dual cpu xeons and ram

[Kippa]

Just wondering, does all the ram on a dual xeon cpu system get used by both cpus at the same time. So does both cpus have access to 64gb of ram, or does each cpu only have access to half of the ram like one cpu have access to 32gb and the other cpu have access to the other 32gb?

 

[zir_blazer]

Check NUMA. You have two Memory Banks, one for each Processor Integrated Memory Controller. If Processor 1 wants to access data from the Memory Bank of Processor 2, it has to use the QPI link to go to the other Processor first.

 

[dave_the_nerd]

A process running on CPU1 should, theoretically, be able to see and address RAM controlld by CPU2, and visa versa (through the shared bus) but it would usually prefer not to.

And you would still have to have a second CPU installed for the CPU2-controlled DIMM slots to work. (Assuming a modern CPU anyway, the memory controller's on the CPU. If you're talking Socket 771 dualies, it's a little different.)

 

[Subyman]

Like the others have said, they will both have access to all RAM. However, CPU A will not have fast access to CPU B's RAM. This is why some tasks are actually slower on dual CPU systems, such as certain types of matrix operations.