MacPro5,1 RAM configuration question

Discussion in 'Mac Pro' started by whwang, Dec 20, 2017.

  1. whwang macrumors regular

    Joined:
    Dec 18, 2009
    #1
    Hi,

    I have a 2010 MacPro5,1, with six 16GB RAM modules in it. Recently I found one of them is broken, undetectable by the system. OWC told me that it’s better to return all the six modules and get them replaced together, to ensure they are mateched.

    I plan to wait until my iMacPro arrives and then return the RAM for replacement. This is because if I return them now, I will be left with only 24GB of RAM (those originally came with the MacPro) for a substantial period. And this won’t be enough.

    So in short, I will keep running my MacPro for a while with the five good 16GB modules. My question is, should I install all five of them, or just four (two for each CPU)? 80GB of RAM and 64GB aren’t that different for me, at least for a while. Will there be any issue if the two CPUs have different amounts of RAM modules installed for them?

    Thanks.
     
  2. h9826790 macrumors G3

    h9826790

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    #2
    80GB should be fine
     
  3. bsbeamer macrumors 6502a

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    Sep 19, 2012
    #3
    I do remember YEARS ago that it was recommended to keep the same number of sticks per CPU for a dual processor. Unsure if that's still the case, but keep in mind in case you do run into any problems. (Doubt there will be any major issues.)

    If you see a red light through the front grates it means there's an error. Had a bad stick that I was unaware of for at least a few weeks...
     
  4. whwang thread starter macrumors regular

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    Dec 18, 2009
    #4
    Thanks. I even remember for this generation of MacPro, three RAM modules per CPU can be faster than four or two. But I don’t know exactly why. I think my memory about this makes me wonder if I should install three for one CPU and two for the other. What will happen if the two CPU/RAM combinations are running at slightly different speeds? Can this drag the overall performance down?
     
  5. bsbeamer macrumors 6502a

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    Sep 19, 2012
    #5
    In theory, it could cause a bottleneck which would slow the system down. That would likely require both processors being actively under load and unable to "keep up" at the same speed, which would also require your given application(s) actually utlizing dual-CPUs properly. Hard to know for sure if this will happen or not.

    I know for sure the 6-core single processor Xeon MacPro's are at 48GB RAM limit, which is a limitation of the CPU itself. In a single configuration, this is not a problem. Personally, I would go with two & two.
     
  6. h9826790 macrumors G3

    h9826790

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    #6
    The max is 56GB (3x16+8) at this moment, not 48GB. Also no one try the 32GB DIMM yet, those hex core processor may able to boot with more than 56GB RAM.
     
  7. pl1984 macrumors 6502a

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    Oct 31, 2017
    #7
    The processors use triple memory channels meaning it can access three modules at a time. Thus a three module configuration is optimized to the processor.
     
  8. bsbeamer macrumors 6502a

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    Sep 19, 2012
    #8
    Have you run 3x16 + 8 in your system yet?

    FYI - this is specifically posted at OWC for their RAM modules:
    OWC 16GB modules require that all installed modules be of the same p/n OWC 16GB modules. Other existing Apple or 3rd party 1GB, 2GB, 4GB and 8GB modules are not supported for use with these kits and need to be removed when these modules are added.

    Have not seen 32GB modules stocked anywhere that include MacPro5,1 compatibility. There's a few off-brand names on Amazon, but prices are too steep for an experiment.
     
  9. h9826790 macrumors G3

    h9826790

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    #9
    It's been tested by members here.

    Dual processors model can go up to 160GB RAM (5x32).

    Anyway, none of Apple, Intel, and OWC get this limitation right. And I may treat Apple or Intel's limitation as the official information. But OWC's info means absolutely nothing. Their 16GB DRIMM may not work with other size DIMM is because they never told you RDIMM cannot work with UDIMM, and they sell 16G RDIMM, but 1,2,4,8 UDIMM.
     
  10. kohlson macrumors 65816

    Joined:
    Apr 23, 2010
    #10
    Optimal *memory* speed comes from 3 sticks per CPU, installed correctly. IIRC, there is a 5-7% performance penalty (memory, not overall throughput) for installing 4 sticks per CPU. The presumption here is that the user installs exactly the amount of memory needed (remember, these are server parts). But if the workload requires more memory than is available from the 3 sticks, adding a 4th stick is desirable over hitting swap.
    In my observation, 5,1 MPs are somewhat flexible on what is installed. A friend has a dual-socket system with 15 GB installed. 3x4 plus 3x1. Before I met him I would have bet more than a beer this wouldn't work. A mutual friend recently upgraded to 32 GB, and gave him his 3x4GB. So hope is on the horizon.
     
  11. h9826790 macrumors G3

    h9826790

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    #11
    The Mac Pro is very flexible on RAM indeed.

    https://forums.macrumors.com/threads/can-i-mix-memory-size.1903411/#post-21638766
     
  12. orph macrumors 65816

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    #12
    the potential gains from 3 channel are lost by most real life use and if you ever go over your installed ram then the slowdowns are massive, stick it all in.
     
  13. fendersrule macrumors 6502

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    Oct 9, 2008
    #13
    "If" you ever, I guess. For me, 24GB of Ram whilst leaving a slot open is optimal since I've yet to come close to using it all.
     
  14. orph macrumors 65816

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    Dec 12, 2005
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    UK
    #14
    @fendersrule use case is important, whwang is talking about 80GB of RAM so he must be using some of the few apps that actually eat ram which means more is better.

    use case is relay important, i have 32GB of ram and at times i can go past that with video work on more complex projects but if i ever look at AE then my ram just fills in no time.

    id gess as use case is not given it's valid to point out but for some workloads it is needed to have large amounts of ram
     
  15. whwang thread starter macrumors regular

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    Dec 18, 2009
    #15
    Yes, I once hit a point that even my old 96GB RAM was not enough. Then I decided to re-write my program and split the job. It's less ideal, but at least it did not take forever to complete the jobs. Swapping the HD and even the SSD is much slower.

    That does not happen often, though. Hopefully I won't need to face it again until my 128GB iMac Pro arrives. In the mean time, I think I can accept either 80GB (3*16 + 2*16) or 64GB (2*16 + 2*16). It seems that the 64GB case will give a small speed advantage, based on what people said here, as long as the RAM usage does not exceed 64GB.
     
  16. h9826790 macrumors G3

    h9826790

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    #16
    Just try both config in Geekbench, check the memory score, then you will know if only run 4x16 is faster than 5x16. If there is a slow down, the score difference should be very significant (way outside normal error margin). e.g.

    3x16GB is like this
    3x16.jpg
    and 4x8GB is like this
    4x8.jpg
    As you can see. The multi-core memory performance will be 50% faster if only use 3 slots but not 4. Of course, the real world performance difference will be much much smaller than this because we are not always memory bandwidth limiting, also the system should use cache normally. However, it’s a good indicator if you want to know which RAM configuration is “faster”.
     
  17. whwang thread starter macrumors regular

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    Dec 18, 2009
    #17
    Stupid me. Why I haven't thought about giving it a test?
     
  18. whwang thread starter macrumors regular

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    Dec 18, 2009
    #18
    Let me report what I got. I ran Geekbench 4.2 64bit. Here are the key scores.

    ---------------------------------------------------
    64 GB (16GB * 2 + 16GB * 2)

    Single-core score: 2641
    Crypto Score 1721
    Integer Score 3145
    Floating Point Score 2357
    Memory Score 2164
    Memory Copy 1870 5.18 GB/sec
    Memory Latency 4398 98.4 ns
    Memory Bandwidth 1233 6.59 GB/sec

    Multi-core score: 21599
    Crypto Score 13889
    Integer Score 28412
    Floating Point Score 24313
    Memory Score 4123
    Memory Copy 4864 13.5 GB/sec
    Memory Latency 4413 98.1 ns
    Memory Bandwidth 3267 17.4 GB/sec
    ---------------------------------------------------
    80 GB (16GB * 3 + 16GB * 2)

    Single-core score: 2652
    Crypto Score 16u4
    Integer Score 3173
    Floating Point Score 2358
    Memory Score 2163
    Memory Copy 1711 4.74 GB/sec
    Memory Latency 5816 74.4 ns
    Memory Bandwidth 1018 5.44 GB/sec

    Multi-core score: 14244
    Crypto Score 3616
    Integer Score 18151
    Floating Point Score 18318
    Memory Score 1997
    Memory Copy 1630 4.52 GB/sec
    Memory Latency 3500 123.7 ns
    Memory Bandwidth 1397 7.46 GB/sec
    ---------------------------------------------------

    You can see that the 80GB configuration gets substantial punishment in memory categories, especially the multi-core ones. I think I will stay with 64GB, wait for my iMacPro to arrive, and have OWC replace all the six 16GB modules.
     

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