The rankHandlers[ ] matrix for a three-rank communicator. Each cell (i, j) is the Handler that rank i holds for rank j. Diagonal cells (blue background) are self-handlers using plain allocated memory. Off-diagonal cells (orange background) are remote handlers backed by RDMA-registered memory. Each Handler is an SPSC ring buffer: P shows the particle buffer with dark cells indicating occupied slots, and the labels H and T denote the current head and tail counters. All handlers are lock-free.