2R Dual rank vs 4R Quad rank RAM on cMP 5,1

[pixelatedscraps]

I just installed 64Gb (4x16Gb) of PC3-10600 DDR3-1333 4R (quad ranked) RAM into the cMP 5,1 in my signature (slots 1-2, 5-6) and its displaying as running at 1066Mhz not 1333Mhz in System Report.

Would I be advised in exchanging these for 2R dual ranked chips instead of 4R in order to gain the optimum performance in the run up to 128Gb?

I picked these up for cheaper than their 2R equivalent but the seller is happy to exchange the RAM. I've read through the very good Macrumors Guide on Understanding Intel Mac RAM and Buying RAM and can't seem to find a concrete answer.

I saw another thread here that said RAM speeds get down clocked when you aren't using triple channel or by filling all 8 slots but the performance loss is offset by the gain in total system RAM.

What's the general wisdom on purchasing RAM in order to get the optimal and maximum performance on a cMP 5,1?

 

[itdk92]

Yes just get Dual Rank.

 

[leon771]

Can't read your signature. What CPUs do you have?

 

[h9826790]

Can't read your signature. What CPUs do you have?

X5680 (if you using mobile browser like iPhone, turn your Phone 90 degree will able to display the signature).

To OP,

The exact issue caused by Quad rank DIMM still unknown. All we know is sometimes won't boot.

e.g. If there is a total rank limit.

8x 2R = 16R

therefore if limit is 20R

6x 4R = 24R may cause no boot.

But no definite answer if this limit really exist.

Also, Quad rank DIMM may run at lower speed at certain condition.

e.g. 3x 4R DIMM can run at 1333, but 4x4R DIMM will run at 1066. However, again, don't know the exact rule yet.

To make it simple, better just get the 2R DIMM, they are cheap nowadays anyway.

For Max performance, there are 2 type of max performance. For max max speed, that's 6x 1333 identical DIMM in the dual processor model. However, max demonstrated is just 96GB so far.

To run 128GB RAM, there are 2 known options.

1) 8x16GB
2) 4x32GB

In fact, you can go up to 5x32GB = 160GB (no one successfully go beyond that at this moment)

But no matter which config you choose. The max speed will be a bit lowered than 6x16GB.

Therefore, when your actual memory usage is less than 96GB (including use the free RAM as cache). Your system will run tiny bit faster with 6x16GB. But once go beyond that. 128GB or even 160GB should be faster overall.

In general, for memory, the more the better. Optimising triple channel architecture will give you higher score in benchmarks. But for real world. Sure more cache has benefit in most situation.