Hello, Cafe, I'm currently reading Parallel and Concurrent Programming in Haskell. I tried to implement an n-queens problem as an exercise, but a little bit overwhelmed by the strategy mystery. The code is listed at http://pastebin.com/ySgc3VSR. I implemented the higher order skeleton of search and try to add parallelism. My original idea was spawning a few threads while the search tree was small and each thread handles the following expansion of the tree. Hence, I gave a "maxDepth" variable to restrict the parallelism. But this did not go very well if I set the maxDepth to very small value, say, 2, 3, 4 etc. The speed up was very small. Viewing in threadscope, it seems there was very small amount of parallelism. Then I tried to let it go, which means I set the tree depth to a value larger than the maximum possible depth, say 20. Then it speeds up about 1.4 times on 2-cores. Viewing in threadscope, it seems the 2 cores are fully utilized. But the report is a bit annoying: SPARKS: 4686728 (100456 converted, 3461 overflowed, 0 dud, 4540817 GC'd, 41994 fizzled) with no restrictions on the parallelism, a lot of sparks are not converted and are GC'd. I wonder if there is some way to further improve the results, reducing un-convereted sparks and achieving better speed-up ratio. Thanks.