1066Mhz on a 3.46Ghz 12 Core?

Discussion in 'Mac Pro' started by sammyman, May 11, 2015.

  1. sammyman macrumors 6502a

    Mar 21, 2005
    I was hoping once I upgraded my processors to 3.46Ghz, that I would be able to utilize all 1333Mhz of RAM. It is still showing 1066Mhz. Any ideas?

    Attached Files:

  2. MacVidCards Suspended

    Nov 17, 2008
    Hollywood, CA
    Ram has to al be 1333 and you need to do a PRAM reset
  3. sammyman thread starter macrumors 6502a

    Mar 21, 2005
    Does it have to all be matching? I believe all my RAM is 1333, but 2 of the sticks aren't matching.

    Doing another PRAM reset now and pulling everything out to verify.
  4. sammyman thread starter macrumors 6502a

    Mar 21, 2005
    So I thought for sure they were all 1333. It appears after testing that these 3 sticks of RAM are all 1066.

    Is it better to run 48GB on 1333 or 64gb on 1066?

    I guess I'll have to eBay these sticks and find more RAM if I want the full 64GB at 1333.

    Attached Files:

  5. rusinu macrumors newbie

    May 11, 2015
    These 3 sticks are 1066 only and 24GB.
    Better is when you put ram to slots: 1, 2, 3, 4-must be free. Then you have tripple-channel memory.
    When you put RAM to all 4 slots, you have only double channel memory.
    It does not matter the speed of memory, 1066 is only 2-3% slower then 1333.
    It's better to run 48GB (3 slots) at 1333.
  6. sammyman thread starter macrumors 6502a

    Mar 21, 2005
    Thank you! I don't understand the difference between double channel and triple channel, but I'm going to listen to your advice and keep 1-3 and 5-7 installed, leaving 4 and 8 open.
  7. h9826790 macrumors G5


    Apr 3, 2014
    Hong Kong
    Same situation here. When running 3 sticks, I got 1333, but only 1066 when use all slots. (All 4 sticks are the same)

    Anyway, after some tests. It's no way to tell the difference in speed for normal real world usage.

    For benchmarking, yes, triple channel will shows better result. However, for real world usage, more RAM usually will win (because more cache avail for the system).

    So, if 48G is good enough for you, of course you can run them in triple channel configuration. However, if you able to use all of them, that means go for 64G is the better choice. Otherwise, if the system start to use HDD because running out of RAM, that will be 1000 times worst than running the RAM in 1066.
  8. benjaprud, May 11, 2015
    Last edited: May 12, 2015

    benjaprud macrumors member

    Apr 9, 2015
    If you use 4 sticks of 1333MHz 1,35V (low power) RAM it will run at 1066MHz and 2-way interleave (3 sticks should run at 1333MHz), if you use quad-rank RAM it will run at 1066MHz and 4 sticks will run at 800MHz and 2-way interleave whatever the frequency supported by the RAM. 4 identical sticks of single or dual-rank memory at 1333MHz/1,5V should run at 1333MHz (and again 2-way interleave). More information here.
  9. 666sheep macrumors 68040


    Dec 7, 2009
    Nope. It does not work like this.
    When all slots are populated with identical sticks you still have triple channel, only bandwidth between slots 3 and 4 is shared. Dual channel is when stick are in 2 slots. It was said many times before BTW ;)
    And IMHO it's better to have a sufficient amount 1066 RAM than insufficient 1333 :D
  10. iamMacPerson macrumors 68030


    Jun 12, 2011
    Ok this is interesting because I tested this theory in my 4,1->5,1 machine. It sees my DIMMs as 1333MHz no matter how many I have in, but with three DIMMs it actually scores higher then with four even though I have more memory with the latter.

    These are OWC DIMMs too, and supposed to be for the 5,1 MP.
  11. Umbongo macrumors 601


    Sep 14, 2006
    This is how it works with the Mac Pro and 4 DIMMs.

    It's slightly faster than dual-channel bandwidth, but of course you likely have more capacity so it's basically irrelevant in the real world, as is 1333MHz vs 1066MHz. On a dual CPU system remember you are often going to be access memory from one CPU with the other also so QPI interconnect is also a factor - but again irrelevant for creative workstation usage.

    With 1.5v 1333MHz DIMMs you should get that speed no matter how many DIMMs you use.


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