12-core Mac Pro - Number of RAM slots?

Discussion in 'Mac Pro' started by mustgroove, Nov 7, 2011.

  1. mustgroove macrumors regular

    Aug 14, 2006
    I'm aware that current Mac Pros have triple-channel memory interfaces, such that installing 3 RAM modules gives faster performance than installing 4, on the single-socket machines.

    On the dual-socket machines, do you need to install 6 RAM modules for best performance? Or is 3 also OK for these?
  2. minifridge1138 macrumors 6502a

    Jun 26, 2010
    The dual-socket MP comes with 8 slots.

    There is a benefit from installing 6 (3 in each of the 2 banks) but the user will not notice the difference.

    Let's discuss...

    Scenario 1:
    Lets say you install 6 gigs (1 gig in 3 slots of each bank). Call this setup A.
    Then you install 2 more gigs (1 gig in each of the remaining two slots). Call this setup B.

    Yes, setup A is going to use 6 gigs more efficient than setup B, but setup B has 2 more gigs of ram. The extra ram in B is going to help you much more than the presence of 2 more chips will hurt you. Overall, you still do better with more.

    Scenario 2:
    You install 6 4GB ram chips for a total of 24GB. Setup A.
    You install 8 3GB ram chips for a total of 24GB. Setup B.

    Setup A will perform slightly better than B. The amount of ram is the same in both, but A benefits from the configuration.

    I've never scene a test that shows a user-noticeable-difference of using 6 vs 8 ram slots (or 3 vs 4 in the single socket machine).
  3. bizzle macrumors 6502a

    Jun 29, 2008
    Except B wouldn't work at all because 3GB RAM modules don't exist.
  4. Zwhaler macrumors 604


    Jun 10, 2006
    Or you can put in 4x2 + 4x4 for a total of 24GB.
  5. Keebler macrumors 68030

    Jun 20, 2005
    I find the even number of slots and the 3 memory channel reading interesting b/c the math doesn't add up :)

    I just ordered a new pro and telequest thankfully suggested OWC. He suggested 24 GB in 6 x 4 GB sticks (now $206.99) so I could do that and have

    Bank A 1 - 4 GB
    Bank A 2 - 4 GB
    Bank A 3 - 4 GB

    Bank B 1 - 4 GB
    Bank B 2 - 4 GB
    Bank B 3 - 4 GB

    Bank A 4 - 1 GB (from original 6 coming with the machine
    Bank B 4 - 1 GB

    Or buy 32 GB for $272.99 and fill all slots with 4 GBs

    Would I really notice that much of a performance 'hit' ?

    Using Final Cut, DVD Studio Pro, Photoshop (which I hear as a limit anyways) and Aperture

  6. Zwhaler macrumors 604


    Jun 10, 2006
    I'm running 24GB and I rarely see it go above 16-18GB active while using Final Cut Pro X. For if you are going to upgrade once you might as well get 32GB.
  7. Umbongo macrumors 601


    Sep 14, 2006
    Don't bother with the 1GB DIMMs. If you are getting excessive page outs on 24GB it is unlikely an extra 2GB will be of much use anyway.
  8. Keebler macrumors 68030

    Jun 20, 2005
    Thanks. I received this response from a ram dealer. :

    (looks like ill install 24 to begin)

    "The way the MacPro memory banks work is:

    If you install 2 matched modules in a bank, it will run in dual channel mode

    If you install 3 matched modules in a bank AND leave the top socket empty, it will run in Triple Channel mode. This results in a 52% improvement in memory bandwidth performance -- which makes a marginal improvement in the real-time application performance of the machine because it is bottlenecked by other factors such as software that doesn't multi-thread effectively.

    If you install two matched modules in slots 1 and 2 and two matched modules in slots 3 and 4 in a bank, then the machine will drop back to dual channel mode.

    Worst configuration is three matched modules in slots 1 - 3 and then putting a mis-matched module in slot 4. This drops the machine to dual channel on the first 2 slots and all the way down to single channel on the 3 and 4 slots.

    If you have a machine with 8 memory sockets, matching the configuration in each bank also gets you a memory performance benefit.

    Double sided modules are also faster than single sided modules by about 20% on raw memory bandwidth, so 4 GB and 8 GB modules are faster than 1 GB and 2 GB modules.

    So the fastest possible memory configuration for an 8 memory slot machine is six 4 GB or six 8 GB modules, installed in slots 1 - 3 of each bank"
  9. Umbongo macrumors 601


    Sep 14, 2006
    A lot of people assume this as the performance is similar, but it isn't "dual channel" mode.

    Never heard anyone claim this - because that isn't how it works. There is some performance loss. Matched DIMMs aren't that important, but you should always go for them as it is simple to do so.

    1GB and 2GB dual-ranked DDR3 DIMMs are plentiful, so that's a bit of upsell marketing spiel.

    In order of optimum speed from best to last it's something like:

    6x DDR3 dual-ranked unbuffered DIMMs
    6x DDR3 quad-ranked registered DIMMs
    6x DDR3 dual-ranked registered DIMMS
    8x DDR3 dual-ranked registered DIMMs
    8x DDR3 dual-ranked unbuffered DIMMs

    You probably wouldn't notice much real world performance for creative applications though and capacity is nearly always going to be superior than optimising for speed if you are constantly running out of memory.
  10. Keebler macrumors 68030

    Jun 20, 2005
    Thanks umbongo. Im not that technically savvy so all the feedback i get is important

    So what you are saying is that i should load it with the 8 x 4 gb sticks i have coming instead of the 6x 4 gbs with 2 empty slots? :)
  11. Umbongo macrumors 601


    Sep 14, 2006
    Yeah :)

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