new i5/i7 iMacs & triple channel memory

You should follow the type of memory apple suggest for the iMac. That doesn't mean you have to order it from Apple you can get OWC memory for much less that meets Apple's specifications. It's nothing more complex than that.

Regards,

Roger

While the Mac Pro (Gainstown codenamed processors) does use triple channel memory, they in fact have 4 slots in the quad core and 8 slots in the 8 core models. It is faster to use memory in 3 or 6 slots in these models. On the other hand, the new iMac can use the new Core i7-860 Lynnfield processor (also part of the overall Nehalem family) which uses two wide memory. So you need to put memory in in pairs.

I hijack this tread as it's in the same vein as a question I have.

I have ordered a 27" i7 imac with 4GB (2 x2GB) to expand the memory do I have to put two more chips in or can I put just one more.

Also does all the memory have to be the same size, would it have to be 4 x 2GB or can I have say 2 X 2GB and 2 X 4GB to give me 12 GB , or can I have 2 X 2GB and 1 4GB with a free slot to give me 8GB

Core i5/i7 CPUs using the P55 chipset do not support triple channel memory, only dual channel. This applies to the 27" iMac only. The Mac Pro Core i7 systems use the x58 chipset. They support triple channel memory only if you use 3 dimms. If you use 4, you will be running in Dual channel mode.

I think it is always better to install RAM in pairs.

The TRIPLE channel memory part has nothing to do with the individual memory slots or DIMMS but rather with the controller. If you want the system to take advantage of triple channel capabilities of the controller you WILL have to keep the DIMMS to three which requires you to buy the larger sized DIMMS. Tests show that triple channel boost performance significantly so you have to decide whether you want more memory or faster memory, in the latter leaving that 4th slot empty. I think that is why Apple supplies that slot... for most people the additional RAM to run more apps will outweigh the need for extreme speed... this is after all a consumer machine. If four DIMMS are installed the controller will work in dual channel mode... still not shabby.

To learn more about triple channel memory go to.
http://www.valueram.com/europe/triplechannel/

Not sure exactly what this means for us all, but over at Crucial, they seem to think that our yet-to-arrive beauties don't even support dual channel memory mode.

Anyone know what implications that has?

rafsanchez-

I think you've gotten a little confused on this point. As a couple of the posters have noted, the particular Intel quad-core processors that Apple is using in the 27" imacs are capable of only dual-channel memory mode; 3-channel is not a feature. (the i7 model available in the imac is the "entry level" version of that processor family) So no matter how many slots you fill, you will be operating in 2-channel mode.

I was talking on the phone to the Apple Store (1-800-MY-APPLE) about configuring 12 GB memory for the 27" iMac with an i7 processor. I suggested 2x4GB and 2x2GB so that the memory would be installed in pairs. He said two things. First: 12GB was not a recommended configuration ergo not an option in the Online Apple Store. Second: it has not been necessary to install memory in pairs for some time now so he would recommend I configured it as 3x4GB in case I eventually wanted to bump the memory to 4x4GB for a total of 16GB in the four slots. He then went to talk to his tech support who confirmed the 3x4GB was acceptable i.e. no worries about installing pairs of chips. Any other experts care to comment?

srcollins
I'm still confused. As you can see from my earlier post, according to Crucial, the i7 iMacs don't seem to use double channel memory mode at all.
If Apple stores are telling us not to worry about installing RAM in pairs, that would seem to imply the same thing.
Can anyone make sense of the voodoo for us dullard consumers.

The i7 860 quad-core processor used in the late 2009 iMacs only support dual channel access to the memory. That should be clear. That does not in itself mean the same thing as requiring memory be installed in pairs; it just means the processor can support data transfer over two simultaneous paths. For example, for the processors that support triple channel access to memory, such as in the Mac Pro, the configurations that use 3 or 6 memory slots are going to execute more efficiently than the systems that use 2 or 4 or 8 memory slots. But they will still work. I'm hoping an expert can confirm that the iMac i7 will function efficiently in a 3x4GB configuration or should I go with 2x4GB and 2x2GB for a total of 12GB installed memory.

I took a look at the Crucial memory configurations for the new iMac & I can see how that is confusing, but I think I know what is going on. I see where they say the memory is not dual-channel; note that they also say the memory does not have to be installed in pairs. I went to the PC side of the Crucial memory configuration & picked out a high-end HP system at random; there, the memory was identified as dual channel and that the modules have to be installed in pairs.

As Rick L points out, we know that the processors in the new iMacs incorporate dual memory channels (BTW, Intel has a really neat site that gives all kind of details on their processors at:
http://www.intel.com/products/desktop/processors/index.htm?iid=processorsbody+dtcore

My conclusion is that when Crucial classifies memory as dual channel, they are really talking about whether or not the memory controller in the computer requires the modules to be installed in pairs. I'm thinking that some vendor's implementation of the dual channel memory controller is so rigid that bad things will happen if the memory is not configured in that manner. I know some PCs will have color coded memory slots to guide one to put the modules in the proper slots in the proper order. I think the iMacs will function regardless of how the memory slots are filled; hence, Crucial does not classify the memory as dual channel. (Again, the memory module itself is not "dual channel" - dual channel is a function of the processor's or motherboard's memory controller.) This is consistent with the information Rick got from Apple.

I remember that when the latest Mac Pros came out with the triple channel memory controller, there was a lot of discussion (& testing) about whether a 3 module X 2 GB/module configuration had a performance advantage over a 4 X 2 configuration. I can't remember which had the performance advantage (maybe the 3 X 2 was a little better), but perhaps you could search out the Mac Pro discussions on the web; I would think the results there could apply to the iMac case.

Rick-
Just a suggestion; why not contact OtherWorldComputing with your question about memory. They are very sharp on memory for Macs. For instance, those guys validated that 4GB modules would work in the latest, low-end Mac Pro long before Apple approved that configuration.
(BTW, I didn't notice you were all over the MacPro memory until after I posted my last message)

Many thanks srcollins; much less confused now.
Thanks for the Intel link too... will go for a jaunt there presently.