Out of my 4GB only 3GB is shown

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Out of my 4GB only 3GB is shown

Postby gocool » Wed Aug 28, 2013 3:36

Hi,
I recently upgraded my RAM from 2 GB to 4 GB.

In my BIOS I do see 4GB being recognized.

When I login to Sabayon it shows only 3GB

Code: Select all
[email protected] ~ $ free -h
             total       used       free     shared    buffers     cached
Mem:          3.0G       2.7G       282M         0B       179M       427M
-/+ buffers/cache:       2.1G       888M
Swap:         9.0G       136M       8.8G


I know that to recognize the entire 4GB you need a 64 bit kernel. But I also know that if you have PAE, your 32 bit should be able to use the entire 4GB.

Code: Select all
[email protected] ~ $ grep --color=always -i PAE /proc/cpuinfo
flags           : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov clflush dts acpi mmx fxsr sse sse2 ss ht tm pbe nx constant_tsc arch_perfmon bts aperfmperf pni monitor est tm2 xtpr pdcm dtherm
flags           : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov clflush dts acpi mmx fxsr sse sse2 ss ht tm pbe nx constant_tsc arch_perfmon bts aperfmperf pni monitor est tm2 xtpr pdcm dtherm


Please do help me in this.
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Re: Out of my 4GB only 3GB is shown

Postby wokko7 » Wed Aug 28, 2013 4:11

You can only use the 4g on a 64 bit not 32 bit
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Re: Out of my 4GB only 3GB is shown

Postby gocool » Wed Aug 28, 2013 10:19

No that's not true. If you have PAE enabled kernel, then it should take more than 3 GB.
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Re: Out of my 4GB only 3GB is shown

Postby linuxfluesterer » Wed Aug 28, 2013 11:09

Maybe this is a good explanation:
http://stackoverflow.com/questions/9808014/whats-the-relation-between-32-64-bit-application-os-and-processor

If you don't know, which (Sabayon) Kernel (32bit/64bit) you have installed, you can check with:
Code: Select all
uname -a

Check cpu with:
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lscpu

When you see: x86_64 ==> 64-bit kernel
When you see: i686 ==> 32-bit kernel

Conclusion: best result to receive most of your installed RAM is a 64bit Sabayon on a 64bit processor system.

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Re: Out of my 4GB only 3GB is shown

Postby gocool » Wed Aug 28, 2013 15:45

Thank you linuxfluesterer.

That breaks it down for me.

I was concerned with the link here where it mentioned something about PAE.

Then what is that PAE all about?
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Re: Out of my 4GB only 3GB is shown

Postby Stupot » Wed Aug 28, 2013 16:52

PAE means you have up to 64 GB of address space on your 32bit system. Sabayon ships with PAE enabled kernels (assuming you are using the 3.4 kernel or higher). You should be able to use 4 GB of RAM with Sabayon 32 bit.
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Re: Out of my 4GB only 3GB is shown

Postby gocool » Wed Aug 28, 2013 17:12

Thank you Stupot.

I am using kernel 3.7.0

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sabayon user # uname -r
3.7.0-sabayon


But I don't see all the 4GB

Code: Select all
sabayon user # free -h
             total       used       free     shared    buffers     cached
Mem:          3.0G       2.5G       505M         0B       144M       959M
-/+ buffers/cache:       1.4G       1.6G
Swap:         9.0G         0B       9.0G
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Re: Out of my 4GB only 3GB is shown

Postby Stupot » Wed Aug 28, 2013 18:40

Make sure that PAE is enabled via your BIOS settings. Some motherboards have an option for it to be turned on or off, although I'm assuming you already looked around your BIOS settings.

Other than that, I'm really not sure. I run nothing but 64 bit linux installs myself, so I've never personally dealt with PAE. But my understanding is that it should just work. I certainly don't see anything wrong with your approach in general.
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Re: Out of my 4GB only 3GB is shown

Postby linuxfluesterer » Wed Aug 28, 2013 21:39

Hallo Stupot.
I assume, PAE is set in BIOS already.
Code: Select all
uname -r
should contain reference to PAE if the kernel is PAE enabled on a 32bit architecture.
Anyway, though it is enabled, I have read, this PAE mode slows down system speed.
But though you might have access to 4GByte Ram, you seem to loose about 1 GByte Ram, see reason here:

Image[/URL]

I would see a benefit, if there were more than 4 GBytes installed, provided, the computer (BIOS) supports more than 4 GBytes of Ram.

I'm using a 64bit OS (Sabayon 64bit) on a 64bit architecture (Core i5), so I'm not expert for 32bit topic, but you may remember, that 32bit Windows was not be able to address more than 3.xx GByte of Ram.

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Re: Out of my 4GB only 3GB is shown

Postby Stupot » Thu Aug 29, 2013 16:00

linuxfluesterer, it seems to me you keep talking purely about non-PAE 32 bit kernels.

2^32 is exactly 4GB, but not all of that can be used for RAM, as your diagram shows. Address space is used for more than just RAM. That's why only 3 GB shows up in a standard non-PAE 32 bit kernel.

With PAE, this is the design:
"x86 processor hardware-architecture is augmented with additional address lines used to select the additional memory, so physical address size increases from 32 bits to 36 bits. This increases the physical memory addressable by the system from 4 GB to 64 GB."

So, for PAE kernels, 64 GB of address space is available. 64 GB of RAM can't be used though, as you've still got BIOS, PCI space, APIC space, etc to account for, but it's feasible that you could use perhaps 63 GB of RAM on a 32 bit PAE kernel.

32 bit Windows (at least XP) did not have PAE. The kernel gocool is using does.

Please, read about PAE, because you seem to be missing the point of it. http://en.wikipedia.org/wiki/Physical_Address_Extension

I would like to note that each individual application is still limited to a 32 bit addressing space. Only the OS is keeping track of the larger amount. Perhaps whatever application is being used to see the memory is only seeing 3 GB because of this, even though the kernel can actually use more. I wouldn't think this is the case though, since each application should have access to up to 4 GB, not 3 GB.
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