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Mac Pro memory performance

I’m not a fan of theoretical technical discussions about memory bandwidth (transfer speed) [1], [2], [3], except as a starting point for investigation—which I’ve now done*! Also, measured results are more persuasive than theoretical ones. While performance should ideally be measured for actual usage scenario(s), such results lack broad applicability, and are tedious to perform.

The number of memory modules, their “rank”, and their proper installation all affect memory bandwidth (speed) on the Mac Pro. See Recommendations.

For some programs, memory bandwidth will have little or no effect on performance. But for programs that make extensive access to memory, especially large amounts of it, performance could vary substantially between optimal and non-optimal memory configurations. Of course, an adequate amount of memory of any speed has far more impact; this discussion is largely irrelevant if Mac OS X must swap memory contents to and from the disk (“virtual memory”).

Results

I had 8 FB-DIMMS available: four (4) Apple 512MB modules (single-rank), and four (4) SATech 2GB modules (dual rank), for a total of 10GB. I tested the speed of various combinations. “Speed” here means the speed of large-block memory copying.

Mac Pro Memory Bandwidth
Configuration Installed In Memory Copy Speed
(aggregate, 4 threads)
Relative Speed
2 X 512MB A1, A2 1440 MB/sec  49
4 X 512MB A1, A2
B1, B2
2640 MB/sec  90
2 X 2GB A1, A2 1724 MB/sec  59
4 X 2GB A1, A2
B1, B2
2924 MB/sec 100
2 X 2GB +
2 X 512MB
A1, A2
B1, B2
2640 MB/sec
1724 MB/sec
 90
 59
4 X 2GB +
4 X 512MB
A1, A2, B1, B2
A3, A4, B3, B4
2924 MB/sec
2560 MB/sec
100
 87


Recommendations

Modules must be installed in pairs of matched size (see Apple’s Replacement Instructions and RAM Expansion Product-Specific Details). So there can be 2, 4, 6 or 8 modules installed. The riser boards are nicely labeled as to which slot is which (slots 1/2/3/4 on each riser board).

  1. Aim for a configuration of either 4 or 8 modules, not 2 or 6. Each group of 4 modules should be the same size, and occupy the same slots on each riser board.
  2. Avoid single-rank modules. Apple’s 512MB modules are single-rank. However, 4 of them do offer 90% of the performance of the dual-rank modules, so they’re still speedy in groups of 4.
  3. If you have mixed sizes, install the fastest modules in the lowest-numbered slots. Mac OS X allocates memory from the lowest-numbered slots first.

Keep in mind that the performance of any specific program might or might not benefit from the optimal configuration. On the other hand, there is no reason not to purchase the optimal configuration, assuming your budget allows it.

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