Excerpts from t.r.willingham's message of Sun Nov 02 17:28:08 -0600 2008:
What would it take to implement a -j equivalent for, say, GHC? Or if this is not possible, what is wrong with my reasoning?
Thanks, TW
Hi, The main issue has to do with the decisions the compiler needs to make in order to generate adequate code for a general case. The problem is the compiler has to make decisions about the *granularity* of the threading in particular - the generated code for example may waste a lot of time sparking off threads using 'par' or something, for computations that take less time than the thread-creation itself, meaning the overall cost of that thread being spawned was negative. So your code would need the threading to be more coarse-grained. On the other hand, if you have some computation, the compiler may not adequately sprinkle enough par's throughout the program, and therefore we miss opportunities where the parallelism could give us a speed up - in this case, we need more fine-grained threading. So the problem is (in the general case): given an arbitrary input program, how do you adequately decide what should and should not be parallelized in that program? Even then, how do you decide the granularity at which the threads should operate? It's a pretty tough problem, and I don't think we're even close to full-blown automagically-parallelizing compilers (although with things like parallel strategies and DPH we can get close.) But there is work in this area using profiling information to guide these optimizations, see "Feedback directed implicit parallelism" here: http://research.microsoft.com/~tharris/papers/2007-fdip.pdf Austin