[fzzzy]

Are they really preemptive? How does the scheduler decide when to switch?

[Peaker]

Yeah, they are preemptive (though there had been a long-standing bug where threads that have no allocations don't get preempted, I believe it is fixed now).

This is some documentation of the scheduler: http://blog.ezyang.com/2013/01/the-ghc-scheduler/