In January 2012, OWC released the second generation of their 16GB memory modules, which currently operate at full-bus speed of 1333 MHz*, and execute as nicely as 8GB memory modules.
These 16GB modules allow a full 1333Mhz bus overall performance and are quicker in both 1066MHz and 1333Mhz memory bus Mac Pros with the Dual-Rank design than the Quad-Rank module alternatives they replace.
I tested the OWC 16B Mac Memory modules on both a 6-core 3.33 GHz and 12-core 3.33 GHz Mac Pro machine. The modules on both machines showed up as 1333 MHz, and tested at the very same speed as the 8GB memory modules.
OWC released the first Version of their 16GB memory modules for the 2009/2010/2011 Mac Pros in May of 2011. These modules worked fantastic, but did have 1 draw back of downclocking to 1066 MHz or even 800 MHz.
Available Memory settings
Max for 12-core
A 4-core or 6-core Mac Pro can work with up to 48GB.
An 8-core or 12-core up to 96GB.
More than that is acknowledged in hardware, but Mac OS X Lion 10.7.2 will not use more than 48/96GB respectively. The very same Mac booted into Windows boot can address 128GB, so this is plainly a Mac OS X bug, not a hardware error.
making use of a 64-bit memory tester, the alloc command can allocate about 91GB of that 96GB prior to severe virtual memory paging starts. So the absolute limit to addressable memory for a solitary program about 91GB, but it would be clever to limit any program to 85GB or so.
Reduce those statistics in half for a 4/6-core Mac Pro which can take 3 X 16GB memory modules: about 43-44GB of 48GB, with a sensible limit of 40GB.
Enough memory is vital for really big files in Photoshop CS5, video processing, and other memory hogging programs.
Mix and match
One can also mix 16GB and 8GB memory modules freely.
For instance, 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 increase two 16GB modules to go to 80GB, thus preserving the investment in existing 8GB modules.
Optimum setup for optimum memory bandwidth remains a triple-channel memory configuration:
- 3 modules in the 4/6-core Mac Pro (fill 3 of 4 slots), 48GB max.
- 6 modules in the 8/12-core Mac Pro (fill 6 of 8 slots), 96GB max.
Nevertheless, the real-world effect of triple channel vs dual channel memory bandwidth is quite small, so this can be safely overlooked for practical purposes, and if the additional memory is actually needed, then additional Mac Ram is a very good concept.