[Haskell-cafe] Can't seem to get `par` working appropriately with
lists
Luke Andrew
luke.haskell-cafe at la.id.au
Thu Feb 21 06:58:05 EST 2008
Hopefully an easy one here; after reading Don Stewart's blog posts about
parallel Haskell and with a shiny new quad-core cpu begging for a
workout, i thought I'd give Haskell a try. I've also been meaning to
write a ray-tracer, so I started with that. I've got the initial
ray-tracer working, and am now looking to parallize it. I tried using
the `par` function to evaluate things in parallel, but I couldn't get it
to work with lists. I simplified my problem down into the following 2
test cases:
(there are two fibonacci functions to ensure haskell treats them as 2
independent computations to be spread across 2 cores)
test1.hs:
import Control.Parallel
fib1 n = if n == 0 then 0 else if n == 1 then 1 else fib1 (n-1) +
fib1 (n-2)
fib2 n = if n == 0 then 0 else if n == 1 then 1 else fib2 (n-1) +
fib2 (n-2)
main = do print $ (fib2 37 `par` fib1 37) + (fib2 37)
compilation & testing:
luke at Sonata:~/mcls$ ghc -O2 -threaded --make test1
[1 of 1] Compiling Main ( test1.hs, test1.o )
Linking test1 ...
luke at Sonata:~/mcls$ time ./test1 +RTS -N1
48315634
real 0m5.856s
user 0m5.816s
sys 0m0.004s
luke at Sonata:~/mcls$ time ./test1 +RTS -N2
48315634
real 0m3.450s
user 0m6.734s
sys 0m0.024s
As expected, running it with 2 cores is substantially faster. Doing
almost the same thing, but with lists, doesn't seem to have any
significant speed difference:
test2.hs:
import Control.Parallel
fib1 n = if n == 0 then 0 else if n == 1 then 1 else fib1 (n-1) +
fib1 (n-2)
fib2 n = if n == 0 then 0 else if n == 1 then 1 else fib2 (n-1) +
fib2 (n-2)
fiblist1 n = [fib1 x| x <- [1..n]]
fiblist2 n = [fib2 x| x <- [1..n]]
main = do print $ zipWith (+) (fiblist2 37 `par` fiblist1 37)
(fiblist2 37)
compilation & testing:
luke at Sonata:~/mcls$ ghc -O2 -threaded --make test2
luke at Sonata:~/mcls$ time ./test2 +RTS -N1
[2,2,4,6,10,16,26,42... ...405774,18454930,29860704,48315634]
real 0m15.294s
user 0m15.196s
sys 0m0.013s
luke at Sonata:~/mcls$ time ./test2 +RTS -N2
[2,2,4,6,10,16,26,42... ...405774,18454930,29860704,48315634]
real 0m15.268s
user 0m15.169s
sys 0m0.013s
I've tried using bang patterns in various places, but to no avail. Any
ideas?
Luke Andrew.
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