Slice is a three socket Opteron machine with two PCIe slots and two Infiniband 4x ports. It will take up to 16G of RAM, but how they do that evenly with six DDR400 slots is a little beyond me. In any case, this is much more of a blade than a full system.
Why would you want three sockets rather than four? Easy, latency. Any CPU in a 3S system is one hop away from any other CPU. In a 4S system, you can be two hops away. This adds latency, and more importantly, you take a big hit on cache coherency latency. This kills performance.
Because of this, a 3S machine may end up faster than a 4S under some workloads. Given an equal CPU count, a cluster of 3S machines will be faster than 4S machines. You only pay for it in density and the odd looks your co-admins give you when you tell then you have 3S machines. µ
Sign up for INQbot – a weekly roundup of the best from the INQ