## Parallelization with Haskell – Easy as can be

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 f1 and f2 in parallel, return the sum as the result

where

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.