (and thus the hardware supports it...?) but cannot utilise it. Wouldn't the 64bit kernel automatically use the full 4Gigs? Is an extra 'server kernel' installation needed, even with a clean installation of a 64bit edition?

Thanks for the help,
G.
 
Old 08-22-2009, 02:02 AM   #2
xeleema
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Greetingz!

Assuming your ThinkPad isn't doing you the "favor" of chopping off 512MB of your RAM (some vendors do this because they think you'll never run a 64-bit OS), you'll need to ensure you're running a PAE-enabled kernel.

Physical Address Extension (PAE) is something that should be enabled on almost any 64-bit kernel. However, I'm not an Ubuntu or Mint user (though I'll give Mint a try!).

Now in Ubuntu, I know the "Desktop" version of their distributions do *not* support PAE. However the server-builds do. If you're running the "Desktop" version, you can look into recompiling your kernel, which is basically a rite-of-passage for any Linux user.
There's a good write-up on how to do that here.

Hope this helps!!

(And if it does, feel free to click that little "Thumbs-Up" on my post!)
 
Old 08-22-2009, 04:02 AM   #3
syg00
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PAE is irrelevant for 64-bit kernels - but Ubuntu (and derivatives) certainly supports it in (32-bit) Desktop kernels.

What does the BIOS report when you reboot the box ?.
 
Old 08-22-2009, 07:04 AM   #4
congogr
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Quote:
Originally Posted by syg00 View Post
PAE is irrelevant for 64-bit kernels - but Ubuntu (and derivatives) certainly supports it in (32-bit) Desktop kernels.

What does the BIOS report when you reboot the box ?.
Thanks for the info. Indeed, to my knowledge, PAE would only 'work' on 32bit architectures/OSs.

The BIOS correctly reports 4096MB or RAM.

It's a bit strange why sudo lshw would show 4GiB on two 2GB modules (but not use them all) whereas lshw w/out sudo, only gives 3070.

Also, to make sure of the kernel I'm using (64bit), here's the uname -a output:

Linux UsErNaMe 2.6.28-11-generic #42-Ubuntu SMP Fri Apr 17 01:58:03 UTC 2009 x86_64 GNU/Linux

Suggestions Please???
 
Old 08-22-2009, 08:03 AM   #5
johnsfine
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First verify that the problem is in the bios/motherboard by checking the output of dmesg as discussed in many previous posts, such as
http://www.linuxquestions.org/questi...88#post3649388
Notice some related explanation at:
http://www.linuxquestions.org/questi...90#post3652690

I'm pretty sure that will show your problem is in the bios/motherboard, not in Linux, because you already have a 64 bit kernel and it is almost impossible for this problem to be on the Linux side if you have a 64 bit kernel.

Next check through the bios options to see if there is any way to enable "memory remapping" or turn off "OS install mode" or change some other name for the bios feature of hiding the ram that doesn't fit in the first 4GB of address space.

If you have a url for a reasonably accurate version of the motherboard manual, post that. Maybe someone else can make a better guess at which bios option needs to be changed.

Maybe you need a bios upgrade.

Maybe solving it is impossible.

Quote:
Originally Posted by congogr View Post
PAE would only 'work' on 32bit architectures/OSs.
PAE is a way of configuring the MMU in 32 bit mode that is more similar to the way the MMU is always configured in 64 bit mode. "PAE" is a meaningless term in 64 bit mode, because all the changes from ordinary 32 bit MMU programming to PAE MMU programming are unconditionally included in 64 bit MMU programming (the 64 bit MMU programming goes a little further than PAE, but first includes all the changes from ordinary 32 bit to PAE 32 bit).

Quote:
Originally Posted by xeleema View Post
Physical Address Extension (PAE) is something that should be enabled on almost any 64-bit kernel.
The internal operational details of PAE are enabled in any x86_64 kernel (not "almost any"). That is the only way the MMU works in 64 bit mode. But nobody calls that mode "PAE". It is 64 bit mode and is a superset of "PAE".

Quote:
The BIOS correctly reports 4096MB or RAM.

It's a bit strange why sudo lshw would show 4GiB on two 2GB modules
Even if the memory controller chip on the motherboard is unable to access a full 4GB of ram (so nothing the bios could do could make the extra available) the bios or any Linux program decoding DMI information can see what ram is installed.

The fact that the bios and other programs see the 4GiB does not mean you can use 4GiB. Hopefully, you just need to change some bios setting, but maybe your motherboard just doesn't support a full 4GB of ram. Most motherboards that support a max anywhere above 2GiB but below 4GiB have spec sheets that lie and say they support 4GiB.

Last edited by johnsfine; 08-22-2009 at 08:19 AM.
 
Old 08-22-2009, 12:46 PM   #6
skriefal
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There is nothing you can do about this. It's a limitation of the Intel 945 chipset, which was crippled by Intel to allow only 3GB of adressable RAM. You can install 4GB to enable dual channel mode, but you'll still be able to address only 3GB.
 
  


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