AnandTech has an interesting article “Understanding FB DIMMS”, as part of its discussion of the new Apple Mac Pro (FB-DIMMS are the technical name for the type of memory module used by the Mac Pro).
It’s really an article for nerds, and I confess I’m not nerdy enough to really appreciate it all, but there are a few choice points made which are worthwhile to understand:
- Filling all 8 memory slots will in theory reduce memory performance over using just 4 slots due to the
nature of the serial data transfer in an FB-DIMM—increased latency of 3-5 nanoseconds. Of course, the slots must
be filled appropriately in both cases. So the ideal configurations are 4 X 512GB,
4 X 1GB, or 4 X 2GB. For most users, the price of buying 2GB FB-DIMMS is prohibitive. Whether any of this matters in
practice is another matter—and the effects are likely to be very small.
- AnandTech states that each FB-DIMM consumes between 3 and 6 watts resulting in “~40W” with all 8 slots populated. The math varies because, according to AnandTech, “there’s a range is because how active the AMB is depends on how close it is to the memory controller”.
Since heat is a good way to get a system crash or other failure, it seems unwise to buy cheap memory that doesn’t have heat sinks—40 watts is a lot of heat in the small space occupied by the memory modules. I won’t be buying Mac Pro memory without the heat sinks.
Bottom line: stick to 4 or 8 modules, installed correctly, unless you have an unlimited budget. If you really only need 4GB, buy 4 X 1GB and be happy. If you need 8GB, buy 8 X 1GB and be happy. If you need 16GB (first verify that you need 16GB), be happy until the bill comes due.