The revised (Jan 2012) OWC 16GB memory modules now run at full speed of 1333 MHz and one can also mix 16GB and 8GB memory modules (“Dual-Rank 16GB ECC 240 Pin DDR3”).
According to OWC—
These modules do allow full 1333Mhz bus performance and are faster in both 1066MHz and 1333Mhz memory bus Mac Pros with the Dual-Rank design than the Quad-Rank module options they replace.
A 12-core Mac Pro can take 96GB total with six (6) modules. More than that DOES NOT WORK— the Mac Pro will show 128GB installed, but only 96GB is accessible.
I attribute the 96GB limitation to system software issues (bug?), not a hardware limitation. OWC tells me that a Windows boot can address 128GB on the same Mac Pro, so this is clearly a Mac OS X BUG.
Using 64-bit MemoryTester, the alloc command can allocate about 91GB of that 96GB before severe virtual memory paging begins. So the absolute limit to addressable memory for a single program about 91GB.
Cut those figures in half for a 4/6-core Mac Pro which can take 3 X 16GB memory modules: about 43-44GB of 48GB.
Mix and match
Since the OWC 16GB and 8GB modules can be freely intermixed.
For example, existing 4/6-core Mac Pro users with 3 X 8GB (24GB) could add a 16G module to go to 40GB
Or 12-core users with 6 X 8GB modules could add two 16GB modules to go to 80GB, thus preserving the investment in existing 8GB modules.
Optimal setup for maximum memory bandwidth remains triple-channel memory configuration:
- 3 modules (not 4) in the 4/6-core Mac Pro (fill 3 of 4 slots).
- 6 modules in the 8/12-core Mac Pro (fill 6 of 8 slots).
However, the real-world impact of triple channel vs dual channel memory bandwidth is very small, so this can be safely ignored for practical purposes, and if the additional memory is actually needed, then more is far faster.
Example in my 12-core 3.33GHz Mac Pro
Shown below is one memory configuration in my 12-core 3.33 GHz Mac Pro.
While 128GB is recognized by the hardware, Mac OS X Lion will only use 96GB.
Evidently, Apple engineers don’t test for large memory configurations. This would not be the first reporting bug I’ve seen in Activity Monitor.