RAM all over the place

October 30, 2009 8:10 PM

RAM configurations for the new iMacs - permutations galore!

I'm going to be buying a new quad iMac probably sometime in January 2010 (either the i5 or i7, I have not decided as yet, I'm waiting on reviews and tests). I will be buying it with the stock 4GB RAM, and I'll buy more RAM on my own and install that. However, I'm not clear on a few issues with optimal RAM configurations. The new iMacs have 4 RAM memory slots, and the stock configuration is two 2GB sticks (2GB + 2GB). There's a BTO option of getting additional 4GB RAM in one of two configurations, as 2GB sticks, so the end result would be 2GB+2GB+2GB+2GB=8GB, or two 4GB sticks, so the final configuration would be 4GB+4GB=8GB (much more expensive). Now, come January if I'm lucky RAM prices might drop, but regardless, it will still be cheaper to buy RAM myself and install it, instead of buying from Apple BTO. Issues:

1)Is there a performance penalty with 2GB+2GB+2GB+2GB=8GB versus 4GB+4GB=8GB? I realize that there is an expansion difference, in that using the 4GB sticks, I leave myself the option of upgrading to 16GB.

2)Is it possible to do: 2GB+2GB+4GB=8GB or is it contraindicated to mix 4GB sticks and 2GB sticks? If it's possible, is there a performance penalty vs 4GB+4GB=8GB?

3)Similarly is it possible to do 2GB+2GB+2GB+4GB=10GB or is there an issue? It is my understanding that as Apple has it configured, both the i5 750 and the i7 860 are dual channel Lynnfields, and the i7 is not 3 channel (1156 socket)

4)And again similarly what about 2GB+2GB+4GB+4GB=12GB

5)Final option, what about 2GB+4GB+4GB+4GB=14GB.

Of course, my decision will be driven by pricing, and this may pertain to quite some time down the road, so I may add gradually over the course of a year depending on prices. Thanks!

posted by VikingSword to computers & internet (12 answers total)

Buy the RAM yourself, double check the pin count and all that to make sure - you can get Apple-guaranteed RAM from ifixit.com, I think.

Don't mix DIMM sizes. If you have 4 slots, you should have the same DIMM size (i.e. all 2GB sticks) in all the slots. But really, 8GB of RAM is going to be sufficient for 99% of tasks possible on an iMac, so I wouldn't worry too much about more. If in a year you feel you need more, a few 4GB sticks are easy to come by.
posted by BlackLeotardFront at 8:24 PM on October 30, 2009

...Sorry, I didn't mean to presume what you'd be doing with the mac, it was just a general statement. But if you're concerned about RAM limitations, the best thing with macs is to get the cheapest option when ordering from them, and add your own RAM afterwards, even if it means throwing the original RAM away (you can always sell it, plenty of people (like you) are looking for extra mac-compatible RAM).

And as for 2x4GB vs 4x2GB, the latter is actually better all other things being equal; it shares the load better if I'm not mistaken.
posted by BlackLeotardFront at 8:30 PM on October 30, 2009

Thanks, BlackLeotardFront, I got the use figured, and I am definitely going the DIY route, as I've done for RAM all my life, mac and PC. I was more interested in the technical side of it, performance issues - and I find your claim of 4x2GB being better than 2x4GB quite intriguing... I would have thought the opposite, so that was quite useful, thank you.

Anyhow, I'm looking exactly for that - technical indications of what's the best way to mix and match, based on solid knowledge (because while speculation is amusing, I got that covered :)).
posted by VikingSword at 8:39 PM on October 30, 2009

Remember, RAM is the #1 markup item in new computers. Get the stock ram from apple but definitely check out as many 3rd party dealers for ram that you can. I've used Ramseeker.com to find out what the best prices are. They compare about 15 different sites that sell memory for Macs and PCs.

Caveat: cheap ram is not always good ram. I usually look for the 3rd lowest one, and go with that. That takes out the "bargain basement" competition. Check the reputation of the company selling, and you're all set. I've been doing that for about 10 years, and it's worked fine for me. Apple Certified ram is always the best, and I'll take that over any other, when the price is right.
posted by chambers at 9:25 PM on October 30, 2009

Crucial has good ram for Macs too. My friend works for Apple, and that's all he buys for his computers.
posted by andrewcilento at 9:49 PM on October 30, 2009

Apple's markup on RAM is stupid. Before you try a configuration with more than two DIMMs, I think the iMac (all flat panel models) has only two SODIMM slots. 4GB SODIMMs are stupid expensive right now, about $350 a piece, so you may be practically limited to 4GB of RAM unless you want to shell out a lot of cash. It would probably be better to get a Mac Pro and load it up with RAM if you really need 12GB+.
posted by thewalrus at 2:37 AM on October 31, 2009

BlackLeotardFront is likely to dispute this, but I'll give it a shot. IANAEE...

I can't think of any reason that mixing and matching sizes would be dangerous or bad. Certainly, consistency (256 and 256 vs. 128 and 384) is best when possible as it provides an equal spread of data across this sticks, which in turn will increase the chance of memory access alternating between the sticks, yielding to quicker pulls into the registers. This is the same explanation for 2x4 beating 4x2.

Mixing and matching memory speeds and other "features" is generally bad for business. In most cases, the speed of the slowest memory will become the speed all the memory operates at. It's not unlikely that motherboard manufacturers have done or will do things to mitigate this, but I think it is still an issue.

I also agree on the point about not needing more than eight for most tasks. Unless you are the sick kind of person that enjoys opening all of office, iphoto, itunes, and memory-leaking-old firefox at the same time, usage of that much memory will be limited to artists (rendering, editing, recording, etc), scientists (large modeling), and virtual machine junkies.
posted by yeoldefortran at 5:39 AM on October 31, 2009

"Don't mix DIMM sizes."

In this case that's a bunch of hokum. You can most certainly use different-sized sticks without penalty. These are not multichannel pairs, they're SODIMM slots.

There are plenty of devices for which sizes must be matched. The device in question is not one of them. As long as the timings are equivalent, it should be no trouble to mix and match memory.
posted by majick at 6:54 AM on October 31, 2009

Thanks majick, that's the kind of thing I'm interested in.

So mixing 2GB with 4GB sticks is fine, with no penalty - that's good to know. What about the other concerns? Is there an issue with having to have only certain amounts of total memory, like f.ex. 4GB, 8GB, 16BG is fine, but not 6GB, 12GB, 14GB? And occupying 2 slots or 4 is OK, while 1 or 3 is suboptimal? I ask, because I thought there is an issue with dual vs triple channel memory and so you can have f.ex. 12GB in a MP due to triple channel memory, but both the quads in the new iMacs will have dual channel (socket 1156). I'm sorry to ask such niggling questions, but unfortunately I don't have a firm grasp of how memory is addressed and used in these iMacs.
posted by VikingSword at 9:44 AM on October 31, 2009

In some cases, 2x2x2x2 should be a little faster than 2x2x4, but I doubt you'd be able to tell the difference in real world use. The most applicable benchmark tests I found were here, although it doesn't yet cover the latest models. Buy as much RAM as you can afford, in whatever configuration is cheapest.

A discussion of the topic on Apple's support forum indicates that all of the new iMacs can only operate in dual channel mode. Though the i7 is capable of triple channel access, none of the iMacs support it. So, again, buy as much RAM as you can afford, in whatever configuration is cheapest.
posted by paulg at 10:44 AM on October 31, 2009

The performance gain/loss from trying to match RAM modules is minimal, its not even worth fretting over. Even not installing in pairs won't affect much.

Here's a Toms hardware article showing benchmarks between dual channel and single channel.
posted by wongcorgi at 1:56 PM on October 31, 2009

As noted above, you can mix and match. I'd buy a 2x4gb sticks and leave 2 slots empty incase I wanted to upgrade later.
posted by chunking express at 8:26 AM on November 2, 2009