Ketil, For your particular problem, unevaluated thunks should be easy to check: dump a heap profile and look for a decreasing allocation of thunks. That being said, IntMap is spine strict, so that will all be evaluated, and if your threads are accessing disjoint keys there should be no contention. If there is, yes, threads will be blocking on evaluation, I don't have a good sense for how slow that tends to be. (Cache effects may be swamping you.) You may be interested in repa, if your maps are dense. Edward Excerpts from Ketil Malde's message of Wed Oct 05 17:00:11 -0400 2011:
I don't know if this is relevant to your problems, but I'm currently struggling to get some performance out of a parallel - or rather, concurrent - program.
Basically, the initial thread parses some data into an IntMap, and then multiple threads access this read-only to do the Real Work.
Now, there appears to be a lot of overhead incurred when using multiple threads, and I suspect that this is caused by the map storing unevaluated thunks, which then are forced by accesses by the worker threads. Ideally, the evaluation should be performed in parallel, but perhaps there are issues (of synchronization, say) that makes this less performant?
-k