Writing "fib" to run in parallel:

I'm learning Haskell and trying write code to execute in parallel, but Haskell always runs it sequentially. And when I execute with the -N2 runtime flag it take more time to execute than if I omit this flag.

Here is code:

import Control.Parallel
import Control.Parallel.Strategies

fib :: Int -> Int
fib 1 = 1
fib 0 = 1
fib n = fib (n - 1) + fib (n - 2)

fib2 :: Int -> Int
fib2 n = a `par` (b `pseq` (a+b))
    where a = fib n
          b = fib n + 1

fib3 :: Int -> Int
fib3 n = runEval $ do
                a <- rpar (fib n)
                b <- rpar (fib n + 1)
                rseq a
                rseq b
                return (a+b)

main = do putStrLn (show (fib3 40))

What did I do wrong? I tried this sample in Windows 7 on Intel core i5 and in Linux on Atom.

Here is log from my console session:

ghc -rtsopts -threaded -O2 test.hs
[1 of 1] Compiling Main             ( test.hs, test.o )

test +RTS -s
331160283
          64,496 bytes allocated in the heap
           2,024 bytes copied during GC
          42,888 bytes maximum residency (1 sample(s))
          22,648 bytes maximum slop
               1 MB total memory in use (0 MB lost due to fragmentation)

  Generation 0:     0 collections,     0 parallel,  0.00s,  0.00s elapsed
  Generation 1:     1 collections,     0 parallel,  0.00s,  0.00s elapsed

  Parallel GC work balance: nan (0 / 0, ideal 1)

                        MUT time (elapsed)       GC time  (elapsed)
  Task  0 (worker) :    0.00s    (  6.59s)       0.00s    (  0.00s)
  Task  1 (worker) :    0.00s    (  0.00s)       0.00s    (  0.00s)
  Task  2 (bound)  :    6.33s    (  6.59s)       0.00s    (  0.00s)

  SPARKS: 2 (0 converted, 0 pruned)

  INIT  time    0.00s  (  0.00s elapsed)
  MUT   time    6.33s  (  6.59s elapsed)
  GC    time    0.00s  (  0.00s elapsed)
  EXIT  time    0.00s  (  0.00s elapsed)
  Total time    6.33s  (  6.59s elapsed)

  %GC time       0.0%  (0.0% elapsed)

  Alloc rate    10,191 bytes per MUT second

  Productivity 100.0% of total user, 96.0% of total elapsed

gc_alloc_block_sync: 0
whitehole_spin: 0
gen[0].sync_large_objects: 0
gen[1].sync_large_objects: 0


test +RTS -N2 -s 
331160283
          72,688 bytes allocated in the heap
           5,644 bytes copied during GC
          28,300 bytes maximum residency (1 sample(s))
          24,948 bytes maximum slop
               2 MB total memory in use (0 MB lost due to fragmentation)

  Generation 0:     1 collections,     0 parallel,  0.00s,  0.00s elapsed
  Generation 1:     1 collections,     1 parallel,  0.00s,  0.01s elapsed

  Parallel GC work balance: 1.51 (937 / 621, ideal 2)

                        MUT time (elapsed)       GC time  (elapsed)
  Task  0 (worker) :    0.00s    (  9.29s)       0.00s    (  0.00s)
  Task  1 (worker) :    4.53s    (  9.29s)       0.00s    (  0.00s)
  Task  2 (bound)  :    5.84s    (  9.29s)       0.00s    (  0.01s)
  Task  3 (worker) :    0.00s    (  9.29s)       0.00s    (  0.00s)

  SPARKS: 2 (1 converted, 0 pruned)

  INIT  time    0.00s  (  0.00s elapsed)
  MUT   time   10.38s  (  9.29s elapsed)
  GC    time    0.00s  (  0.01s elapsed)
  EXIT  time    0.00s  (  0.00s elapsed)
  Total time   10.38s  (  9.30s elapsed)

  %GC time       0.0%  (0.1% elapsed)

  Alloc rate    7,006 bytes per MUT second

  Productivity 100.0% of total user, 111.6% of total elapsed

gc_alloc_block_sync: 0
whitehole_spin: 0
gen[0].sync_large_objects: 0
gen[1].sync_large_objects: 0

---

I think answer is that "GHC will optimise the fib function so that it does no allocation, and computations that do no allocation cause problems for the RTS because the scheduler never gets to run and do load-balancing (which is necessary for parallelism)" as wrote Simon in this discussion group. Also I found good tutorial.

链接地址: http://www.djcxy.com/p/57264.html

上一篇: Matplotlib：从多个子图中获取单个子图

下一篇: 写“fib”并行运行：