Fedora core 3 - 4GB memory limit?

[Joel Jaeggli]

On Wed, 12 Apr 2006, Matt Roth wrote:

>>> Eric Persson wrote:
>>>
>>> Hi,
>>>
>>> I have a Fedora core 3 machine:
>>>
>>> Fedora Core release 3 (Heidelberg)
>>> Linux localhost.localdomain 2.6.9-1.667smp #1 SMP Tue Nov 2 14:59:52 EST
>>> 2004 i686 i686 i386 GNU/Linux
>>>
>>> It currently have 3gb of memory in it, and I have two spare slots which
>>> I intend to fill with an additional 2gb of memory to get to a total of
>>> 5gb in the machine.
>>>
>>> Is this possible or is there a 4gb limit in the kernel, which would make
>>> something like PAE needed. If so, is that already included in the stock
>>> kernel used?
>>>
>>> Any other apparent caveats?
>>
>> Alexander Dalloz wrote:
>>
>> Does your motherboard support such an amount of RAM? You did not tell us
>> which one you run. You should look into the manual. On non server i386
>> x86 boards the amount of RAM is often limited to max. 4 GB, where you
>> even loose the amount which is required by I/O components. Thus you
>> often only have max. 3.5GB RAM even if 4GB modules sticked in.
>
> Eric,
>
> In addition to Alexander's points, there are a few other things to consider 
> when installing more than 4 GB of RAM:
>
> 1) 32-bit processors (without 64-bit extensions, such as Intel EM64T) are 
> limited to 4 GB of physical memory.

sort of, intel pae gets you 16 x 4GB pages (36 bit addressing), and a 
slight performance hit.

> 2) 32-bit operating systems and applications use 32-bit pointers than can 
> only address up to 4 GB of physical memory.

on linux the actual result is an indivual process can't have more than 
3GB.

> 3) 32-bit hardware (such as PCI ethernet cards) can be problematic.  I have a 
> Level 5 Networks EtherFabric EF1-21022T that required a custom driver for our 
> system.  I believe it was a DMA bounce buffering issue.

This more problematic with systems that can do memory hoisting (move 
reserved regions above the 4GB address space limit) then systems that 
can't. On systems that can't you just end up with a (potentially) large 
hole in your memory between 3 and 4GB that you can't use. you wonder "why 
when I add a gig of memory to go to 4GB do I only get an additional 500MB" 
for example.

> If you don't have a 64-bit processor or a processor with 64-bit extensions, 
> you are limited to 4 GB of RAM.  Since according to your post you are running 
> Fedora Core 3 i386, this is the most likely scenario.
>
> If you do have a 64-bit processor (and assuming you want to keep running Core 
> 3), I believe you'll need to install Fedora Core 3 x86_64.  This will give 
> you the 64-bit OS and applications needed to take advantage of the additional 
> memory.

if you have a single process that's going to have a giant memory 
footprint, it's time to go 64bit.

> Finally, if you decide to upgrade from 32-bit to 64-bit, evaluate the 
> hardware in the system beforehand to insure it will make the transition 
> smoothly.  Hardware that is labeled as having a 64-bit interface should be 
> fine, but  I'd recommend doing some compatibility research on anything 
> labeled as 32-bit before making the change.
>
> To give you an idea of where I'm coming from, my 64-bit architecture consists 
> of a Dell PowerEdge 6850 with 4 Intel Xeon processors and 20 GB of physical 
> memory running Fedora Core 3 x86_64.  That said, I am by no means an expert 
> on the differences between 32-bit and 64-bit architectures.  I hope this post 
> was helpful, but if I have made any factual errors please post corrections 
> back to the list.
>
> Thank you,
>
> Matthew Roth
> InterMedia Marketing Solutions
> Software Engineer and Systems Developer
>
>

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Joel Jaeggli  	       Unix Consulting 	       joelja at darkwing.uoregon.edu
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