An interconnect is the network fabric that moves data between compute nodes, or between sockets within a node, in a parallel system. Whatever the scale, the interconnect exists to answer the same question: once a computation is split across more than one processor, how does data get from where it was produced to where it's needed. At small scale that's an on-chip bus or ring between cores or sockets; at cluster scale it's a dedicated fabric like InfiniBand connecting whole machines.

An interconnect is characterized by two largely independent numbers: latency, the fixed time to get any message across regardless of size, and bandwidth, the sustained rate of large transfers once they're underway. A workload dominated by many small messages (fine-grained synchronization, small MPI messages) is latency-bound, and no amount of extra bandwidth helps it. A workload dominated by large bulk transfers (checkpoint writes, big collective operations) is bandwidth-bound, and latency barely matters once the transfer is large enough to amortize it. HPC interconnect design (and application tuning) largely comes down to figuring out which regime a given communication pattern falls into and optimizing for that one.

How nodes are wired together shapes both cost and worst-case latency. A fat tree connects nodes through a hierarchy of switches with increasing bandwidth toward the root, giving good bisection bandwidth at the cost of more switches and cabling than a simpler topology. A torus or mesh connects each node directly to its geometric neighbors, which scales cheaply (wiring grows linearly with node count) but means far-apart nodes communicate through many hops, so worst-case latency grows with system size rather than staying flat. Real supercomputers pick a topology based on their expected communication pattern: a workload that's mostly nearest-neighbor exchange (a physical simulation on a spatial grid) fits a torus naturally, while an all-to-all collective favors a fat tree's flatter worst case.

The interconnect's hardware topology mostly stays invisible to application code, which talks to it through a library like MPI or directly through RDMA. RDMA is what lets that library move data between nodes without invoking the OS kernel or copying through an intermediate buffer on every transfer, which is where a fast physical interconnect actually translates into an application seeing low latency, rather than the fabric's speed being wasted on software overhead layered on top of it.