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OWC 128GB Memory Kit: Bandwidth
The OWC 128GB memory kit for the 2013 Mac Pro is manufactured with 1333 MHz parts, but runs at 1066 MHz when installed (this is a technical limitation). By comparison, the 64GB runs at full speed—1866 MHz—a substantial theoretical difference.
On the server-class CPUs (12-core sold by Apple, 8-core 3.3 GHz, 10-core 3.0 GHz), the modules unfortunately run at 800 MHz. But on the stock 4-core, 6-core, 8-core models as shipped by Apple, they run at 1066 MHz.
The difference in memory bandwidth as tested via the system call memcpy()* is substantial, as seen in the graph below, but in real-world applications the CPU on-chip cache largely masks the difference in memory speed; see the tests that follow.
Moreover, when more memory is needed, more memory always trumps memory speed. By the same token, if 64GB is already ample, going to slower-speed 128GB memory is not a win. In short, 128GB is for computing tasks with large data sets, such as huge Phtooshop files, large 3D modeling jobs, databases with large in-memory caches, etc.
Additional notes on the OWC 128GB memory kit
- The OWC 32GB modules (4 X 32GB = 128GB) are manufactured using 1333 MHz parts, but run at 1066 MHz due to technical limitations.
- The 32GB modules can be mixed with 16GB modules, so (for example), a 96GB configuration is possible (2 X 32GB + 2 X 16GB).
- While a single 32GB module should function (mix) with the 16GB modules, OWC advises pairs of matched modules.