Dual channel access

Discussion in 'iMac' started by phobos, Feb 4, 2013.

  1. macrumors regular

    Joined:
    Feb 25, 2008
    #1
    Hey guys
    I was wondering how the memory on the 27" iMac needs to be installed in order to take advantage of the dual channel speeds.
    In my old MacPro we had to do it in pairs. Do I need to do that on the iMac as well!
    I'm going to keep the 8GB (which I saw is 2x4GB) and also install 2x8GB.

    Should I install them as 4-8 4-8 or as a 4-4 8-8?

    Thanks in advance for your answers.
     
  2. bigus7674, Feb 4, 2013
    Last edited: Feb 4, 2013

    macrumors member

    Joined:
    Jan 4, 2005
    #2
    Dual-Channel Memory Info

    Hope this helps (from another post and definition from Crucial):

    2x2 + 2x4 is fine so long as you populate the banks correctly. Both memory stick in the same BANK should match in order to get dual channel support. The BANKs are arranged horizontally.

    Install it as:
    --2gb-- --2gb--
    --4gb-- --4gb--

    Do NOT install it as:
    --2gb-- --4gb--
    --2gb-- --4gb--

    What is dual-channel memory?
    The terminology "dual-channel memory" is being misused by some in the memory industry, which can mislead the consumer. The fact is there's no such thing as dual-channel memory. There are, however, dual-channel platforms.

    When properly used, the term "dual channel" refers to the DDR or DDR2 chipset on certain motherboards designed with two memory channels instead of one. The two channels handle memory-processing more efficiently by utilizing the theoretical bandwidth of the two modules, thus reducing system latencies, the timing delays that inherently occur with one memory module. For example, one controller reads and writes data while the second controller prepares for the next access, hence, eliminating the reset and setup delays that occur before one memory module can begin the read/write process all over again. Think of it like two relay runners. The first runner runs one leg while the second runner sets up and prepares to receive the baton smoothly and carry on the task at hand without delay. While performance gains from dual-channel chipsets aren't huge, they can increase bandwidth by as much as 10 percent. To those seeking to push the performance envelope, that 10 percent can be very important.

    If you have a dual-channel platform and you want to take advantage of the performance gain it offers, our advice is to simply purchase your DDR or DDR2 memory in pairs. However, be very careful to order two modules with the exact same specifications; the modules must be identical to each other to perform correctly.
     
  3. macrumors 6502a

    turtlez

    Joined:
    Jun 17, 2012
    #3
    the pre installed RAM from Apple is in the right spot for dual channel already so just put your other two sticks you buy from 3rd party in the empty slots :)

    I have the same as you btw - 24GB

    [​IMG]
     
  4. phobos, Feb 4, 2013
    Last edited: Feb 4, 2013

    thread starter macrumors regular

    Joined:
    Feb 25, 2008
    #4
    Thanks guys for your answers.
    Apple's ram diagram is very confusing! But since I need to just install them on the empty slots and compare it to the diagrams and photos from you guys its going to be an easy job.
    It's just that Apple's memory snapshot is so confusing! I didn't know how I was supposed to read it! As rows or as columns?
    Now, if only the iMac could get here sooner!
     
  5. macrumors regular

    Joined:
    Sep 20, 2007
    #5
    What I don't understand is that if you install the RAM this way, in the system report, it shows the matching pairs as being in different banks for the same DIMM number. Is that the way it's supposed to be?

    I somehow thought that the matching pairs should be on the same banks with different DIMM numbers for each. Do I have this wrong?

    edit: screen shot of my setup. is this wrong? how do you guys know what the right configuration is?

    [​IMG]
     
  6. macrumors 6502a

    turtlez

    Joined:
    Jun 17, 2012
    #6
    andddd now i'm confused. Seems half the people on this forum tell you to do it one way and the other half say opposite lol
     
  7. macrumors regular

    Joined:
    Sep 20, 2007
    #7
    Sorry, didn't mean to confuse anyone. I actually don't know the correct way to do it. Does anyone have an explanation of why you should do it one way vs. the other? I feel like I haven't read anything concrete. I don't really have time to call AppleCare to ask about it though.
     
  8. macrumors 6502a

    turtlez

    Joined:
    Jun 17, 2012
    #8
    I am in the same boat as you but I remember about a month ago reading about it and the majority of people seemed to agree that the way Apple had their memory installed when it ships is already set for dual channel so you just use the empty slots.
     
  9. thread starter macrumors regular

    Joined:
    Feb 25, 2008
    #9
    I've also seen both versions claiming dual channel depending on the website.
    I really don't know what to believe!!
    Is there any way we can test both versions to see what works and what doesn't?
     
  10. macrumors 6502a

    turtlez

    Joined:
    Jun 17, 2012
    #10
    If you do find a test for memory you would have to see which combination clocked faster
     
  11. macrumors 6502a

    Joined:
    Oct 29, 2008
    #11
    Just populate the slots that Apple left empty. You will get dual channel.

    The confusion stems from the definition of bank vs channel. Difference is like row vs column in a grid.
     
  12. macrumors 6502

    NYY FaN

    Joined:
    Jun 22, 2009
    Location:
    New York
    #12
    I read some place to remove the existing 2x4GB, insert the new ram (in my case 2x8GB) and insert the old 2x4 into the previously empty slots.

    so from top to bottom my config is:

    4
    8
    4
    8

    I got the imac as:

    -
    4
    -
    4

    I haven't tried it in reverse yet..
     
  13. macrumors 6502a

    turtlez

    Joined:
    Jun 17, 2012
    #13
    I don't think it would change anything if you had it as (top)8484(bottom) or 4848. The only time it is wrong is (top)4488(bottom) and 8844
     
  14. macrumors member

    Joined:
    Sep 3, 2007
    Location:
    United Kingodm
    #14
    OK, the memory bay on the back of the 27" is configured in slots:

    1
    2
    3
    4

    Slots 1 & 3 were already populated by Apple.
    Adding a MATCHING PAIR of memory sticks to slots 2 & 4 will allow dual channel to work properly.

    Essentially the total RAM in slots 1 & 2 must equal the total RAM in slots 3 & 4
     

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