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Haskell - Parallelism
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  The Eval Monad

Parallelism in Haskell can be expressed using the Eval Monad from Control.Parallel.Strategies (opens new window), using the rpar and rseq functions (among others).

f1 :: [Int]
f1 = [1..100000000]

f2 :: [Int]
f2 = [1..200000000]

main = runEval $ do
  a <- rpar (f1) -- this'll take a while...
  b <- rpar (f2) -- this'll take a while and then some...
  return (a,b)

Running main above will execute and "return" immediately, while the two values, a and b are computed in the background through rpar.

Note: ensure you compile with -threaded for parallel execution to occur.
  rpar

rpar :: Strategy a executes the given strategy (recall: type Strategy a = a -> Eval a) in parallel:

import Control.Concurrent
import Control.DeepSeq
import Control.Parallel.Strategies
import Data.List.Ordered

main = loop
  where 
    loop = do
      putStrLn "Enter a number"
      n <- getLine

      let lim = read n :: Int
          hf  = quot lim 2
          result = runEval $ do
            -- we split the computation in half, so we can concurrently calculate primes
            as <- rpar (force (primesBtwn 2 hf))
            bs <- rpar (force (primesBtwn (hf + 1) lim))
            return (as ++ bs)

      forkIO $ putStrLn ("\nPrimes are: " ++ (show result) ++ " for " ++ n ++ "\n")
      loop

-- Compute primes between two integers
-- Deliberately inefficient for demonstration purposes
primesBtwn n m = eratos [n..m]
  where
    eratos []     = []
    eratos (p:xs) = p : eratos (xs `minus` [p, p+p..])

Running this will demonstrate the concurrent behaviour:

Enter a number
12
Enter a number

Primes are: [2,3,5,7,8,9,10,11,12] for 12

100
Enter a number

Primes are: [2,3,5,7,11,13,17,19,23,29,31,37,41,43,47,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70,71,72,73,74,75,76,77,78,79,80,81,82,83,84,85,86,87,88,89,90,91,92,93,94,95,96,97,98,99,100] for 100

200000000
Enter a number
-- waiting for 200000000    
200
Enter a number

Primes are: [2,3,5,7,11,13,17,19,23,29,31,37,41,43,47,53,59,61,67,71,73,79,83,89,97,101,102,103,104,105,106,107,108,109,110,111,112,113,114,115,116,117,118,119,120,121,122,123,124,125,126,127,128,129,130,131,132,133,134,135,136,137,138,139,140,141,142,143,144,145,146,147,148,149,150,151,152,153,154,155,156,157,158,159,160,161,162,163,164,165,166,167,168,169,170,171,172,173,174,175,176,177,178,179,180,181,182,183,184,185,186,187,188,189,190,191,192,193,194,195,196,197,198,199,200] for 200

-- still waiting for 200000000
  rseq

We can use rseq :: Strategy a to force an argument to Weak Head Normal Form:

f1 :: [Int]
f1 = [1..100000000]

f2 :: [Int]
f2 = [1..200000000]

main = runEval $ do
  a <- rpar (f1) -- this'll take a while...
  b <- rpar (f2) -- this'll take a while and then some...
  rseq a
  return (a,b)

This subtly changes the semantics of the rpar example; whereas the latter would return immediately whilst computing the values in the background, this example will wait until a can be evaluated to WHNF.

Parameters

Type/Function Detail
data Eval a (opens new window) Eval is a Monad that makes it easier to define parallel strategies
type Strategy a = a -> Eval a (opens new window) a function that embodies a parallel evaluation strategy. The function traverses (parts of) its argument, evaluating subexpressions in parallel or in sequence
rpar :: Strategy a sparks its argument (for evaluation in parallel)
rseq :: Strategy a evaluates its argument to weak head normal form
force :: NFData a => a -> a evaluates the entire structure of its argument, reducing it to normal form, before returning the argument itself. It is provided by the Control.DeepSeq module

Remarks

Simon Marlow's book (opens new window), Concurrent and Parallel Programming in Haskell, is outstanding and covers a multitude of concepts. It is also very much accessible for even the newest Haskell programmer. It is highly recommended and available in PDF or online for free.

Parallel vs Concurrent

Simon Marlow puts it best (opens new window):

A parallel program is one that uses a multiplicity of computational hardware (e.g., several processor cores) to perform a computation more quickly. The aim is to arrive at the answer earlier, by delegating different parts of the computation to different processors that execute at the same time.

By contrast, concurrency is a program-structuring technique in which there are multiple threads of control. Conceptually, the threads of control execute “at the same time”; that is, the user sees their effects interleaved. Whether they actually execute at the same time or not is an implementation detail; a concurrent program can execute on a single processor through interleaved execution or on multiple physical processors.

Weak Head Normal Form

It's important to be aware of how lazy-evaluation works. The first section of this chapter (opens new window) will give a strong introduction into WHNF and how this relates to parallel and concurrent programming.

 

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