does matching memory sizes still matter? (PLUS Apple gives me a free upgrade!)

Discussion in 'iMac' started by Gherkin, Oct 12, 2013.

  1. Gherkin macrumors 6502

    Apr 9, 2004
    I wasn't sure...

    I just got a new 27" and ordered it with 8GB from Apple (2 x 4 GB sticks).

    I ordered an 8GB stick of Crucial memory from Amazon. I have it installed and OS X shows 3 of 4 slots being used with 4GB, 4GB and 8GB.

    would it have made my system faster if I bought two 4 GB sticks instead? giving me 4 x 4 x 4 x 4.

    as a side note....

    I ordered a BTO i7, 8 GB RAM, 1 TB Fusion, 4 GB graphics and had it shipped to the Apple store. After I picked up and turned it on for the first time, the fans went on at full blast within 10 seconds and would not turn off. I took it back immediately to the Apple Store. They looked at it over night and told me they were just going to do a return because it was dead.

    So to do a BTO I was going to have to wait another 10 or so days.... or not! I asked what they actually had in stock for iMacs, and he looked on his inventory app and they had my exact configuration except with a 3 TB Fusion. He told me he felt bad about getting a bum iMac and sent me home with the upgraded model at no extra charge!
  2. emseven macrumors regular

    Sep 19, 2013
    Yes, pairs should match.

    RAM 2GB--2GB--4GB---4GB
    Slots 1-----2------3------4

    RAM 4GB--2GB--4GB---2GB
    Slots 1-----2------3------4
  3. dmrowley macrumors 6502


    Aug 4, 2010
    Dortches, NC
    This is why we love Apple... Most of the time the Geniuses (Genii?) will bend over backwards to help you.
  4. Gherkin, Oct 12, 2013
    Last edited: Oct 12, 2013

    Gherkin thread starter macrumors 6502

    Apr 9, 2004
    ok... well I'll probably eventually get another 8GB for this computer, so then I'll have 4GB--4GB--8GB--8GB. Good to know.

    exactly. Yesterday I was cursing Apple and pretty mad, but now I'm just so happy. They know what they are doing.
  5. Micky Do macrumors 68000

    Micky Do

    Aug 31, 2012
    An island in the Andaman Sea.
    Dunno for sure, but my experience is.......

    I asked the local Apple dealer to install 4 GB of RAM in my early 2009 base model Mini (and do some other things including loading Mountain Lion.... with a very slow internet connection and no credit card I could not easily do it myself). It came back with 4GB installed along side the original 1 GB, so now has a total of 5 GB of RAM. I am well pleased with the upgrade; it works fast and just fine.
  6. Gherkin thread starter macrumors 6502

    Apr 9, 2004
    maybe someone has better information, but the more I look into it, I'm seeing people say you get a 10% performance increase if you match sizes. Dual channel mode.

    so yes, you got a real nice boost, and in your case, going with the 4 GB was better than matching with 1 GB.
  7. drambuie macrumors 6502a

    Feb 16, 2010
    Actually, the correct and incorrect are reversed.

    If you have two 4GB DIMMs and are adding two 8GB DIMMs to the late 2012/2103 iMacs the modules would be installed from top to bottom as follows:

    Slot 1 - 4GB (Bank0/DIMM0)

    Slot 2 - 8GB (Bank0/DIMM1)

    Slot 3 - 4GB (Bank1/DIMM0)

    Slot 4 - 8GB (Bank1/DIMM1)

    From the factory, the two 4GB DIMMs will be installed in slots 1 and 3. This is the standard layout since the introduction of dual channel memory.

    Proper installation can be confirmed by looking at the memory module layout in About This Mac. From the above example the 4GB pair would be in the first row, (DIMM0), with the 8GB pair in the second row, (DIMM1).

    In actual use, the order of DIMM installation wouldn't noticeably affect performance. If you add a single 8GB DIMM to an iMac with two 4GB DIMMs, the first 8GB would work in dual channel mode, only memory from 8GB to 16GB would run in single channel mode with the slight performance hit.
  8. emseven macrumors regular

    Sep 19, 2013
    Apologies, didn't consider banks. Was going for more universal matched pairs concept.
  9. drambuie macrumors 6502a

    Feb 16, 2010
    Apple's terminology is a bit confusing. If you replace "BANK" with "Channel", it's easier to follow. It would have made it easier if Apple had used color coded DIMM sockets like most motherboards do.

    The reality is that installing the DIMMs in the wrong order would have little impact on real life performance, especially with 4GB and 8GB DIMM pairs. The worst that would happen is that the lower 16GB would operate in dual channel mode and the rarely used top 8GB from 17 to 24 would run in single channel mode.

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