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I've just upgraded my girlfriend's MacBook (I think it is a 2011 Model, one of the last white plastic ones) to Mavericks, and it has slowed down significantly. I'd like to upgrade the RAM on the MacBook to get it running faster. Currently, it has a total of 2GB installed in two separate RAM slots.

If I take one of these DDR3 modules out, what are the performance implications of replacing it with a bigger module to run alongside the remaining 1GB module?

Or should I make sure that the RAM modules are installed in pairs?

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My answer generalized for all macs of the 2011 era and not the plastic models. For you, I'd get the most total RAM that you can afford from a seller that has friendly advice and/or good return policies. If it's plastic and not unibody - you really need to know your exact model and check a site like ramjet.com/collections/macbook-memory/products/… to determine what your options are. My guess is you have the 2009 model since that's the only one that runs Mavericks. If so, 4 GB might run $120 and 6 GB $150 so my choice would be to add a 2 or a 4. –  bmike Feb 28 at 17:01
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Matched pairs is the optimal configuration.

However, you will still see a performance increase by adding more memory. It is important to note that an increase in RAM ,whether in matched pairs or not, will give a much bigger performance increase than a move from unmatched pairs to matched pairs.

Really, the matched pairs issue only happens when there is an option of two sticks of size 2X, or one stick of size X.

In your current scenario, having an unmatched pair is fine, and will still lead to a good performance increase.

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I’d add that the gain of having matching pair is really marginal. –  Édouard Feb 28 at 16:24
    
Yes, I should state that more clearly. –  wrossmck Feb 28 at 16:24
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In a testing lab with unlimited funds, matching RAM (matching CAS, the controllers and the size perfectly) can benchmark faster than non-matched RAM of equal total size.

In the real world, you get a max of two RAM slots on the MacBook line and most people don't a) wait on their RAM for any significant amount of time ever b) don't throw away good RAM just because it doesn't match another chip they might put inside. (Also, I'm not saying no one waits on RAM, but that the matched vs unmatched situation doesn't cause people to wait)

For MacBook that are now vintage (especially the plastic MacBook with the removable batteries in black and white case) - the VRAM was part of the RAM and having matching chips was actually noticeable when you didn't get it right. Games might even be unplayable vs playable in some cases.

For anything Mac that is Unibody (not with a removable battery / frame is solid Aluminum) - just buy the most RAM you can afford from a vendor that will accept a return within a week if you mess up picking the right CAS latency and it doesn't work when you insert it. I run Mac Mini servers hard with constrained RAM and just buy a pair of 8 GB sticks for each pair of Macs. They come with 2x2GB so the macs have 10 GB of RAM and there is no measurable speed difference between a matched 8 GB and a non-matched 10GB and since the initial 2 GB stick is free I would never go with a pair of 4 GB sticks in that computer.

As a rule of thumb - adding more total but unmatched memory is always faster than adding less total matched memory on aluminum case Apple products released between 2009 and 2014. (And let's exclude the Mac Pro since that can have enough RAM slots and CPU to complicate the generalization I just made)

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First of all, even 3GB of RAM is not enough for OSX (and I'm not talking only about Mavericks), you should get 8 GB (2x4GB) to have the Macbook work smoothly, without sending data to a swap file on the HDD (which has a significant impact on performance). Also, you can use 'purge' command to clean up inactive memory.

Now, back on the memory modules question. You don't have to worry about the performance loss of not having two of the same sized modules. You won't notice any difference. Dual-channel doesn't offer too much of an improvement, but having more RAM in your system is.

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I know the OP said 2011, but if it's plastic, then it doesn't meet Mavericks minimum specs: apple.com/osx/specs Depending on the CPU in it / actual model it might not even work past 2 GB of RAM if it's the original plastic MacBook. –  bmike Feb 28 at 17:04
    
@bmike, Wiki says MB Uni 2009 supports 8GB, MB Uni 2010 up to 16GB. en.wikipedia.org/wiki/MacBook#Unibody_polycarbonate_model, even if there is no upgrade support from Apple (but that is the case with some MBPs as well, and I know they can support 16GB, as I've got one). –  Radoo Feb 28 at 18:17
    
Yes - Wiki does conflict from my go-to source - ramjet.com - they state 6 so I should have added my source. Definite +1 for that link to 8 GB in that model. –  bmike Feb 28 at 18:19
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Matched pairs is an opitmal configuration - but really, I'd get a 4 or an 8GB SO-DIMM. You'll see a nice speed increase and unless you are running a lot of memory intensive apps you will generally not need to concern yourself with matching the SO-DIMMs.

Alternatively if you take it into a shop and have it done for you there, they MIGHT give you a credit for trading in the two 1GB DIMMs and you could put a couple of 4GB modules in (I like to go with 8GB minimum these days, if at all affordable) which would be a nice memory AND speed boost.

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See my comment on Radoo's answer - the most a plastic MacBook can run is 6 GB at best. I don't know if plastic is correct or if 2011 is correct though. –  bmike Feb 28 at 17:05
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