SBE Max Memory & Performance
For the Sandy Bridge-E processors (currently the i7-3930k and the i7-3960X), Intel lists the max memory as 32gb. However it's generally understood that 64gb works for these, and 8x8gb memory kits are available for SBE systems.
On the Intel forums, you can see Speedy2's comment on this thread citing an Intel representative saying:
"Speedy, to keep you informed processor can take only upto 32GB of maximum RAM memory and consider you are using 64 GB of maximum memory installed in your motherboard 64 GB of memory will be split ie first 32 GB will be processed and second 32 GB will be queued."
Now I'm not too familiar with how the CPU and memory act as the lowest levels. Does this mean that, if my six-core processor happens to need data stored in different parts of the RAM, the cores which need data from the second 32gb section of the data will have to idle twice as long while waiting for the RAM to return the requested data?
Practically speaking I'm a grad student trying to make a cheap workstation. I'd love a 64gb box for the RAM disk, which would help my CAD to run faster (confirmed through testing), but the gain just isn't worth it if it'd make my RAM operations 33% slower (since half the RAM operations should run at regular speed while the other half would have idle twice as long).
Sorry, forgot to include my rig's components. I've already made it, just thinking of returning the 32gb RAM for 64gb.
-Intel i7-3930k (Intel, NewEgg),
-ASUS P9X79 Deluxe (NewEgg),
-Corsair Vengeance 16GB (4x4GB) DDR3 1866 (x2, for 32GB total) (NewEgg),
-SeaSonic Platinum-1000 (NewEgg),
-Radeon HD 6970 (NewEgg),
-Crucial m4 256gb (NewEgg),
-ASUS VH242H screen (x3) (NewEgg),
-Corsair H100 water cooler (NewEgg),
along with a good case, BluRay burners, internal USB hubs for dongles, etc.
I wouldn't take their word for it. I've not heard about Intel's memory controllers working that way and it seems strange.
Intel's memory support is often not what will actually work, for example LGA 1366 memory controllers can address 48GB the same way they address 24GB - or at least no benchmarks I've seen ever showed anything resembling some wait to address more. There just weren't 8GB DIMMs around to certify them originally. I've seen 16GB DIMMs work on Xeon 3680s in the correct manner too yet Intel don't support registered DIMMs so the limit was 24GB, not 96GB.
I guess I wouldn't take that evidence that the 3690 memory controller is limited in some way. Since the move to Nehalem it seems to be that memory controllers can handle as much as you can get in there. The E5 2600 ones should handle 256GB per controller easily and I don't see why they would be using different controllers.
Easiest thing would be to contact one of the smaller companies, like GSkill, who are selling 64GB kits and get their take on it.
An email to Intel customer service, pleasantries truncated:
"For the i7-3930k and all other factors being equal, is there a performance hit for using 64gb vs. 32gb?
It's understood that the i7-3930k can use 64gb, yet Intel's specs show that only 32gb are supported. The reason is unclear, and I would like to know if there's a performance hit associated with the 64gb route, i.e., the processor would have to consider 64gb as two separate 32gb spaces and access them separately or something."
Response from Intel, pleasantries truncated:
"Please bear in mind that the motherboards may be supporting 64GB memory and different speeds and voltages. However, the memory is not longer controlled by the motherboard.
Since the memory controller is integrated on the Processor, we need to follow certain specifications on size, speed and voltage. In this case, the maximum memory size supported for Intel® Core™ i7-3930K Processor is 32GB.
Working over these specifications may affect the performance and also may damage the Processor."
I've sent Intel other emails, too, so let's see what else is said.
A consistent theme so far is that Intel doesn't support RAM over 32gb, and that while it may work, there may be performance degradation and/or damage to the processor. I wish that I knew the technical details behind this though.
Another Intel rep who seemed particularly knowledgeable (he was able to answer other questions which no one else had been able to):
We will have some limitations on memory and performance. Since the memory controller is integrated on the Core i7, i5 and i3 we need to follow certain specifications as voltage, speed and size. For example, the maximum memory size for these new items is 32GB. On different processors, we are limited to use 1066 and 1333Mhz only. Also, we need to use memory designed at 1.5 volts to avoid any overvolting that may affect the system and damage the Processor. For performance and to avoid using the Processor out of its specifications we strongly recommend to work with 32GB as max.
To me the bottom line is that using more than 32GB is an at-your-own-risk sort of thing. As Intel puts it using more than 32GB of RAM can lead to processor damage and reduced performance.
I need my computer to work reliably, so I'm not going to risk it. 32GB it is.