Fast and optimally-reliable application-specific multiprocessor-synthesis is critical in system-level design, especially in medical, automotive, space, and military applications. Previous work in multiprocessor-synthesis and task-allocation for performance and reliability requires exponential time, and therefore, is useful only for small examples. We present the first deterministic and provably-optimal algorithm (RELSYN-OPT) to synthesize real-time, reliable multiprocessors using a heterogeneous library of N processors and L link types. We prove that for a series-parallel graph with M subtasks and nested-depth d, the worst-case computational complexity of RELSYN-OPT Is O(M/spl middot/(L+N)/spl middot/N/sup d/). For tree-structured task graphs, RELSYN-OMT runs in O(M/spl middot/(L+N)), and is asymptotically optimum, RELSYN-OPT, because of its speed, applies to static and dynamic task allocation for an ultra-reliable distributed processing environment for which, until now, research has produced only suboptimal heuristic solutions.