DDR3 RAM in late 2009 iMac - 12GB or 16GB?

As usual, OWC has done the testing.

http://eshop.macsales.com/Reviews/Framework.cfm?page=/Benchmarks/iMacSnowBench.h tml

Kenichi, because the results for Photoshop (running alone) do not vary much with additional memory in the OWC benchmarks, I suspect they did not tune Photoshop to take advantage of the RAM above 4 GB which could be used for scratch files and should have given significantly higher results for the memory configurations above 4 GB, the base machines. Perhaps I am wrong. One would think they would have tuned the software because it would help sell more memory when using Photoshop CS4!

Barry, I wonder if you are interpreting DD2 and DD3 to imply dual-channel and triple-channel access to the memory? Sorry I have not researched how the Core 2 Duo processors use memory other than they use dual-channel memory. Here's my understanding of memory access for triple and dual access memory on the Mac Pro and high-end iMacs. The Mac Pro and the iMacs both use DDR3 memory at a frequency of 1066 MHz.

The current Mac Pros use triple-channel memory i.e. they allow three concurrent access paths to the memory when memory is installed in 3 or 6 slots depending on the processor - one quad-core versus the high-end two quad cores. If memory is installed in 2 or 4 or 8 slots, the memory access falls back to dual-channel access, i.e. two concurrent paths to the memory. So it sounds like it's always best to use 3 or 6 slots; generally true. However sometimes if more memory is very important to the application, the slower dual access will give the better overall results because the greater memory installed prevented having to use the hard disks as much.

The iMac (late 2009) with the i5 or i7 processors do not use triple-channel memory access at any time. They use dual-channel memory access as long as the memory is installed in pairs, i.e. 2 or 4 slots filled. If 3 slots are filled, the first 2 slots or a pair is still accessed using two concurrent paths, the 3rd slot would be accessed using one path and that access would be slower. So it is not a complete penalty to have unpaired memory with the i5 and i7 processors but anything using that last stick you install will suffer.

As for iMac (late 2009) configurations then, the memory access speed of 4 GB, 8 GB, 12 GB, and 16 GB can be identical if they are always installed in pairs. For example, to use 12 GB, the most efficient install would be 2x4GB sticks and 2z2GB sticks. 3x4GB sticks would hamper the access time of that last 4GB stick.

On the Mac Pro, 3x4GB sticks or 6x2GB and 6x4GB would be the most efficient access most of the time, but not on the iMacs. Hope that helps.

8GB is certainly the value "sweet spot." 4x2GB is a lot less expensive than any config involving those 4GB modules.

Hi Barry,

This may be a little bit off-topic but Apple states that we can use 2GB or 4 GB modules. http://support.apple.com/kb/HT3912
I would also like to use 1GB modules from a macbook.
Is your machine performing well with the additional 2x1GB?

Thanks,
Carlos

Barry, happy you are happy! What type of 1 GB RAM did you install in your new iMac? The Apple-installed memory is DDR3 1066 MHz SODIMM (PC3-8500).

Yes, they should be fine as I think the RAM speeds are identical in the Macboks and the iMacs, DDR3 1066 MHz. Very good you didn't waste the original sticks.