I just read Kazu Yamamoto's article on a high performance web server in the latest Monad.Reader, and I came across a statement that doesn't sound correct to me. He says: "When a user thread issues a system call, a context switch occurs. This means that all Haskell user threads stop, and instead the kernel is given the CPU time. " Is this right? I thought that when a system call is made by a Haskell thread being run by a particular worker thread on a CPU, other runnable Haskell threads in the run queues of the HECs for other CPUs can continue running concurrently (provided we've run our Haskell program with multiple CPUs using the -Nx RTS argument). That's what I understood from the discussion of foreign calls in "Runtime Support for Multicore Haskell". Andreas