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I have a Mac Pro tower with a 2 x 2.66 GHz 6-Core Intel Xeon chip and 8 RAM slots.

I have been told because of the processor's triple channel memory capacity that I should be installing RAM modules in sets of three. I do a lot of compiling on this machine and want to maximize all the hardware I've got. Does it follow that I should avoid installing RAM in the last two slots?

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It could be me, but 8 slots suggest dual-channel... I'd have thought that triple-channel would have 6 slots. Is it the machine from apple.com/macpro/specs.html ? I'd suspect that Apple have only connected up 2 of the three memory controllers available on the chips in these systems, as it says "Eight memory slots (four per processor)" and there's no way you could get a triple-channel system using 4 slots. it could be that the last memory channel is used for inter-processor communications and memory access using NUMA: en.wikipedia.org/wiki/Non-Uniform_Memory_Access –  Mokubai Jan 25 '11 at 17:52
@Mokubai - it's the 12-core option –  fbrereto Jan 25 '11 at 19:58
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The latency increase (if there would even be one on a high-quality motherboard) would be outweighed by the increase in availible memory. Don't sweat the slot voodoo and get the RAM you need.

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Do you have anything empirical to back this up? Otherwise I'm puzzled Intel would add another memory channel to begin with, instead of just advising users to buy more RAM. –  fbrereto Jan 25 '11 at 19:57
I have seen consumer motherboard from Intel with 4 memory slots. The first three on the same channel and able to run as single, dual and triple channel mode. The fourth slot was present (and would be single). Slot 5 and 6 were not ion the board. Probably as a cost issue. So it is not MAC HW specific. –  Hennes Sep 7 '13 at 17:10
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