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Lucid Nonsense

Mac Pro Memory Musings

Apple released new Macs at the beginning of the month including new Mac Pros based on the Intel Nehalem architecture. This is a fairly big architecture change for Intel, and will be marketed under the i7 brand for consumer chips while for workstations and servers, the Xeon brand will continue to be used. The new Mac Pros are using either one or two quad core Nehalem Xeon chips.

There are several big differences that these chips have brought to the Mac Pros, including hyperthreading and a new interconnect known as QuickPath (QPI). One thing that has made a big difference though is how this new architecture handles memory. The Nehalem chips have an on-board memory controller, instead of this being in a separate chip (commonly known as a Northbridge). This approach is something AMD have been benefitting from for years, and brings a significant boost in memory performance. That in itself is one of the big factors that gives the Nehalem based Xeons an edge over their predecessors, although it’s not the only change.

Unlike the previous Mac Pros that used the fairly unusual FB-DIMM standard, the current models have reverted to using standard DIMMs, while moving to the faster DDR3 RAM in the process. As with the previous models the RAM is still ECC (Error Correcting Code) RAM. One fairly controversial difference is that the quad core Mac Pro now also contains only four RAM slots (down from the last quad core’s eight). The current eight core Mac Pro retains eight slots and it’s 32GB memory limit. Apple state that the quad core Mac Pro is only capable of supporting 8GB of memory, using 2GB DIMMs. This doesn’t appear to be technically accurate, but it’s fairly clear why they don’t offer an option to buy a quad core Mac Pro with more than 8GB of RAM. From a quick check of the Apple store at the prices Apple charge for the 4GB RAM modules it would be cheaper to spec an eight core Mac Pro with 16GB of RAM (8 x 2GB) than it would be to spec a quad core Mac Pro with 16GB of RAM (4 x 4GB). Fortunately other companies will be selling the 4GB RAM sticks cheaper than Apple fairly shortly and the quad core Mac Pros seem to accept them fine, although even from third parties they cost significantly more than 2 x 2GB sticks.

To add to the fun and games the memory system on these models has one further change; triple interleaving. Most Mac users will be using dual interleaving even if they’re not aware of it; it’s been used on Macs for years. Triple interleaving is just the next, fairly logical, step. Interleaving works to reduce latency to RAM by splitting requests across several banks of matched RAM. With dual interleaving you need two sticks of matched RAM, with triple interleaving, unsurprisingly, you need three. One thing that is not entirely clear is how this is handled when four, or on the eight core Mac Pro, all eight slots are used. Triple interleaving doesn’t work over four or eight sticks, you need three or six sticks, so the obvious answer is that when the maximum RAM is installed in a Mac Pro, it drops to dual interleaving, rather than triple, with a slight speed decrease as a result. Apple certainly don’t make this clear and they were the first company to receive the Nehalem based Xeons, so documentation is currently a little thin on the ground. From reading some of Intel’s i7 documentation this looks like the correct reading to me. Other people however seem to think that four RAM sticks are used as a set of triple interleaved RAM plus a single stick. I’ve not found anything to support that. Yet.

I’m trying to get an official answer from Apple on this one, but certainly it looks like to get the best RAM performance out of a Mac Pro the RAM now needs to be installed in threes. Certainly early benchmarks seem to bear that out as well. As mentioned in that article some applications may perform better with larger amounts of RAM at the (marginally) slower speed, but it’s certainly something to think about if you’re looking at a Mac Pro and speed is absolutely of the essence for your work.