new i5/i7 iMacs & triple channel memory

Just a bit of additional clarification.

The i5/i7 CPUs that Apple is using in the 27" are use socket LGA 1156 and a special BIOS to support it in Wintel computers.

The Mac Pros use a completely different socket with different capabilities.

Of all the LGA 1156 motherboards Newegg sells only one has six ram slots, and it won't work right with all slots filled.
See the first review of someone who tried here: http://www.newegg.com/Product/Product.aspx?Item=N82E16813128403.

Bottom line two or four pairs of DIMMs work best.
More or less (ie one DIMM or three DIMMs) will cripple your performance.

Hope this helps.

Thanks for the further clarification srcollins. Your conclusions about Crucial's site's approach seem reasonable and now I feel confident if I order 12GB of RAM that 3x4GB installs will work and not be degraded in terms of performance versus 2x4B + 2x2GB installs. Now I need advice if Final Cut and Photoshop CS4 with onOne will be happy with 8, 12 or 16 GB! Somewhat embarrassed to admit my old PowerMac (R.I.P) only had 1 GB and I was still productive but I want to speed things up considerably now on a new iMac.

You were asking about benchmarks on the Mac Pro when filling 3 or 6 slots versus filling 4 or 8 slots. My recollection is that the theory stated filling 3 or 6 slots was preferable on the current Mac Pro but the actual benchmark results were very close because some of the tests were helped by simply having more total RAM available when the 4 or 8 slots were filled. I am fairly sure they did not have the same total GB in the comparison as they were addressing the question of which would perform better, not which configuration gave more efficient access to the identical total RAM. Once again, if the RAM is needed by programs, more RAM is usually better. I suspect if the total RAM had been identical, the configuration with 3 or 6 slots filled would be quicker. Sorry I don't have a URL to the test results.

I did some more reading about the i5/i7 processors and how Intel is accessing memory. The processor supports FLEXIBLE dual channel access meaning there are two paths to the memory that can be used simultaneously if the memory is paired. If the memory installed has the same performance characteristics, and the memory is installed in matched pairs, the access to that memory is symmetrical and provides the highest performance. Once memory is detected in one bank that is not matched, the access to that memory is single channel and hence degraded. But the other installed memory will continue to be accessed at the higher rate via dual access paths. So a system with 4x4 GB or 4x2 GB RAM will have all memory access using dual paths. If a system had 3x4 GB which I was considering, the first 8 GB RAM would be accessed at the higher speed dual access and the additional 4 GB would be single channel and slower. An iMac system with 2x4 GB and 2x2 GB RAM would have all 12 GB accessed via the dual channels. This is likely what I'll go with (subject to reviewing benchmark results when they are published), installing a 2 GB and a 4 GB stick in each of the iMacs two memory banks. Sounds like a plan?

Rick-
I think your plan of installing the 2 pairs of RAM - 2x4 GB and 2x2 GB - is sound. Like you, I'm waiting to see real published results on the quad-core iMacs. Good luck.

Rick--I was SO glad to see you corrected earlier assumptions in your second post. I was reading from the beginning of the thread and was about to have a fit trying to get to end of this madness so I could help.

This thread is about best utilization of RAM in quad core iMacs, namely i5 and i7. I can clear this all up. (I have one on my desk and have run the gamut with it and RAM)

Crucial and others memory sites are reporting the correct chips to buy--anything to do with channel comes from the hardware logic board and not the memory chips.

Here is the simplest explanation--FOR BEST PERFORMANCE INSTALL RAM IN MATCHED PAIRS, 2 or 4GB pairs. Using an odd amount will degrade performance. If you want to know why, google "dual channel memory"

iMac i5 and i7 machines do NOT support triple channel memory, and only one i7 currently does and that chip alone costs >$1000 and will Not soon appear in iMac. Even with machines that do support triple channel, there has been no distinguishable benefit in use outside synthetic benchmarking.

So do not buy odd amounts, regardless what salespeople tell you on the phone. AppleCare is still in training on this issue themselves.

Configure your machine for a total of 4, 8, 12, or 16 GB and as long as chips are in pairs you have optimal performance of DUAL CHANNEL memory. Not triple--but don't feel bad because noone is seeing benefits from 3pl anyway.

Lang I should also add I wasn't directing that post at you but to everyone else who might still be wondering.

And if anyone wants to better grasp dual versus triple, go here:

http://www.tomshardware.com/reviews/intel-core-i5,2410-6.html

Crucial seemed to know that the machine uses duel channel when they sold me a kit containing two memory modules.

I agree with your post after I had done research reading Intel's datasheets on the i7/i5. One thing you said is intriguing:

'This thread is about best utilization of RAM in quad core iMacs, namely i5 and i7. I can clear this all up. (I have one on my desk and have run the gamut with it and RAM)'

Have you done any benchmark or productivity application testing with 4 GB, 8 GB, 12 GB, and 16 GB RAM installed in your new iMac? At this point I have not seen any results comparing performance using different amounts of RAM in the same machine.

A quick follow-up for those on this thread -- Has anyone gone with 1333 (PC3 10600) over the 1066 in the i7? Some threads have warned against it while others say there should be a performance boost (though I haven't seen any tests). For $170 I can get 8GB (4x2GB) of Crucial RAM (http://www.newegg.com/Product/Product.aspx?Item=N82E16820148317&cmre=ram_1333_204_pin-_-20-148-317--Product)

I too have not heard of anyone installing 1333 MHz memory in the latest Macs including the new iMacs with the quad processors. The i5 and i7 processors themselves can handle 1066 and 1333 GHz memory but do not know if Apple has done anything that restricts use to their recommended 1066 memory. I wonder if your memory supplier would be able to answer you best.

I have been on "tuner" sites where they have indicated this could be done along with overclocking the i7 processor considerably but those were PC compatible machines, likely custom built, by those whose hobby seems to be pushing the limits. Most of us stay within the recommended configuration to avoid potential problems.

Of course I meant to say 1333 MHz memory, not 1333 GHz memory. The later may be available in twenty years...

Looks like there isn't much difference in memory performance looking at the 1066 vs. 1333 vs. 1600 (http://browse.geekbench.ca/geekbench2/chart/185106) -- am I missing something? Does this suggest that it is EFI limited and no real benefit to using anything higher that 1066?

I just received my 16GB RAM order from RamJet...I ordered exactly what was recommended (1066 MHz) but it seems they sent 1333 MHz. The machine booted fine and seems to be running fine as well. I'm going to call RamJet tomorrow, anyway, if for nothing else but to ask why they sent the 1333.

Can you do a Geekbench test and upload the results ? Please et us know your Geekbench score. Thanks.