Min-cut program decomposition for thread-level speculation

Abstract

With billion-transistor chips on the horizon, single-chip multiprocessors (CMPs) are likely to become commodity components. Speculative CMPs use hardware to enforce dependence, allowing the compiler to improve performance by speculating on ambiguous dependences without absolute guarantees of independence. The compiler is responsible for decomposing a sequential program into speculatively parallel threads, while considering multiple performance overheads related to data dependence, load imbalance, and thread prediction. Although the decomposition problem lends itself to a min-cut-based approach, the overheads depend on the thread size, requiring the edge weights to be changed as the algorithm progresses. The changing weights make our approach different from graph-theoretic solutions to the general problem of task scheduling. One recent work uses a set of heuristics, each targeting a specific overhead in isolation, and gives precedence to thread prediction, without comparing the performance of the threads resulting from each heuristic. By contrast, our method uses a sequence of balanced min-cuts that give equal consideration to all the overheads, and adjusts the edge weights after every cut. This method achieves an (geometric) average speedup of 74% for floating-point programs and 23% for integer programs on a four-processor chip, improving on the 52% and 13% achieved by the previous heuristics.