Mixing RAM sizes?

Discussion in 'Mac Pro' started by CH12671, Dec 30, 2013.

  1. CH12671 macrumors 6502

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    #1
    I'm considering just ordering the minimum (12g) of RAM in my nMP, and promptly ordering a 16g upgrade kit from crucial (2x8 gig). My understanding is that the 8 gigs are still non-registered, so they could technically mix with the two 4 gig factory chips, couldn't they? I would only do it for a few months until I can order another 2x8 chips to complete the 32 gig set....I just didn't want to waste the two factory chips in the meantime, or the two open slots.
    Would it work?

    Thanks
     
  2. jav6454 macrumors P6

    jav6454

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    #2
    I would not recommend mixing sizes if you care about the Quad Channel advantage. Basically, a boost in memory I/O speed.
     
  3. Umbongo, Dec 30, 2013
    Last edited: Dec 30, 2013

    Umbongo macrumors 601

    Umbongo

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    #3
    Yes it should work. See my comment below on performance.
     
  4. Anim macrumors 6502a

    Anim

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    #4
    I'm pretty sure that the Xeon CPU memory controller does not support mixing of ram. I read it somewhere on the intel website when looking at the chip features. What this does in the real world I have no idea unless somebody else has tested it using the same CPU. I would treat it as risky until you get some solid info.
     
  5. Anim macrumors 6502a

    Anim

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    #5
  6. Umbongo, Dec 30, 2013
    Last edited: Dec 30, 2013

    Umbongo macrumors 601

    Umbongo

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    #6
    Flex is something for dual-channel boards, don't think it is on any triple or quad channel boards, probably because of how it works. It lets you mix capacities and part of the larger capacity DIMM, up to the size of the smaller one, work in dual channel and the other part in single channel. So 8GB and 4GB would be 2x4GB dual and 4GB single. With 8, 8, 4, 4 in a Mac Pro it looks like it will all go in single channel, but the extra capacity may well offer more overall performance than 16GB in dual-channel mode would.

    Probably worth someone benchmarking.
     
  7. deconstruct60 macrumors 604

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    #7
    Crucial Vendor-> Product -> specific version selector doesn't show them at first but if press the "show all compatible offerings" button it shows. As long as both subsets are Unbuffered ECC and meets Apple's guidelines it should work.

    Holding onto the 1-2 DIMMs you pulled for a couple of weeks wouldn't hurt so that have something to fall back to and/or use for elimination of glitch (root cause) testing.
     
  8. Anim, Dec 30, 2013
    Last edited: Dec 30, 2013

    Anim macrumors 6502a

    Anim

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    #8
    Good post, yes, Flex comes in when dual/single channel mode is being used and allows mixed ram. From what I understand, all ram slots on the nMP will be used for 4 channel mode but 12GB (3x4) will be triple channel mode? So, if you buy 2x8 and use the 2x4 (removing one of the 4's) that would give you 24GB of total memory with dual channel on the 2 x 8 and 1 channel on the 2 x 4?

    There is a good breakdown here:
    http://www.intel.com/support/motherboards/desktop/sb/cs-011965.htm

    PS. Sorry for the edits
     
  9. Umbongo macrumors 601

    Umbongo

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    #9
    Honestly don't know. The documentation suggests the following would occur with 8, 8, 4, 4:

    However we know from the 2009 Mac Pro that there are other things possible. than Intel document (2, 1, 1 DIMMs per channel giving an interleave with dual-channel performance). It could be that 8, 8, 4, 4 would act like 8+4 on one channel and 8+4 on a second channel rather than only allowing access to one DIMM at a time.

    12GB/s read/6GB/s write or whatever the real world performance of a single DIMM is probably won't be a bother for a temporary solution although I wouldn't do it long term.

    I only have one capacity of DIMM for my workstations or I'd test it, hopefully someone will as I'm curious and it is a logical thing many will consider - getting 2x8GB DIMMs and then another 2 later on. Annoyingly if it had 8 slots it wouldn't even be an issue, you'd easily get dual-channel performance :/
     
  10. CH12671 thread starter macrumors 6502

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    #10
    So, Umbongo, you are saying that I may have to install the chips as 8,4 and 8,4 rather than 8,8 and 4,4? I guess the machine will let me know quickly if it doesn't like the way they are matched.

    And I definitely plan to hang onto the one 4 gig chip I pull out. I am going to just go with the base model, and upgrade everything but the ram. It makes more sense to me to spend 229 dollars and have 24 gigs of ram than to spend 100 dollars and have 16 gigs of ram......plus it gives me cheaper upgrade options later. I don't see myself needing more than 32 gigs ultimately.
     
  11. Umbongo macrumors 601

    Umbongo

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    #11
    Yeah, OS X may tell you how to place them when you boot with mixed capacities. Someone may have tested this before you buy anyway so keep an eye out.
     

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