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Old 07-29-2009, 07:41 AM   #1
nuwen52
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64 bit CentOS not recognizing all 4GB


I'm not a newbie, but kinda have a newbie question.

I am running CentOS 64 bit on my workstation. I've installed the 64 bit version and have since upgraded the memory to 4GB. The bios recognizes all 4GB. Conky recognized 3.05 GB, so I checked the /proc/meminfo and it showed MemTotal: 3194880 kB

A uname -a shows: 2.6.18-128.2.1.el5.centos.plusxen #1 SMP Thu Jul 16 07:55:16 EDT 2009 x86_64 x86_64 x86_64 GNU/Linux

After checking the net and the forums, what I have found is dealing with people running the 32bit version. It's probably something simple, but if anyone happens to know something I should check, I would appreciate it.

Thanks
 
Old 07-29-2009, 08:12 AM   #2
mek
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64 bit kernel should map your 4 gbs without problem

Have you tried booting with kernel SMP, it should be installed by default, and you can choose it in the grub menu, and also, do you have installed the latest version of centos 4, and updated? because there was a bug related to grub, and how it maps the memory, with something similar to your problem:

http://bugs.centos.org/view.php?id=1905

in order to check that the bug is the same that your problem, what version of grub are you currently running?

rpm -qa | grep grub
 
Old 07-29-2009, 08:41 AM   #3
johnsfine
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The problem is not in Linux, it is in the BIOS or the motherboard.

Even if the BIOS says it recognizes 4GB that doesn't mean the motherboard really supports a full 4GB. The BIOS can tell 4GB is installed even if the motherboard can't support that much.

More likely, the motherboard can support that much, but the BIOS has some option set wrong. Often that option is something about "memory mapping". If there is any option like that, it must be enabled.

Do you have a URL for a PDF of the motherboard manual. If I look through the documented BIOS options, I'm likely to recognize the one you need to change (of course that only helps if the actual BIOS menus are similar to the documented ones, which is not the case in many motherboards including my own).
 
Old 07-29-2009, 08:43 AM   #4
nuwen52
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I'm using CentOS 5.3, and it's updated as of this morning. The version of grub is: grub-0.97-13.2

According to the uname -a, I am running the SMP version, even though I don't think the computer has a need for SMP. According to the discussion that you pointed to, (from what I can tell) there might be a problem with grub and a Xen enabled kernel. I'll try one that doesn't have Xen (since I don't need it) before I mess with grub.
 
Old 07-29-2009, 11:12 AM   #5
nuwen52
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I took a look in the bios and there aren't very many things that I can actually change. The only one that had any mention of memory was the "OS Install" option. Which said that some OS's can't install with more than about 2GB of memory, so enabling this option will limit the amount of ram visible to the OS. That option was in the "off" setting, which (according to the docs) says that all 100% of the ram should be visible. I didn't get a chance to do more research yet, but thanks for the input.
 
Old 07-30-2009, 08:48 AM   #6
nuwen52
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johnsfine,

I did get your message and the above is what I did to check it out. The box is a Dell Optiplex GX520. The exact motherboard, I'd have to look up.
 
Old 07-30-2009, 12:53 PM   #7
lazlow
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I am not sure if this applies to your motherboard or not, but Intel cheated on some of their early 64bit systems. What they did was put a 32bit memory controller on 64bit motherboards. On these motherboards one is stuck with using a 32bit OS with PAE in order to access ram over 3Gb(roughly).
 
Old 07-30-2009, 12:59 PM   #8
johnsfine
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I did a few web searches on the GX520 and see many places where it says the max memory supported is 4GB. The same is said for the Intel 945 chipset that is used and includes the memory controller.

A few sites support my guess that everything in the specs, the BIOS, and Windows Vista which tells you 4GB is supported and/or present is basically lying. The chip really only supports three point something GB of ram.
For example:
http://www.ecs.com.cn/ECSWebSite/Sup...uID=65&LanID=0

Quote:
Originally Posted by lazlow View Post
put a 32bit memory controller on 64bit motherboards. On these motherboards one is stuck with using a 32bit OS with PAE in order to access ram over 3Gb(roughly).
Do you have any links to support that? It contradicts everything I think I know about the relationship between the cpu and the memory controller.

If only 32 bits of physical address pass from the CPU to the memory controller then only three point something GB of actual ram can be supported.

If more than 32 bits of physical address pass from CPU to the memory controller then both 64 bit mode and 32 bit PAE mode should be equally able to access the memory. Viewed from the interface outside the CPU chip, there is no difference between PAE and 64 bit for the first 64GB of address space.

Many motherboards quote a max memory of 4GB. I think those are mostly lies. I'm not certain, because that limit can be true. A motherboard might support only two dimms and be wired or programmed to not support dimms over 2GB each so it might have a true limit of 4GB. But I don't know whether a memory controller chip can have a true limit of 4GB of memory. A memory controller chip can have a limit of 4GB of physical address space, which limits it to 3.xxGB of actual memory. That is common and then they usually lie and say the limit is 4GB of memory. But a limit of 4GB of actual memory for a memory controller chip doesn't make any technical sense. So if the motherboard has a "4GB" limit because of the memory controller chip, it is probably really a 3.xxGB limit.

Last edited by johnsfine; 07-30-2009 at 01:14 PM.
 
Old 07-30-2009, 01:16 PM   #9
lazlow
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I could be wrong about the PAE section but I do seem to recall a thread here that one of the guys with this issue got around it this way. As far as the 32bit controller issue:

http://www.ifrankie.com/?p=70
 
Old 07-30-2009, 01:46 PM   #10
johnsfine
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Quote:
Originally Posted by lazlow View Post
I could be wrong about the PAE section
Yes. Nothing there talks about 32 bit PAE solving the problem. It wouldn't. Nothing less than a better motherboard would solve that problem.

Quote:
but I do seem to recall a thread here that one of the guys with this issue got around it this way.
Again a URL would help, but so far as I know:

A) If the BIOS and memory controller chip and motherboard all support more than 4GB of physical address space and you install 4GB of actual ram then:
1) Linux or Windows 32 bit non PAE will support only 3.xxGB
2) Common licenses for 32 bit Windows PAE will support the same 3.xxGB
3) Any Linux 32 bit PAE and any Linux or Windows 64 bit and uncommon licenses for Windows 32 bit PAE all support the full 4GB.

B) If the BIOS or memory controller or motherboard are limited to 4GB of physical address space then all possible OS's are limited to the same 3.xxGB of ram (but Vista has been "upgraded" to detect the unusable extra ram and lie to you to make you think it can be used).

In many situations people think my use of the term "lie" is too blunt.
If a motherboard supports just 4GB of physical address space and "supports" 4GB of ram, but because of the limit of physical address space there is no possible way for any OS to use more than 3.xxGB of ram, are they lying when they say it supports 4GB of ram, or are they just misleading you (it supports the 4GB, you just can't use more than 3.xxGB of it) or is the buyer just misunderstanding the terminology (it depends on what the meaning of the word "supports" and/or "max memory supported" doesn't mean you can use that much memory).

Last edited by johnsfine; 07-30-2009 at 01:51 PM.
 
Old 07-30-2009, 01:58 PM   #11
lazlow
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John

I do not know what is up with you lately. Clearly I stated I could be wrong on the PAE issue and I also clearly stated that the link I posted had to do with the 32bit controller and not the PAE issue. Personally I read several thousand threads per week across a multitude of forums, as this particular issue peaked well over a year ago it is not the "freshest" thread that I remember. If I had been able to find the thread quickly, I would have posted it. As I could not, I posted that I could be wrong on the PAE issue. If you would quit wasting everyone's time by attacking people needlessly, we would all be better off. You had already made you point, I admitted that I could be wrong. What information is there to gain in attacking after that point? If I happen to find the thread (at some point) I was referring to, I would have/will post it.
 
Old 07-30-2009, 03:23 PM   #12
johnsfine
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Quote:
Originally Posted by lazlow View Post
I do not know what is up with you lately.
I really thought I was just clarifying things for the OP.

Quote:
I read several thousand threads per week across a multitude of forums, as this particular issue peaked well over a year ago it is not the "freshest" thread that I remember.
Same here and I usually fail to find the URL when it becomes relevant for another thread. So when I said
Quote:
Again a URL would help
I didn't mean that any more than literally and certainly not as an attack. I expect you agree with me that a URL would help and I agree with you that it is often hard to remember where you saw information.

Quote:
If you would quit wasting everyone's time by attacking people needlessly, we would all be better off. You had already made you point
I may have failed to realize I had already made my point. I thought the link you posted added some extra confusion for any ordinary person reading the thread and I thought some comment could further clarify things. If I thought I was only talking to you, I'm sure I would have realized I had already said enough.

I'm not great at knowing when I said enough even talking to just one person. Talking to whoever might be reading the thread makes that much harder.
 
Old 07-31-2009, 01:03 AM   #13
chrism01
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This may be useful: http://kbase.redhat.com/faq/docs/DOC-6559.
I think basically the system reserves approx 1GB for the kernel itself.
 
Old 07-31-2009, 07:00 AM   #14
johnsfine
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Quote:
Originally Posted by chrism01 View Post
This may be useful: http://kbase.redhat.com/faq/docs/DOC-6559.
I think basically the system reserves approx 1GB for the kernel itself.
That is an unrelated issue. That is a limit on per process virtual memory and only for 32 bit kernels.

We were talking about a restriction on whole system physical memory that may (depending on cause) also apply to 64 bit kernels.
 
  


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