Buying 2x4GB sticks in Mac Pro. How to make them optimal?

Discussion in 'Mac Pro' started by itou, Apr 20, 2009.

  1. itou macrumors regular

    Jan 16, 2008

    I'm ready to upgrade my Mac Pro after a year with the factory 2GB's (2x1GB). I'm wondering what's the best way to install the new 2x4GB sticks and why? I'm excited and quite sure I won't notice the speed difference anyhow but it's a nice feeling to know that it's optimal!

    I've heard that the 4GB sticks should go first? As in A1, then B1, and then the 1GB sticks should be in A2, and B2?

    Is that correct?

    A=top riser
    B=bottom riser

    Oh yes! I have the early 2008 model.
  2. itou thread starter macrumors regular

    Jan 16, 2008
  3. Sun Baked macrumors G5

    Sun Baked

    May 19, 2002
    That is the best way to install it, putting 4 of the same (aka two matching pairs) is the best way.

    Failing that, two pairs of the biggest DIMMs in the first two slots. Paired with the same size DIMMs on each riser. 4GB A1&A2 and 1GB B1&B2


    The machine has 4 channels of memory, two dual channel memory controllers, one for Riser A and one for Riser B.

    Only DIMMs 1-2 on each riser are active, or they are turned off and slots 3-4 are used.

    This switch exacts a penalty. So anything in slots 3-4 on either riser will take longer.



    This above picture is why Apple says colors denote same size dimms, because each riser has its own dual channel memory controller. And to run in dual channel mode you need to pair them up on the same controller, and quad channel means 2 pairs of the same DIMMs.

    Attached Files:

  4. itou thread starter macrumors regular

    Jan 16, 2008
    please excuse my ignorance, so to confirm:

    both 4GB sticks should go on Riser A1 and A2
    both 2GB sticks should go on Riser B1 and B2?

    what about now? the mac pro came stock with 2GB (2x1GB sticks) on A1 and B1? would i see a performance increase if i put both sticks (1GB) on Riser A?


    looking at this , it seems that splitting up equal ram to both risers is the way to go? i'm just wondering should i put both 4GB's on 1 riser and the other two 1GBs on the second riser.
  5. Tesselator macrumors 601


    Jan 9, 2008
    Just like the attached graphic above your post shows. A1 and A2, B1 and B2, and it doesn't matter at all which one gets the 2x4 or the 2x1 set. Your 1st 4GB will be accessed at the higher speed quad-channel and the remaining 6GB at a slightly slower dual-channel access.
  6. Sun Baked macrumors G5

    Sun Baked

    May 19, 2002
    Yes, or the other way around.


    Branch 0 is one memory controller, Branch 1 the second, each controls a single riser.

    The link you showed basically stated that adding memory 4 dimms at a time is best. 4 matched dimms best (aka, quad channel), but 2 pairs of any size dimms not much worse (2 dual channel pairs, slightly slower than quad channel).


    The new Mac Pro 2008 memory map, with the first pair in A1 and B1 would lead to many people getting confused. So just ignore it since you have more than 2 dimms.

    Likely Apple changed the chart when they found that splitting the dimms up and putting one on each memory controller worked better and boosted the performance for 2GB and 2 dimm systems.

    Once you get beyond 2 you have to switch back to the old 2006 Mac Pro memory chart.
  7. itou thread starter macrumors regular

    Jan 16, 2008

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