Hello, Below is a program that seems to block indefinitely with ghc in a multicore machine. This program has a loop that does not produce allocations, and I understand that this may grab one of the cores. The question is, why can't the other cores take the blocked thread? The program was compiled with: $ ghc --make -O -threaded test.hs and it is run with: $ ./test Program text follows. Thanks, Facundo -------- import Control.Concurrent import Control.Monad import System.Environment main :: IO () main = do y <- getArgs mv0 <- newEmptyMVar mv1 <- newEmptyMVar forkIO $ do takeMVar mv0 putMVar mv1 () loop (y == ["yield"]) putMVar mv0 () takeMVar mv1 loop :: Bool -> IO () loop cooperative = go where go = when cooperative yield >> go