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In general and specifically in my MacBook pro (2012). I'm confused on if the answer is yes or no because I have heard both answers. So I have come here to get things cleared up.

Can I have two different capacity RAM sticks, why or why not? I would be using the exact same brand and speed and frequency.

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the word is CAPACITY 'cos size could also mean dimensions, though no doubt you don't mean different dimensions. but the word is CAPACITY (and generally if you don't think in strict terms you might mess up when buying technical things online) –  barlop Jan 3 at 9:01
    
Yes i meant capacity –  cojomojo Jan 3 at 9:05
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3 Answers 3

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Will be perfectly fine.

Perhaps there may be some advantage of them being 100% identical in every day. There is the factor of Timings.. like CAS latency. though in theory you could get RAM with identical CAS latency too, where the only difference really is capacity.

AFAIK The main thing with performance is they should be the same speed. RAM Timings / CAS latency play a role.. RAM is cheap, you could replace all your RAM with higher capacity.

People are commonly using different capacity RAM. And they try to match the speeds otherwise they do or can end up running at the speed of the slowest.

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and of course make sure the voltages are the same and the right level for the motherboard's requirements. –  barlop Jan 4 at 16:23
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Two different capacity memory modules will be just fine.

If you get two modules of different speeds then the computer should run at the speed of the slowest modules. In in 99% of all cases will automatically.

In many cases you can also use a few memory modules in a special way and gain some extra speed. There are several ways this can be done (interleaving modules, dual channel, triple channel, quad channel, ...) but the most common on is dual channel and it yields about 5% speed extra speed without needing extra hardware. 5% is not shocking, but 5% speed for free is nice. So many people get told they should buy identical DIMMs.

Having said that, more RAM usually trumps the extra speed from dual channel.

That leaves only these things:

  1. Get memory which your motherboard supports. E.g. difference between SoDIMM and a regular DIMM. No quad-rank modules on non-servers. Do not mix normal unbuffered and buffered/registered modules.
    Most of this you can safely ignore since almost all consumer memory is the same: unbuffered, no ECC, no parity). Unless you spent effort to find something exotic you will get the right stuff.
  2. Get memory which is the same voltage as the other already installed memory. (e.g if your first DIMMs is rated 1.5 to 1.65v and you get a second one rated 1.35 to 1.5 volt things should work fine at the 1.5v setting. (Older modules often work at a higher voltage and are not guaranteed to work at a lower one.)
  3. Make sure you still have an empty memory socket. :)


Memory speed vs amount explanation:

Say I have an encrypted file (e.g. my_secrets.aes) which is 1GiB in size. I want to decode it and I will access the whole file over and over again.

If I do this on a computer with 2 GiB then the computer will boot, load the OS. You can start the cracker program and load the file. The file will be cached in the memory. The first time I to decrypt/break the file I will try to brute force it with some key A. Next I try the following key on the same file, just as long until I find the right key to decrypt it with.

Every time I will need to access the whole file. The OS will have a copy in the disk cache which means it will only need to be loaded from disk once. That is a nice part of having enough memory. And since I choose a 2GiB host and did not start any other programs the file will fit completely into the disk cache.

Secondly I do the same thing with a 4GiB computer. It will not be any faster.

On the third attempt I do the same thing with faster memory. The CPU can read faster from the memory and the whole decoding will go faster.

This shows why faster memory can have advantages.
Not that this is an extreme advantage. I run no other programs which use memory. I had a file size which was significantly larger than the cache on a modern CPU, forcing the program to continuously read from memory.

ON regular day to day usage you will have several program running: E.g. a webbrowser or two, a mail client, a virus scanner, an open spreadsheet, a music player, ... All of these use memory and all of these profit from having some of their data in the disk cache. Actually, not just their data. If I start word, write a shopping list and then save word then the disk cache will have part of the word program. The next time I start it not all of that program needs to be loaded from slow disk. Instead it can use the faster in memory data.

This is another somewhat extreme side where I start a program. Exit it and then start it again.

Now in most cases of day to day usage there is a mix of both. Faster memory helps. More memory helps. But on average for Jane Regular there is an bigger speed advantage with more memory over bigger memory.

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You write "more RAM usually tromps the extra speed from dual channel." I guess you meant trumps. But still not clear to me, do you mean it is faster to have more RAM of different speeds. Or faster to have less RAM but the same speed? It's not clear just looking at that sentence whether you mean that more ram "trumps" the extra speed, in the sense that more ram is better and gets more overall speed. Or more ram "trumps" the extra speed in the sense that you lose the extra speed. Very unclear sentence there. –  barlop Jan 3 at 21:57
    
Yes, I meant "trumps". It was not an obvious typo to a non-native speaker. In most of the cases it is faster to have more RAM rather then less but faster RAM. I'll fix the typo and try to expand that at the same time. –  Hennes Jan 4 at 17:40
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For performance, you should be using 2 matched sticks of RAM.
But most of the time, modern hardware can deal with having mismatched sticks and still work fine, but it won't be the most optimal configuration.

Some older motherboards refuse to boot, some will just run them in single channel mode, some will still work at full speed with mismatched sticks of RAM.

It all comes down to the hardware really.

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-1 You're making general statements about mismatched ram sticks. As if as long as the RAM fits then it's fine, but I don't think this is so. RAM can come in different voltages. As for speeds, you talk of two RAM sticks of different speeds and both running full speed, though i'm aware of two ram sticks both running at the speed of the lower one, and what i'm aware of may be far more common.. because if they're working as a pair they'd pretty much need to work at the same speed(I suppose unless they can somehow synchronize with one as some multiple of speed to the other). –  barlop Jan 3 at 17:27
    
I admit, I didn't take voltages into account. However you can have different speeds of ram and both can run at full speed if they are in different channels. But again this depends on the hardware. –  Lawrence Jan 4 at 13:13
    
You write " some will just run them in single channel mode, some will still work at full speed with mismatched sticks of RAM". When you write a sentence that says "some......., some...." it suggests you're talking about a different some. And when you wrote the second half of that line, "some will still.." did you mean single channel again? –  barlop Jan 4 at 16:19
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