Hybrid programming is a coding strategy to create programs that make the best use of multi-node HPC systems. HPC cluster architectures are necessarily characterized by NUMA: Non-Uniform Memory Access. In multi-node systems, global memory is distributed across all the nodes, but shared within nodes (though divided among sockets within a node).

• A multithreaded programming strategy is effective and efficient for shared memory, but a program that relies only on shared memory can only use the resources of a single node.
• A message passing strategy permits processes on different nodes to communicate with one another but cannot take advantage of the efficiency of shared memory.

A hybrid strategy combines multithreading within nodes and message passing between nodes.