[CentOS] Adding RAM

Kevin Krieser k_krieser at sbcglobal.net
Sun Dec 7 21:21:18 UTC 2008

On Dec 7, 2008, at 1:37 PM, Morten Torstensen wrote:

> Kevin Krieser wrote:
>> At least with regard to the upstream provider, on X86 the desktop
>> version has a limit of 4GB of RAM, regardless of how much more memory
>> you have.  And they removed the hugemem version, so instead of up to
>> 64GB of RAM on 32 bit, you can only get to 16GB for server versions.
> With PAE you can access up to 64GB memory. It works much the same  
> way as
> XMS memory in DOS, where "high mem" is mapped to a low mem window.  
> It is
> just addresses that are mapped, there is no physical copying of memory
> that you had with EMS memory.
> Generally, PAE would not make much sense on >16GB memory machines, as
> you still need the space in the 4GB range to address it. Personally I
> would use PAE on machines with up to 8-12GB memory (assuming x86_64
> wasn't an option). With more than 16GB I would recommend against it,  
> as
> you get a lot of remapping and/or limited space in the 4GB range.
> YMMV depending on specific workload of course.

I'm just going by what the redhat site says for EL 5.  On this  
version, they don't provide the hugemem version for server anymore, on  
the assumption that if you really need to use more than 16GB of RAM  
you should be running 64 bits.  I assume that this also helps with  
reducing sizes of page tables, and testing.

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