Efficient Parallel Self-Adjusting Computation
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Self-adjusting computation is an approach for automatically producing dynamic algorithms from static ones. The approach works by tracking control and data dependencies, and propagating changes through the dependencies when making an update. Extensively studied in the sequential setting, some results on parallel self-adjusting computation exist, but are either only applicable to limited classes of computations, such as map-reduce, or are ad-hoc systems with no theoretical analysis of their performance. In this paper, we present the first system for parallel self-adjusting computation that applies to a wide class of nested parallel algorithms and provides theoretical bounds on the work and span of the resulting dynamic algorithms. As with bounds in the sequential setting, our bounds relate a "distance" measure between computations on different inputs to the cost of propagating an update. However, here we also consider parallelism in the propagation cost. The main innovation in the paper is in using Series-Parallel trees (SP trees) to track sequential and parallel control dependencies to allow propagation of changes to be applied safely in parallel. We show both theoretically and through experiments that our system allows algorithms to produce updated results over large datasets significantly faster than from-scratch execution. We demonstrate our system with several example applications, including algorithms for dynamic sequences and dynamic trees. In all cases studied, we show that parallel self-adjusting computation can provide a significant benefit in both work savings and parallel time.
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