Slot 1: Memory Channel 1, Processor 1
Slot 2: Memory Channel 2, Processor 1
Slots 3 & 4: Memory Channel 3, Processor 1
Slot 5: Memory Channel 1, Processor 2
Slot 6: Memory Channel 2, Processor 2
Slots 7 & 8: Memory Channel 3, Processor 2
Slots 3 and 4, as well as slots 7 and 8, share a memory channel. The best memory performance is achieved when slot 4 (single-processor model) or slots 4 and 8 (dual-processor model) aren’t occupied. By not populating the second of the paired memory slots, you allow each memory module to connect to its own dedicated memory channel.
If you do choose to populate the last memory slots, you may reduce optimum memory performance, but only when memory in the shared slots is being accessed.If you install different-size DIMMs in your computer, follow the order in the tables above. If the DIMM configuration you install doesn’t provide optimized performance, the Memory Slot Utility will appear on screen and recommend an improved configuration. To use the Memory Slot Utility again, go to /System/Library/Core Services.
Preferred Memory Slot Population for Dual-Processor Mac Pro
Two memory modules: Slots 1 and 2.
Three memory modules: Slots 1, 2, and 3.
Four memory modules: Slots 1, 2, 5, and 6.
Six memory modules: Slots 1, 2, 3, 5, 6, and 7.
Eight memory modules: Slots 1, 2, 3, 4, 5, 6, 7, and 8.
Notice that in the above configurations, slots 4 and 8 are the last to be populated, ensuring the best overall memory performance.