How Two-Processor Mac Pro access RAM?

Discussion in 'Mac Pro' started by tant, Dec 29, 2016.

  1. tant macrumors newbie

    Joined:
    Dec 29, 2016
    #1
    Hi All,

    I plan to buy used Mac Pro (mid 2010) with 2 processors but I am not sure how Mac manage RAM between these 2 processors. For example, if I have 32GB (8X4GB) RAM with 8 cores (2 quad-core processors) will I be able to access all 32GB per core, or is it 16GB per processor (in which case a core may access max 16GB)?

    Thanks
     
  2. AidenShaw macrumors P6

    AidenShaw

    Joined:
    Feb 8, 2003
    Location:
    The Peninsula
    #2
    There's one pool of RAM visible to every core/thread. It's just like a single socket multi-core system in that regard - it just has more cores and potentially more RAM.
     
  3. AidenShaw macrumors P6

    AidenShaw

    Joined:
    Feb 8, 2003
    Location:
    The Peninsula
    #3
    To elaborate - although there is one pool of RAM visible to every core, in fact the usual case is that half of the RAM is attached to one CPU socket, and the other half to the other CPU socket. (Half and half is not required, but is a best practice.)

    This means that a given core (or thread) has slightly faster access to RAM that's attached to its socket, and slightly slower access to RAM that's on the other socket. The program accesses all memory exactly the same - no special code to reach "far" memory - but a small performance hit.

    For most applications the cache masks this small imbalance. While contrived benchmarks can show a more significant performance hit - for most applications the effect is negligible.
     
  4. flowrider macrumors 601

    flowrider

    Joined:
    Nov 23, 2012
    #4
    To elaborate a bit further. Each of the CPUs has 3 channel RAM access. Therefore the RAM nearest the CPU is accessed by that CPU. The third and forth slot are tied together, and if RAM is only installed in the fourth slot, you won't see it. The best performance happens when only the first three slots are populated with the same speed and density RAM modules. If you populate all 4 slots, you take a very slight performance hit.

    Lou
     
  5. kschendel macrumors 6502a

    Joined:
    Dec 9, 2014
    #5
    Yes, each core can see all of the memory, all 32Gb in your example. There may be small discrepancies in access time, as described by the preceding posters, but as far as visibility goes, all of the memory can be seen by all cores in all processors.
     
  6. mattspace macrumors 6502

    mattspace

    Joined:
    Jun 5, 2013
    Location:
    Australia
    #6
    here's:

    https://support.apple.com/en-au/HT205043

    Apple's guide to installing ram, which you'll probably want at some stage of the process.
     
  7. tant thread starter macrumors newbie

    Joined:
    Dec 29, 2016
    #7
    Thank you very much guys!
    Now it's much easier to spend the money and get the beast :) !
     

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