Backtracking-based load balancing

  • Authors:

  • Tasuku Hiraishi;Masahiro Yasugi;Seiji Umatani;Taiichi Yuasa

  • Affiliations:

  • Kyoto University, Kyoto, Japan;Kyoto University, Kyoto, Japan;Kyoto University, Kyoto, Japan;Kyoto University, Kyoto, Japan

  • Venue:
  • Proceedings of the 14th ACM SIGPLAN symposium on Principles and practice of parallel programming
  • Year:
  • 2009

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Abstract

High-productivity languages for parallel computing become more important as parallel environments including multicores become more common. Cilk is such a language. It provides good load balancing for many applications including irregular ones; that is, it keeps all workers busy by creating plenty of "logical" threads and adopting the oldest-first work stealing strategy. This paper proposes a "logical thread"-free framework called Tascell, which achieves a higher performance and supports a wider range of parallel environments including clusters without loss of productivity. A Tascell worker spawns a "real" task only when requested by another idle worker. The worker performs the spawning by temporarily "backtracking" and restoring its oldest task-spawnable state. Our approach eliminates the cost of spawning/managing logical threads. It also promotes the reuse of workspaces and improves the locality of reference since it does not need to prepare a workspace for each concurrently runnable logical thread. Furthermore, Tascell enables elegant and highly-efficient backtrack search algorithms with delayed workspace copying. For instance, our 16-queens problem solver is 1.86 times faster than Cilk on a system with two dual-core processors. Our approach also enables a single program to run in both shared and distributed memory environments with reasonable efficiency and scalability.