# [Haskell-cafe] Can't seem to get `par` working appropriately with lists

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 )

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.
```