[Beowulf] Mixing 32-bit compute nodes with 64-bit head nodes

[Joe Landman]

Andrew D. Fant wrote:
> Sean Dilda wrote:
>> Andrew D. Fant wrote:
>>
>>> The 64-bit motivation is mostly about providing adequate memory for
>>> multiple users running gui applications.
>>>
>> How big are the apps?  Assuming they only use one or two gigs of memory
>> a piece, you should be able to have a 32-bit machine with lots of RAM.
>> Or even an opteron with lots of RAM with a 32-bit OS on it.  That would
>> save you from having to worry about any 32 vs 64 bit issues.
> 
> I was lead to believe that PAE on 32-bit systems was prone to causing
> performance  problems.  If that isn't the case, you are certainly right.

PAE is an Intel developed (hacked) method to increase the address space 
of the 32 bit processors.  It is slow due to the way it works.  It 
basically adds another bit of segmentation to the address space.  Anyone 
remember EMS/XMS from DOS days?  Accessing pages through a 64kB window? 
  This is not identical in design to that, but it is close.

AMD64 removes the segmentation of the address space.  You get a flat 
virtual address space.  This means less work per address calculation as 
compared to segmented address space.

If you have the option, AMD64 is a better route than PAE.

> 
> Thanks,
> 	Andy
> 


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Joseph Landman, Ph.D
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