kernel not recognising 4GB of physical RAM

rolypoly · Tux's lil' helper Joined: 30 Nov 2003 Posts: 143

I just upgraded the RAM in my laptop (Toshiba Satellite P100) from 2GB RAM to 4GB RAM.
The BIOS recognises the full 4GB, but when booting into linux, only 3GB shows up. I'm running a 64 bit kernel.

Here is the output of 'cat /proc/cpuinfo'

Hypnos · Posted: Fri Jun 19, 2009 7:00 am Post subject:

First, are you sure you're running a 64-bit kernel? If you are, it will say "x86_64" in the output of "arch".

Next, do you have PAE enabled in your BIOS? If so, turn that off and see how that works.

I don't think the CPU address sizes are relevant.

Hopefully, your motherboard actually supports 4GB of RAM ...

EDIT: typo
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s4e8 · Guru Joined: 29 Jul 2006 Posts: 311

Toshiba Satellite P100 use an ancient i945 chipset, there's no such 4G memory support, 3G is all you get.

Peach · Posted: Fri Jun 19, 2009 8:39 am Post subject:

rolypoly · Tux's lil' helper Joined: 30 Nov 2003 Posts: 143

@hypnos - my bios doesn't have an option for PAE.
Output of uname -a is:

rolypoly · Tux's lil' helper Joined: 30 Nov 2003 Posts: 143

I have also read that I can add a kernel boot switch of mem=4G to try and force the kernel to recognise the full amount of RAM, so will try that next, then the live cd.

Peach · Posted: Fri Jun 19, 2009 9:27 am Post subject:

s4e8 · Guru Joined: 29 Jul 2006 Posts: 311

Full 4G RAM support require memory remap (or memory hole) support, remap PCI overlayed RAM address to address above 4G.
First Intel north bridge(chipset) supporting this feature is 965 family.

rolypoly · Tux's lil' helper Joined: 30 Nov 2003 Posts: 143

I was going to use a 64bit live cd, rather than a 32bit live cd, assuming I can find one

Here is the output of lspci:

Hypnos · Posted: Fri Jun 19, 2009 9:51 am Post subject:

You need to know what precise chipset you have here (seen from "lscpi").

If you have the 945GM, it only supports 2GB system RAM. It may see all 4GB of RAM, but some block of addresses corresponding to ~1GB will be reserved for mmap'ed devices. (More here.)

The problem is that it can address only a certain amount. A chipset that supports 4GB of RAM, such as the 965, can address more than that.
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x22 · Apprentice Joined: 24 Apr 2006 Posts: 208

Some BIOSes have a setting which must be enabled (or disabled?) to work with 4GB or more physical RAM (usually called "memory remap").

x22 · Apprentice Joined: 24 Apr 2006 Posts: 208

Hypnos · Posted: Fri Jun 19, 2009 12:54 pm Post subject:

rolypoly · Tux's lil' helper Joined: 30 Nov 2003 Posts: 143

Well, using the mem=4G kernel boot parameter didn't help.
Booting from a Fedora 11 x86_64 boot CD still only shows 3GB available.

I suspect, after looking at some other similar posts elsewhere, I have a buggy (windows only) BIOS. Before a previous BIOS update, I had to follow the instructions on modifying the DSDT just to get sound working, so it's no surprise to me that there is something stopping me using the full 4GB under linux.
I notice there is a newer BIOS version available on the Toshiba website, but in best Toshiba fashion, there is no description of what's changed. And it's a Windows only executable. I don't really want to install Windows just do flash the BIOS, so I might see if I can use a Windows PE boot CD.

Hypnos · Posted: Sun Jun 21, 2009 3:41 am Post subject:

Does Windows see 4GB of RAM? You might not have to install Windows to determine this, just boot up with an install/recovery CD.
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rolypoly · Tux's lil' helper Joined: 30 Nov 2003 Posts: 143

Good idea. I'll check as soon as I can find a windows CD. Strangely I don't have many of those lying around

x22 · Apprentice Joined: 24 Apr 2006 Posts: 208

oRDeX · Veteran Joined: 19 Oct 2003 Posts: 1325 Location: Italy

Yes, i think that "supports" means "you can install up to..", but doesn't mean "you can use all the ram up to.."

Hypnos · Posted: Mon Jun 22, 2009 12:01 pm Post subject:

Yes, I was using "see" and "address" in the sense that the technical manual uses "support." I should have explained that more clearly. Obviously, the OP's chipset isn't the 945GU/GSE which can only address 2GB (meant for netbooks), but it's useful to know that it's not the 943/940GML, either. So ~3GB of RAM is supported in the sense of being available to the OS.

I stumbled across something that is new to me: when you look at cpuinfo, the listing of "physical" address size does tell you the maximum system memory your architecture can handle. So while the instruction set is full 64-bit, if your OS tries to allocate RAM with addresses beyond the maximum physical address the CPU will raise a general protection fault. For 36-bit physical address size (most x86_64 CPUs) the maximum system RAM is 64GB.

This indeed has little to do with PAE, but is interesting nonetheless.
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rolypoly · Tux's lil' helper Joined: 30 Nov 2003 Posts: 143

OK, looks like I'm gonna have to settle for 3GB

I've flashed the bios to the latest level and it still shows 3GB.

Maybe at some point I'll get a spare hard drive and install a 64bit flavour of windows and see what that does.

Thanks for help and suggestions.

Roland.

Evincar · Apprentice Joined: 13 Feb 2007 Posts: 217 Location: Madrid

I have an Aspire 5672, which is based on the same chipset, I think, and I can confirm that the BIOS will not see anything above 3G. Doesn't matter what you do in the kernel, because the hardware can not use more.
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energyman76b · Advocate Joined: 26 Mar 2003 Posts: 2048 Location: Germany

I am not conviced about that - look at dmesg - the first stuff after the kernel is loaded is about memory.

I am sure the bios know about 4gb - but everything between 3gb and 4gb is reserved for pci space, dma etc pp.
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rolypoly · Tux's lil' helper Joined: 30 Nov 2003 Posts: 143

I've spoken to a Toshiba product manager about this issue and he says under Windows 64bit, the whole 4GB should be available. He's got no experience with linux so can't comment about that, but says any 64bit OS should see the full 4GB.

I'll have a look at the start of dmesg next time I boot and see what I can find. Maybe I'll ask the LKML?

Since updating the BIOS, the fan on my nvidia chipset is no longer working, which causes excessive heat and eventually screen corruption and shutdown, so now I've got to downgrade by BIOS to the previous level too

That's assuming the Toshiba product manager can find a copy for me, since their website only provides the latest.

Hypnos · Posted: Thu Jun 25, 2009 4:05 am Post subject:

It's rather mysterious how a 64-bit OS would be able to use 4GB when the chipset can only address 32-bit and there must be ~1GB of addresses reserved for mmap'ped devices.

The only thing I can think of is that the chipset does "support" (in the sense of available to the OS) 4GB of RAM, there exists a memory remap option as x22 wrote, and you have to enable it.

Keep us posted!
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gentoo_ram · Posted: Thu Jun 25, 2009 5:55 am Post subject:

I agree, you're only going to see 3GB in the O.S. It's a limitation of the hardware addressing of the chipset. Keep in mind if you were to try Vista SP1, it would report "4GB" since it reports installed RAM, not usable RAM. But, again, only a little over 3GB would be usable. So it would appear that Windows sees all 4GB, but it won't use it for the same reasons Linux can't. The hardware.