Ticket: http://hackage.haskell.org/trac/ghc/ticket/4859 I think these functions are implementation-independent enough to add to the main `Control.Concurrent` API: {{{ {- | Like 'forkIO', but lets you specify on which CPU the thread is created. Unlike a `forkIO` thread, a thread created by `forkOnIO` will stay on the same CPU for its entire lifetime (`forkIO` threads can migrate between CPUs according to the scheduling policy). `forkOnIO` is useful for overriding the scheduling policy when you know in advance how best to distribute the threads. The `Int` argument specifies the CPU number; it is interpreted modulo the value returned by 'getNumCapabilities'. -} forkOnIO :: Int -> IO () -> IO ThreadId -- | Like 'forkIOWithUnmask', but the child thread is pinned to the -- given CPU, as with 'forkOnIO'. forkOnIOWithUnmask :: Int -> ((forall a . IO a -> IO a) -> IO ()) -> IO ThreadId {- | Returns the number of Haskell threads that can run truly simultaneously (on separate physical processors) at any given time. The CPU number passed to `forkOnIO` is interpreted modulo this value. An implementation in which Haskell threads are mapped directly to OS threads might return the number of physical processor cores in the machine, and 'forkOnIO' would be implemented using the OS's affinity facilities. An implementation that schedules Haskell threads onto a smaller number of OS threads (like GHC) would return the number of such OS threads that can be running simultaneously. GHC notes: this returns the number passed as the argument to the @+RTS -N@ flag. In current implementations, the value is fixed when the program starts and never changes, but it is possible that in the future the number of capabilities might vary at runtime. -} getNumCapabilities :: IO Int -- | returns @Just x@ if the given thread was created with either -- @forkOnIO x@ or @forkOnIOWithUnmask@, or @Nothing@ otherwise. threadIsPinned :: ThreadId -> IO (Maybe Int) }}} In base 4.3.0.0 (GHC 7.0.1) we currently have `forkOnIO` and `forkOnIOUnmasked`, available from `GHC.Conc`. I am about to add the other functions to `GHC.Conc`, and deprecate `forkOnIOUnmasked` (see http://hackage.haskell.org/trac/ghc/ticket/4457). This proposal is to export the above functions from`Control.Concurrent` too. A feature request for `threadIsPinned` was submitted as http://hackage.haskell.org/trac/ghc/ticket/4457. Naming is up for grabs: I'm not at all sure that "capabilities" is a good word here, but I can't think of any better ideas. "processors" or "CPUs" don't seem quite right. Discussion period: 4 weeks (until 19 Jan 2011) Cheers, Simon