Seq no more: Better Strategies for Parallel Haskell

Simon Marlow, Patrick Maier, Hans-Wolfgang Loidl, Mustafa K. Aswad, and Phil Trinder.
To appear in Haskell Symposium 2010, Baltimore, MD, United States, 30th September, 2010.

We present a complete redesign of evaluation strategies, a key abstraction for specifying pure, deterministic parallelism in Haskell. Our new formulation preserves the compositionality and modularity benefits of the original, while providing significant new benefits. First, we introduce an evaluation-order monad to provide clearer, more generic, and more efficient specification of parallel evaluation. Secondly, the new formulation resolves a subtle space management issue with the original strategies, allowing parallelism (sparks) to be preserved while reclaiming heap associated with superfluous parallelism. Related to this, the new formulation provides far better support for speculative parallelism as the garbage collector now prunes unneeded speculation. Finally, the new formulation provides improved compositionality: we can directly express parallelism embedded within lazy data structures, producing more compositional strategies, and our basic strategies are parametric in the coordination combinator, facilitating a richer set of parallelism combinators.

We give measurements over a range of benchmarks demonstrating that the runtime overheads of the new formulation relative to the original are low, and the new strategies even yield slightly better speedups on average than the original strategies.

  author    = {Marlow, Simon and
               Maier, Patrick and
               Loidl, Hans-Wolfgang and
               Aswad, Mustafa K. and
               Trinder, Phil},
  title     = {Seq no more: Better Strategies for Parallel Haskell},
  booktitle = {Proceedings of the 3rd ACM SIGPLAN symposium on Haskell},
  publisher = {ACM Press},
  year      = {2010},
  month     = sep,
  address   = {Baltimore, MD, United States},
  pages     = {91--102}

Available in: pdf

Benchmark programs: Benchmark tarball (.tar.bz2)

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