MP 1,1-5,1 Mac Pro 5,1 quad core: best RAM option?

[cinclodes]

After reading various discussions at this form about RAM on the Mac Pro 2012, I'm still not sure about all of the facts. I have a quad core model with 4x8GB for 32GB. I understand that it can be upgraded to 3x16GB for 48GB. I wonder why it can't be upgraded to 4x16GB for 64 GB? Is there any advantage of having the RAM spread over four slots rather than three? It seems that leaving one of the slots empty could potentially be a disadvantage. Is there a definite advantage to going from 4x8GB to 3x16GB? I use the Mac Pro for processing video. I'm not sure if either configuration would have a noticeable advantage over the other. Since RAM is cheap, I'm going to try it.

 

[tsialex]

First thing, only the Xeons with dual QPI links, the X56xx Xeons and the low-power versions E56xx/L56xx, can access more than 56GB with a single CPU tray.

4x16GB will not work with a single QPI link Xeon, the W35xx or a W36xx, an artificial limitation imposed by Intel for you to spend more with X56xx Xeons. 3x16 works and if you choose carefully the DIMMs (all being 2Rx4), 3x16GB + 8GB will also work, but with poor performance.​

Second, Nehalem and Westmere Xeons are tri-channel, so, slot 4 (and 8 with a dual CPU tray) share the same channel with slot 3 (and 7 with a dual CPU tray), so there is a penalty when you install a DIMM to slot 4 (and 8 with a dual CPU tray).

People over the years tested this extensively and the drop in performance is around 3 to 5% when you use 4 DIMMs per CPU, if the application is not heavily memory access bound. Scientific and ML applications have the biggest penalty, with someone writing that had a 20% drop for a specific app. If you need more memory to avoid swap, more, but slower, memory is always better than going to the disk for swap.​

 

[cinclodes]

Thanks for providing those informative and interesting facts!