Hello All I was going trough Real World Haskell and it says "If we try to put a value into an MVar that is already full, our thread is put to sleep until another thread takes the value out". I wrote a simple code to block main import Data.List import Control.Concurrent fun m = do putMVar m 10 return () main = do m <- newEmptyMVar forkIO $ fun m putMVar m 10 return () What I am expecting that main should be blocked at least couple of times but its behaving more deterministically. [mukesh.tiwari@ Programming]$ ghc-7.4.1 -threaded -fforce-recomp Concurrent.hs [1 of 1] Compiling Main ( Concurrent.hs, Concurrent.o ) Linking Concurrent ... [mukesh.tiwari@ Programming]$ ./Concurrent +RTS -N2 [mukesh.tiwari@ Programming]$ ./Concurrent +RTS -N2 [mukesh.tiwari@ Programming]$ ./Concurrent +RTS -N2 [mukesh.tiwari@ Programming]$ ./Concurrent +RTS -N2 [mukesh.tiwari@ Programming]$ ./Concurrent +RTS -N2 [mukesh.tiwari@ Programming]$ ./Concurrent +RTS -N2 [mukesh.tiwari@ Programming]$ ./Concurrent +RTS -N2 [mukesh.tiwari@ Programming]$ ./Concurrent +RTS -N2 [mukesh.tiwari@ Programming]$ ./Concurrent +RTS -N2 [mukesh.tiwari@ Programming]$ ./Concurrent +RTS -N2 [mukesh.tiwari@ Programming]$ ./Concurrent +RTS -N2 [mukesh.tiwari@ Programming]$ ./Concurrent +RTS -N2 [mukesh.tiwari@ Programming]$ ./Concurrent +RTS -N2 [mukesh.tiwari@ Programming]$ ./Concurrent +RTS -N2 [mukesh.tiwari@ Programming]$ ./Concurrent +RTS -N2 [mukesh.tiwari@ Programming]$ I am expecting to get thread blocked indefinitely on MVar at least half the time. Could some one please tell me why this deterministic behavior ? Regards Mukesh Tiwari