cMP RAM Matched Sets

Discussion in 'Mac Pro' started by Bollockser, Jun 12, 2017.

  1. Bollockser macrumors regular

    Joined:
    Oct 28, 2014
    #1
    Hi I have a 2x2.26 Quad 2009 Mac Pro 4,1>5,1

    My question is, if I purchase 48GB now (16x3), a matched set of 3, can I later on buy another 48GB matched set of 3 to install in the other processor tray, and retain all the benefits of matched sets (and tri-channel)?

    Thanks!
     
  2. AidenShaw macrumors P6

    AidenShaw

    Joined:
    Feb 8, 2003
    Location:
    The Peninsula
    #2
    It's pretty much a myth today that "matched sets" of DIMMs are needed or useful. It's a marketing ploy for memory manufacturers to pull three DIMMs out of the box and sell them for more $ than pulling three DIMMs out of the same box and selling them as three separate items. (Normally there's a quantity discount - but "matched set" creates a quantity premium.)

    Of course, having identical or very similar specs for DIMMs on a channel can be useful - but it doesn't matter if you put a Hynix and a Crucial and a Samsung on the same channel if the specs match.

    Systems today are pretty good at dealing with memory DIMM mismatches. They'll look at the DIMMs on a channel and figure out the maximum performance that all DIMMs can hit, and downclock the faster DIMMs to the capability of the slower DIMMs.

    There are some odd issues (like the cMP having 8 DIMMs in a tri-channel setup), but in general it's far, far better to have DIMMs in a sub-optimal configuration than to have memory pressure causes hard paging to the pagefile.
     
  3. Bollockser thread starter macrumors regular

    Joined:
    Oct 28, 2014
    #3
    OK very good. I just dont want to drop the $530 in one lump sum.
    So, 48GB twice it is!
    Thank you.
     
  4. orph macrumors 65816

    Joined:
    Dec 12, 2005
    Location:
    UK
    #4
    iv never had a problem with mixed ram, and most pleases you can return if needed
     
  5. jscipione macrumors newbie

    jscipione

    Joined:
    Mar 27, 2017
    #5
    Are there and speed benefit to using sets of 3 sticks as opposed to sets of 2 or sets of 1 to feed the tripple channel memory controller?

    I got 2 8gb OWC sticks in mine now and I'm waiting to get a 3rd 8gb HP stick for my single proc model 5,1.
     
  6. h9826790 macrumors G3

    h9826790

    Joined:
    Apr 3, 2014
    Location:
    Hong Kong
    #6
    Yes, if you only consider the memory performance. e.g.

    4x8GB, memory performance, 1927 single, 2059 multi Screen Shot 2018-04-04 at 16.40.31.jpg
    3x16GB, memory performance, 2421 single, 3177 multi
    Screen Shot 2018-04-04 at 16.40.37.jpg
    So, yes, use 3 sticks (optimised config) gives significant better pure memory performance.

    However, if you look at the overall performance.

    4x8GB, 2787 single, 15823 multi
    4x16GB, 2892 single, 16613 multi

    The difference is very insignificant. And this is already a benchmark tool. For most real world task, the performance penalty will be even less.

    So, for memory on cMP, even 3 sticks is the optimum config. I am quite sure for most scenario, still the more the better.

    Especially the modern OS is designed to use the free RAM as cache to speed up the system.
     
  7. MarkJames68 macrumors 6502

    MarkJames68

    Joined:
    Sep 24, 2017
    #7
    Looks like you’re comparing 2 different speeds of RAM too. The difference between 3 and 4 sticks with the same speed should only be 2% or so.
     
  8. h9826790 macrumors G3

    h9826790

    Joined:
    Apr 3, 2014
    Location:
    Hong Kong
    #8
    Those 8GB sticks actually are all 1333MHz, but when installed in all 4 slots, will slow down to 1066 (most likely due to memory rank issue).

    I agree that’s not a good comparison. Different sticks, different speed, different size.

    But AFAIK, even you use all identical 1333MHz DIMM for both test, the memory score on the 3 sticks one should still noticeable higher than the 4 sticks one. Way more than 2%. However, the overall score should be roughly just 2% difference.

    You may try and please prove me wrong. In fact, I am on the side that using all 4 slots are better (In real world). But just the benchmarks shows memory performance (ONLY memory performance) can up to 50% difference. In real world, because we have cache etc, this performance penalty should be very insignificant overall.
     
  9. MarkJames68 macrumors 6502

    MarkJames68

    Joined:
    Sep 24, 2017
    #9
    Interesting about the memory rank issue. In the nMP if you have an 8 or 12 core and 128GB I believe the top speed goes from 1866 to 1066.
     
  10. bsbeamer macrumors 6502a

    Joined:
    Sep 19, 2012
    #10
    If you need (and utilize) large amounts of RAM, there's not really another choice.

    Running 128GB on my 5,1 with dual 3.46 at the moment. Reports at 1333MHz and have reached above that mythical 96GB usage threshold several times. Regardless of what benchmarks say, I have kept the 128GB in because it's being used. If your RAM usage is below 96GB, there is probably an argument to be made.
     
  11. orph macrumors 65816

    Joined:
    Dec 12, 2005
    Location:
    UK
    #11
    compared to say the CPU the ram speed will make almost no difference for 99% of tasks and any task so important that you need 2% extra speed will want a computer that is not 8 or so years old

    + as mentioned the benefit from more ram tends to out way any other slowdown
     

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