Parallel and Concurrent Programming in Haskell

Part I. Parallel Haskell

Now that processor manufacturers have largely given up trying to squeeze more performance out of individual processors and have refocused their attention on providing us with more processors instead, the biggest gains in performance are to be had by using parallel techniques in our programs so as to make use of these extra cores. Parallel Haskell is aimed at providing access to multiple processors in a natural and robust way.

You might wonder whether the compiler could automatically parallelize programs for us. After all, it should be easier to do this in a purely functional language, where the only dependencies between computations are data dependencies, which are mostly perspicuous and thus readily analyzed. However, even in a purely functional language, automatic parallelization is thwarted by an age-old problem: To make the program faster, we have to gain more from parallelism than we lose due to the overhead of adding it, and compile-time analysis cannot make good judgments in this area. An alternative approach is to use runtime profiling to find good candidates for parallelization and to feed this information back into the compiler. Even this, however, has not been terribly successful in practice.

Fully automatic parallelization is still a pipe dream. However, the parallel programming models provided by Haskell do succeed in eliminating some mundane or error-prone aspects traditionally associated with parallel programming:

• Parallel programming in Haskell ...