Parallelization with Haskell - Easy as can be

June 07, 2009

The functional programming language Haskell provides a very easy way of parallelization.

Consider the following naive implementation of the Fibonacci function.

fib 0 = 0
fib 1 = 1
fib n = fib (n-1) + fib (n-2)

This implementation has a bad expontential time complexity, so it should be improved, for example with caching. But this is beyond the scope of this article. We just need a function that takes a while to finish.

In Haskell there are two operators that have to be used for parallelization: par and pseq. par a b is some kind of a “fork” operation: a is started in parallel and b is returned. Keep in mind that Haskell is has a lazy evaluation strategy. a is only evaluated if it is needed The function pseq a b evaluates first a then b.

Equipped with this two operations it is very easy to parallelize fib.

parfib n
  | n < 11    = fib n                        -- For small values of n we use the sequential version

  | otherwise = f1 `par` (f2 `pseq` (f1+f2)) -- calculate in parallel, return the sum

  f1 = parfib (n-1)
  f2 = parfib (n-2)

The code has to be compiled with the -threaded option.

$ ghc -O3 -threaded --make -o parfib ParFib.hs

The number of threads is specified at runtime with the -N command line option.

$ ./parfib +RTS -N7 -RTS

On an Intel Core i7 920 this resulted in a speedup of 4.13 for n=38. This processor has four physical cores. So this is efficient. Haskell is still one of the best programming languages.