Previous 1 2 Next 22 Replies Latest reply: Dec 14, 2012 5:35 PM by Saxman
Saxman Level 2 (175 points)

I recently upped my RAM from 8 GB to 12 GB. System Profiler shows I have 12, and I ran the Hardware Test, and it didn't say anything was wrong. However, since installing it, it seems if anything, my computer is running more slowly, especially my Safari. Would I not be able to tell the difference in speed of windows opening, functioning of programs, etc?

iMac, Mac OS X (10.6.8), 21.5" ,3.2 GHz i3, 12GB Ram
  • Kappy Level 10 (265,914 points)

    Your problem isn't due to the RAM which is apparently working fine.

  • BobHarris Level 6 (17,699 points)

    Do you have matched pairs of DIMM cards installed?


    Accessing RAM has 2 phases.  The first is draining the memroy cell to see if it had anything in it, and the second is refilling the memory cell now that it has been drained.


    If RAM DIMMs are installed as matched pairs (where matched means same size and technical specs), then the hardware will arrange for the RAM to be accessed first from one DIMM, then while it is restoring the drained values, it will access the paired DIMM.  This alternating between DIMMs allows for more data to be transferred from RAM to the CPU at a faster rate.


    If the DIMMs are not matched, the hardware will access RAM serially, waiting between each read for the previous read to refill the drained memory cells.


    Matched pairs does not double speed, as the CPU is not always accessing RAM, as the CPU has its own on-chip cache to defer going to RAM, but it does improve performance.


    If you did not properly pair your DIMMs when you installed them, that might explain some of your issues.


    Or I could be totally wrong, and you are experiencing some other problem.

  • Saxman Level 2 (175 points)

    What do you mean by "matched pairs"? The same sizes, same brand, position in the computer?

    I have one parir of 2GB, and one pair of 4GB, with one of each loaded into the two sides of the RAM storage space in the computer.

  • Saxman Level 2 (175 points)

    OK, if it's fine, why did I notice some slowing down, and no speeding up, once I installed the memory?

  • BobHarris Level 6 (17,699 points)

    What do you mean by "matched pairs"? The same sizes, same brand, position in the computer?

    A match pair are both the same size and have the same specs speed, DDRn etc...).  They do not need to be the same manufacture.


    I have one pair of 2GB, and one pair of 4GB, with one of each loaded into the two sides of the RAM storage space in the computer.

    Applications -> Utilities -> System Profiler -> Memory


    You will see


    BANK 0/DIMM0:  nn GB

    BANK 1/DIMM0:  nn GB

    BANK 0/DIMM1:  nn GB

    BANK 1/DIMM1:  nn GB


    Bank 0 should have a matched pair.

    Bank 1 should have the other matched pair.


    If they are not matched, then you will not get the benefit of memory interleaving.

  • Kappy Level 10 (265,914 points)

    The top two slots should have a matched pair - same size. The bottom slot another matched pair - same size.


    upper slots: 2+2

    lower slots: 4+4

  • Saxman Level 2 (175 points)

    Well, you and BobHarris are saying contradictory things. I have it the way you show it, altho it's the 4s on top, and 2s underneath. However, he says it should be the 2s & 4s vertically, not horizontally.
    So, who's right? I suppose I could change the position on my computer, and see if putting them vertically helps.
    However, another issue are these too tiny screws! Mine are about completely stripped, and I probably should get new ones before trying another in and out... Do others have the same issue with them?

  • Kappy Level 10 (265,914 points)

    Look in your computer's user manual. It's shown there as I've stated. Slot labeling is confusing.

  • BobHarris Level 6 (17,699 points)

    Well I could be wrong.  Wouldn't be the first time.


    You could google "iMac memory banks" and you will most likely find some installation videos and maybe some guides on how to install non-equal sized RAM DIMMs

  • Kappy Level 10 (265,914 points)

    You are right, but the table is a little confusing for the less technically minded.


    I think it's because there are four banks. Bank 0 and DIMMA and Bank 0 and DIMMB are the upper slots. Bank 1 and DIMMC and Bank 1 and DIMMD are the lower slots. For memory interleaving DIMM A and DIMM B should be a matched pair as should DIMM C and DIMM D. However, the upper pair do not have to match the lower pair.


    So, as read your table it's correct.

  • Saxman Level 2 (175 points)

    OK, cause all my manual says is that when replacing installed modules, to replace the ones "furthest away from the display", a pretty odd way of describing what I take to mean the top slots, which is what I did. So it appears, I have the memory in there correctly. Two 4s on top, and two 2s in the lower slots.


    I knew it couldn't be as easy as their being in the wrong slots.... So, I still wonder why my performance decreased when I added 4GBs more RAM.

  • Kappy Level 10 (265,914 points)

    Farthest away from the screen will be the two lower slots since you are looking at them with the computer lying flat on the table, screen facing up.


    The computer comes, stock, with only two installed modules, and they are installed in the upper two slots.


    If you had them installed as:


    upper: 4-2

    lower: 4-2




    upper: 2-4

    lower: 2-4


    Then the memory is not interleaved. Memory performance is degradated about 5-15%.

  • Saxman Level 2 (175 points)

    Not really Kappy, as the manual says to load with the screen face down, which would make the lower & upper reversed. But in any case, I have the memory loaded in pairs, with the upper being equal & the lower being equal. So, I guess the interleaving is not the issue. I'm getting a replacement pair tomorrow, so I'll see if putting the new ones in shows any visible increase in speed...

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