About RAM slots used in 4-,8-, and 12 core machines..

Discussion in 'Mac Pro' started by MrAverigeUser, Feb 19, 2016.

  1. MrAverigeUser, Feb 19, 2016
    Last edited: Feb 19, 2016

    MrAverigeUser macrumors 6502a


    May 20, 2015

    There is no question about the fact that 4- and 8-core classic MacPros should have a configuration with just 3 of4 RAM slots used ... So "3of4-slots-configuration is prferable, BUT:

    I remember the argument why a 6-core or a 12-core MP would profit from 4of4-configuration:

    The Xeons ar able to create a virtual 3rd core. To max that out, you need a sum of real hardre cores that you can divide by 3 so that any pair of the real existing cores can create and use a third one. if this is not the case, the 4th occupied slot will only brake instead of helping the system.

    -----> could someone with a 12-core MBP help,us tomverify this?
    Would be easy for him:

    Just run a performance test with both configurations and report the result....
    Imthinkmthis would be very interesting for a lot of people.

    As for "as much RAM as possible is the best and not enough RAM in 3 slots is worse than enough RAM in 4 slots" :

    i agree, having 32 GB of RaM since I purchased my MP with it (yes, I was really that naive and burned money because at that time this was horribly expensive...)

    Last question:
    Does it matter which of the 3 slots are used for RAM or not?

    Thanks in advance​
  2. h9826790 macrumors G4


    Apr 3, 2014
    Hong Kong
    Use the 4th slot with my W3690 (6 cores) actually will decrease the memory performance (proved in Geekbench).

    So, the magic of multiple of 3 won't work. I am pretty sure it's the same on the 12 cores, but I can't test it by myself.

    And yes, which 3 slots does matter. The 3rd and 4th slots share the same channel. Therefore, for optimum performance, you can to put the RAM in only slot 1,2, and 3. Also, AFAIK, the 4th slot only work when there is a RAM stick in slot 3.

    Since the 3rd and 4th slots are sharing the same channel, no matter how advance the CPU, how many cores it has. I believe use both slots can only result in decrease in memory performance.
  3. r6mile macrumors 6502

    Feb 3, 2010
    London, UK
    Sorry if this sounds naive but what's the point of Apple putting in a fourth RAM slot then?
  4. MacUser2525 macrumors 68000


    Mar 17, 2007
    There was none just marketing BS to claim higher ram capacity it should have been six slots like every other manufacturer correctly did ie. two triple channel channels or twelve on the dual core machines.
  5. jerwin macrumors 68020

    Jun 13, 2015


    Sometimes the performance increase associated with having enough memory for the task at hand outweighs the slight performance decrease associated with using 4 dimms.
  6. MrAverigeUser, Feb 19, 2016
    Last edited: Feb 19, 2016

    MrAverigeUser thread starter macrumors 6502a


    May 20, 2015

    Thanks for sharing ! :)

    I did only mention the 12-core and forgot to mention the 6-core (added the 6-core MB).
    So if the hypothesis I presented in my first posting is right, the 6-core should perform more either with 4 slots…

    Thank you as well.

    I add one more page about RAM-upgrade for MacPro in this article:


    What I didn´t know yet:
    Everybody talked bout performance-loss with 4/4-slot configuration compared to 3/4-slot configuration…. in fact, the difference in performance (in use of PS5 which was not capable at that time - in 2011 - to use all the cores) was only ridiculous 2,7% !!!

    Nowadays apps may use more cores than old apps - so the results in performance could (!) perhaps be the opposite, as well as for more than 24 GB RAM….

    here the citation of the article (INFO: In the article 24GB RAM = 3x8 GB, 32 GB RAM = 4x8GB) :

    "ENOUGH memory means that the particular task involved has its needs fully satisfied with memory to spare for other system activities.

    Using 32GB instead of 24GB reduces performance by 2.7%. That’s because 24GB is already ENOUGH, and 3 X 8GB runs in triple channel mode, yielding faster memory access times the dual-channel mode used with 4 X 8GB.

    HOWEVER, if the problem size were to increase to about 24GB or larger, then the 32GB configuration would be much faster, because the ENOUGH criterion would not have been satisfied with 24GB."

  7. dimobr macrumors member

    Feb 14, 2016
    With triple channel enabled the performance gain is so small that in the end it makes no difference (at least for me). Should be somewhere between 5 ~ 7%.
    I'm currently with 24GB (6x4) but soon i want to upgrade to 64GB (8x8), in my case having more memory is more important.
  8. MrAverigeUser thread starter macrumors 6502a


    May 20, 2015

    I think this is right.
  9. MrAverigeUser thread starter macrumors 6502a


    May 20, 2015
    Thanks - but these guys tested Ram-performance, not overall-performance or with real-life apps - interesting though.

    Your 12-core-configuration is then 6x8 GB? Did you test yourself also 8x8 GB Ram-config?

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