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
