Comment by CaliforniaKarl

The "all nodes connecting to all other nodes" setup reminds me of NUMALink, the interconnect that SGI used on many (most? all?) of their supercomputers. In an ideal configuration, each 4-socket node has two NUMALink connections to every other node. As Jeff says, it's a ton of cables, and you don't have to think of framing or congestion in the same way as with RDMA over Ethernet.