4x512MB - adding 2x1GB: What is the best configuration?

Discussion in 'Mac Pro' started by ikarus79m, Oct 3, 2006.

  1. ikarus79m macrumors member

    Joined:
    Sep 30, 2006
    #1
    Hello,

    I just ordered a MacPro with 4x512MB installed from Apple and an additional 2x1GB from MaxUpgrades.com. (For a total of 4GB)

    What is the best way to install the ram on the riser cards to get the best performance for my memory scenario?

    Looking forward to your suggestions.
     
  2. robbieduncan Moderator emeritus

    robbieduncan

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    Location:
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    #2
    Put the 2x1Gb in the first slots on riser 1 and put the 2x512Mb that you take out of those slots on second pair of slots on the same riser.

    The reason for this is that for the CPUs to access memory on the second pair of slots on a riser they talk to the first pair which then forward the request onto the second pair. This adds latency. As your machine is most likely to use RAM from the "bottom" up you want to put the largest sticks in the first slots to reduce the chance of accessing the higher latency RAM.
     
  3. ikarus79m thread starter macrumors member

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    Sep 30, 2006
    #3
    So I put my two 1gig module in Slot 1 and 2 on Riser Card 1 and move the first pair of my 512MB modues in Slot 1 and 2 on my Riser Card 2, since Slot 3 and 4 on Riser card 1 will already be occupied with the other two 512MB of Ram correct?
     
  4. robbieduncan Moderator emeritus

    robbieduncan

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    #4
    Not quite. Apple should supply 4x512 populated in slots 1&2 on Riser 1 and slots 1&2 on Riser 2. These are the lower latency slots. Also the chipset has 2 memory channels, one is connected to Riser 1 and the other to Riser 2 so you get twice the memory bandwidth using slots 1&2 on both risers than you do using slots 1,2,3&4 on Riser 1.

    So you want to move 2x512 from slots 1&2 on Riser 1 to slots 3&4 on Riser 1 and insert 1x1Gb into slots 1&2 on Riser 1.
     
  5. Transeau macrumors 6502a

    Transeau

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    Location:
    Alta Loma, CA
    #5
    I don't want to step on any toes here... but...

    The 5000X chipset has 4 memory channels, not two.

    The system can run in two or four channel mode.

    If you have even amounts of the same size memory (i.e. 4 x 1GB) loaded 2 on each memory card, the system will run in four channel mode. If you have 4x 1GB and 2x 512MB, you will have an uneven amount of memory and the system will run in two channel mode.

    This chipset can support 64GB too :)

    Please see page 308 for more detail.
    http://download.intel.com/design/chipsets/datashts/31307003.pdf

    Also, keep in mind that Apple's Memory cards (the branches) only have 2 slots per channel, not the maximum supported 4 per channel. They have done this because of space requirements for the heat sinks. It does leave it open for 3rd parties to make new reisers that support more memory. (Intrus Technologies is working on this...)
     

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  6. aiongiant macrumors 6502a

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    Aug 8, 2006
    #6
    planning to buy 2x1gb of ram for my Mac Pro to add with the stock 2x512mb

    if i understand correctly to make the most out of my Mac Pro i need to get 4x1gb and ditch the 2x512mb??
     
  7. robbieduncan Moderator emeritus

    robbieduncan

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    #7
    True, but as you have to install the RAM in pairs it's easier to think of it as two channel, one going to each riser, rather than 2 channels to each riser, 1 channel going to the 1st slot on each riser and one to the second.
     
  8. Transeau macrumors 6502a

    Transeau

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    #8
    Yes, to get the maximum bandwidth (21gbps) from the memory, you need to have each riser loaded exactly the same.

    For example, I have 2x 512M and 2x 1GB in each riser. that's 4 512M DIMMS and 4 1GB DIMMS totalling 6GB of memory.

    Basicly you have have a choice of a 128-Bit or 256-Bit memory path.
     
  9. ikarus79m thread starter macrumors member

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    Sep 30, 2006
    #9
    Honestly, how much of a difference will it make? I am just trying to figure that out because I'm not sure how many more hits my wallet can take... ;-)

    Are we talking about a pretti big improvement in the 10-20% range or just a minor nudge around 1 or 2%?
     
  10. technicolor macrumors 68000

    technicolor

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    #10
    In my machine

    I have on riser 1: 2x512
    on riser 2: 2x1

    Are you saying that I should have

    riser 1: 1x512 and 1x1
    riser 2: 1x512 and 1x1
    ?
     
  11. aiongiant macrumors 6502a

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    #11
    i don't think that would work since the rams needs to be in pairs
     
  12. Transeau macrumors 6502a

    Transeau

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    #12
    Yes, it is big.

    Take a 4GB example. Two 1GB DIMMd and Four 512MB DIMMs = 4GB.
    However, they will be in 3 sets. (Riser 1 having the four 512's and Riser 2 have the two 1G's) This will run in dual channel mode. Each channel has 5.325gbps so you have a total max throughput of 10.650gbps.

    If you have Four 1GB DIMMs (Each Riser having 2 DIMMS) you will be in four channel mode. (Identical config of each riser). 5.325gbps x 4 = 21.3gbps Max.

    that literally translates to a 100% improvement.

    BT
     

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  13. ikarus79m thread starter macrumors member

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    Sep 30, 2006
    #13
    But what if I have the 4x512MB spread out over Riser board 1 and 2 (Slots A and B) and then add the additional 2x1GB on Riser board 1 C and D slots? Shouldn't that give me four channel mode?
     
  14. aiongiant macrumors 6502a

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    Aug 8, 2006
    #14
    if i have the 2x512mb stock ram

    adding another 2x512mb is better than 2x1gb?
     
  15. Transeau macrumors 6502a

    Transeau

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    #15
    No, that will drop it back in to Dual Channel mode. You have nothing in channel 3 and 4 on the board 2.
     
  16. Transeau macrumors 6502a

    Transeau

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    #16
    That is correct. You will have the fill 21.3Gbps bandwidth with for 512M. Where you will only have 10.65Gbps if you add 2x1GB.

    Does this help any?

    Code:
           |  Upper Riser  |  Lower Riser
    ---------------------------------------
    ROW 1  |  CH-1 Slot-1  |  CH-3 Slot-1
    ROW 2  |  CH-2 Slot-1  |  CH-4 Slot-1
    ROW 3  |  CH-1 Slot-2  |  CH-3 Slot-2
    ROW 4  |  CH-2 Slot-2  |  CH-4 Slot-2
    If "Slot 1" of Channels 1,2,3 & 4 have identical memory in them, you are at 256-Bit (21.3Gbps)

    If "Slot 1" of Channels 1 & 2 have 512M and Slot 1 of Channels 3 & 4 have 1GB in them, you are at 128-Bit.

    If "Slot 1 & 2" of Channels 1 & 2 have 512M you are at 128-Bit.

    If "Slot 1" of Channel 1 have 512M you are at 128-Bit.

    In the simplest terms; If your risers DO NOT MATCH, you are at 128-bit.
    If your risers MATCH, you at at 256-Bit.
     
  17. aiongiant macrumors 6502a

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    Aug 8, 2006
    #17
    thanks Transeau! good thing i didn't buy my ram yet!

    now gotta find the best place to buy 2x512mb

    Maxupgrades sells them for $239
    so i think there the cheapest ones with the proper heatsinks
     
  18. Transeau macrumors 6502a

    Transeau

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    #18

    Try Tiger Direct. I got the normal ones... no 5lb heat sinks. They work perfect. I just spent the last 9.5 hours compressing several hours of HD video without a single memory error. The four 3.0GHz cores were hitting over 155F.
     
  19. milozauckerman macrumors 6502

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    Jun 25, 2005
    #19
    512MB DIMMs are single-rank - that gives you half the bandwidth if you read the information above that graphic. You reach your full 21.x bandwidth by filling the eight available slots with dual-rank DIMMs (hence 16 green bars)

    4x512 MB gives you 5.3 GB/s total (2.65 each on two risers).
    2x1GB gives you the same bandwidth on a second riser - and you'll still be running at the .

    The first important benchmark to hit with the Mac Pro is four DIMMs over two - that gives you the full 256-bit memory bus no matter what you do from there on.

    The next biggest benchmark is filling all eight slots with 1GB or 2GB DIMMs (dual-rank), reaching your maximum theoretical bandwidth at the cost of high latency.

    In between, you suffer no performance penalties (latency aside) from having 4x512 + 2x1GB or 2x512 + 2x2GB or whatever combination you want to come up with..
     
  20. aiongiant macrumors 6502a

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    Aug 8, 2006
    #20
    so 4x1gb is better than 4x512 casue of the double rank?
    or will it be the same? as long as it's 256bit?
     
  21. milozauckerman macrumors 6502

    Joined:
    Jun 25, 2005
    #21
    4x1GB is better primarily because it's twice as much RAM. But yes, it should come closer to maxing out the potential bandwidth.

    In real-world practice, performance gains would be most noticeable simply from having 4GB vs. 2GB.
     
  22. aiongiant macrumors 6502a

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    Aug 8, 2006
    #22
    aside from having more ram
    is having 4x1gb vs 4x512mb the memory bandwidth diff is how much more? and is it even noticeable?
    casue i don't think i will ever use 4gb of ram to do anything haha

    the price diff is quite a bit from 2gb total to 4gb
     
  23. amtctt macrumors regular

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    Sep 8, 2006
    #23
    maybe a better question is what is better: 4x1gb or 8x512mb which would fill all the slots. Anyone?
     
  24. apoartar macrumors newbie

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    Sep 30, 2006
    Location:
    Hawaii
    #24
    Yes, this is my question also, except that I'm looking at the choice of 4x2GB vs 8x1GB. I realize that with the 8x1GB there wouldn't be any further expansion options, but I'm curious to know if there is any performance difference.
     
  25. milozauckerman macrumors 6502

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    Jun 25, 2005
    #25
    Again, at that point 'bandwidth' isn't the issue - you're going to realize performance gains simply from having the RAM in the first place.
     

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