{-# LANGUAGE Trustworthy #-} {-# LANGUAGE CPP , MagicHash , UnboxedTuples , ScopedTypeVariables , RankNTypes #-} {-# OPTIONS_GHC -Wno-deprecations #-} -- kludge for the Control.Concurrent.QSem, Control.Concurrent.QSemN -- and Control.Concurrent.SampleVar imports. ----------------------------------------------------------------------------- -- | -- Module : Control.Concurrent -- Copyright : (c) The University of Glasgow 2001 -- License : BSD-style (see the file libraries/base/LICENSE) -- -- Maintainer : [email protected] -- Stability : experimental -- Portability : non-portable (concurrency) -- -- A common interface to a collection of useful concurrency -- abstractions. -- ----------------------------------------------------------------------------- module Control.Concurrent ( -- * Concurrent Haskell -- $conc_intro -- * Basic concurrency operations ThreadId, myThreadId, forkIO, forkFinally, forkIOWithUnmask, killThread, throwTo, -- ** Threads with affinity forkOn, forkOnWithUnmask, getNumCapabilities, setNumCapabilities, threadCapability, -- * Scheduling -- $conc_scheduling yield, -- ** Blocking -- $blocking -- ** Waiting threadDelay, threadWaitRead, threadWaitWrite, threadWaitReadSTM, threadWaitWriteSTM, -- * Communication abstractions module Control.Concurrent.MVar, module Control.Concurrent.Chan, module Control.Concurrent.QSem, module Control.Concurrent.QSemN, -- * Bound Threads -- $boundthreads rtsSupportsBoundThreads, forkOS, forkOSWithUnmask, isCurrentThreadBound, runInBoundThread, runInUnboundThread, -- * Weak references to ThreadIds mkWeakThreadId, -- * GHC's implementation of concurrency -- |This section describes features specific to GHC's -- implementation of Concurrent Haskell. -- ** Haskell threads and Operating System threads -- $osthreads -- ** Terminating the program -- $termination -- ** Pre-emption -- $preemption -- ** Deadlock -- $deadlock ) where import Control.Exception.Base as Exception import GHC.Conc hiding (threadWaitRead, threadWaitWrite, threadWaitReadSTM, threadWaitWriteSTM) import GHC.IO ( unsafeUnmask, catchException ) import GHC.IORef ( newIORef, readIORef, writeIORef ) import GHC.Base import System.Posix.Types ( Fd ) import Foreign.StablePtr import Foreign.C.Types #if defined(mingw32_HOST_OS) import Foreign.C import System.IO import Data.Functor ( void ) import Data.Int ( Int64 ) #else import qualified GHC.Conc #endif import Control.Concurrent.MVar import Control.Concurrent.Chan import Control.Concurrent.QSem import Control.Concurrent.QSemN {- $conc_intro The concurrency extension for Haskell is described in the paper /Concurrent Haskell/ <http://www.haskell.org/ghc/docs/papers/concurrent-haskell.ps.gz>. Concurrency is \"lightweight\", which means that both thread creation and context switching overheads are extremely low. Scheduling of Haskell threads is done internally in the Haskell runtime system, and doesn't make use of any operating system-supplied thread packages. However, if you want to interact with a foreign library that expects your program to use the operating system-supplied thread package, you can do so by using 'forkOS' instead of 'forkIO'. Haskell threads can communicate via 'MVar's, a kind of synchronised mutable variable (see "Control.Concurrent.MVar"). Several common concurrency abstractions can be built from 'MVar's, and these are provided by the "Control.Concurrent" library. In GHC, threads may also communicate via exceptions. -} {- $conc_scheduling Scheduling may be either pre-emptive or co-operative, depending on the implementation of Concurrent Haskell (see below for information related to specific compilers). In a co-operative system, context switches only occur when you use one of the primitives defined in this module. This means that programs such as: > main = forkIO (write 'a') >> write 'b' > where write c = putChar c >> write c will print either @aaaaaaaaaaaaaa...@ or @bbbbbbbbbbbb...@, instead of some random interleaving of @a@s and @b@s. In practice, cooperative multitasking is sufficient for writing simple graphical user interfaces. -} {- $blocking Different Haskell implementations have different characteristics with regard to which operations block /all/ threads. Using GHC without the @-threaded@ option, all foreign calls will block all other Haskell threads in the system, although I\/O operations will not. With the @-threaded@ option, only foreign calls with the @unsafe@ attribute will block all other threads. -} -- | Fork a thread and call the supplied function when the thread is about -- to terminate, with an exception or a returned value. The function is -- called with asynchronous exceptions masked. -- -- > forkFinally action and_then = -- > mask $ \restore -> -- > forkIO $ try (restore action) >>= and_then -- -- This function is useful for informing the parent when a child -- terminates, for example. -- -- @since 4.6.0.0 forkFinally :: IO a -> (Either SomeException a -> IO ()) -> IO ThreadId forkFinally action and_then = mask $ \restore -> forkIO $ try (restore action) >>= and_then -- --------------------------------------------------------------------------- -- Bound Threads {- $boundthreads #boundthreads# Support for multiple operating system threads and bound threads as described below is currently only available in the GHC runtime system if you use the /-threaded/ option when linking. Other Haskell systems do not currently support multiple operating system threads. A bound thread is a haskell thread that is /bound/ to an operating system thread. While the bound thread is still scheduled by the Haskell run-time system, the operating system thread takes care of all the foreign calls made by the bound thread. To a foreign library, the bound thread will look exactly like an ordinary operating system thread created using OS functions like @pthread_create@ or @CreateThread@. Bound threads can be created using the 'forkOS' function below. All foreign exported functions are run in a bound thread (bound to the OS thread that called the function). Also, the @main@ action of every Haskell program is run in a bound thread. Why do we need this? Because if a foreign library is called from a thread created using 'forkIO', it won't have access to any /thread-local state/ - state variables that have specific values for each OS thread (see POSIX's @pthread_key_create@ or Win32's @TlsAlloc@). Therefore, some libraries (OpenGL, for example) will not work from a thread created using 'forkIO'. They work fine in threads created using 'forkOS' or when called from @main@ or from a @foreign export@. In terms of performance, 'forkOS' (aka bound) threads are much more expensive than 'forkIO' (aka unbound) threads, because a 'forkOS' thread is tied to a particular OS thread, whereas a 'forkIO' thread can be run by any OS thread. Context-switching between a 'forkOS' thread and a 'forkIO' thread is many times more expensive than between two 'forkIO' threads. Note in particular that the main program thread (the thread running @Main.main@) is always a bound thread, so for good concurrency performance you should ensure that the main thread is not doing repeated communication with other threads in the system. Typically this means forking subthreads to do the work using 'forkIO', and waiting for the results in the main thread. -} -- | 'True' if bound threads are supported. -- If @rtsSupportsBoundThreads@ is 'False', 'isCurrentThreadBound' -- will always return 'False' and both 'forkOS' and 'runInBoundThread' will -- fail. foreign import ccall unsafe rtsSupportsBoundThreads :: Bool {- | Like 'forkIO', this sparks off a new thread to run the 'IO' computation passed as the first argument, and returns the 'ThreadId' of the newly created thread. However, 'forkOS' creates a /bound/ thread, which is necessary if you need to call foreign (non-Haskell) libraries that make use of thread-local state, such as OpenGL (see "Control.Concurrent#boundthreads"). Using 'forkOS' instead of 'forkIO' makes no difference at all to the scheduling behaviour of the Haskell runtime system. It is a common misconception that you need to use 'forkOS' instead of 'forkIO' to avoid blocking all the Haskell threads when making a foreign call; this isn't the case. To allow foreign calls to be made without blocking all the Haskell threads (with GHC), it is only necessary to use the @-threaded@ option when linking your program, and to make sure the foreign import is not marked @unsafe@. -} forkOS :: IO () -> IO ThreadId foreign export ccall forkOS_entry :: StablePtr (IO ()) -> IO () foreign import ccall "forkOS_entry" forkOS_entry_reimported :: StablePtr (IO ()) -> IO () forkOS_entry :: StablePtr (IO ()) -> IO () forkOS_entry stableAction = do action <- deRefStablePtr stableAction action foreign import ccall forkOS_createThread :: StablePtr (IO ()) -> IO CInt failNonThreaded :: IO a failNonThreaded = fail $ "RTS doesn't support multiple OS threads " ++"(use ghc -threaded when linking)" forkOS action0 | rtsSupportsBoundThreads = do mv <- newEmptyMVar b <- Exception.getMaskingState let -- async exceptions are masked in the child if they are masked -- in the parent, as for forkIO (see #1048). forkOS_createThread -- creates a thread with exceptions masked by default. action1 = case b of Unmasked -> unsafeUnmask action0 MaskedInterruptible -> action0 MaskedUninterruptible -> uninterruptibleMask_ action0 action_plus = catch action1 childHandler entry <- newStablePtr (myThreadId >>= putMVar mv >> action_plus) err <- forkOS_createThread entry when (err /= 0) $ fail "Cannot create OS thread." tid <- takeMVar mv freeStablePtr entry return tid | otherwise = failNonThreaded -- | Like 'forkIOWithUnmask', but the child thread is a bound thread, -- as with 'forkOS'. forkOSWithUnmask :: ((forall a . IO a -> IO a) -> IO ()) -> IO ThreadId forkOSWithUnmask io = forkOS (io unsafeUnmask) -- | Returns 'True' if the calling thread is /bound/, that is, if it is -- safe to use foreign libraries that rely on thread-local state from the -- calling thread. isCurrentThreadBound :: IO Bool isCurrentThreadBound = IO $ \ s# -> case isCurrentThreadBound# s# of (# s2#, flg #) -> (# s2#, isTrue# (flg /=# 0#) #) {- | Run the 'IO' computation passed as the first argument. If the calling thread is not /bound/, a bound thread is created temporarily. @runInBoundThread@ doesn't finish until the 'IO' computation finishes. You can wrap a series of foreign function calls that rely on thread-local state with @runInBoundThread@ so that you can use them without knowing whether the current thread is /bound/. -} runInBoundThread :: IO a -> IO a runInBoundThread action | rtsSupportsBoundThreads = do bound <- isCurrentThreadBound if bound then action else do ref <- newIORef undefined let action_plus = Exception.try action >>= writeIORef ref bracket (newStablePtr action_plus) freeStablePtr (\cEntry -> forkOS_entry_reimported cEntry >> readIORef ref) >>= unsafeResult | otherwise = failNonThreaded {- | Run the 'IO' computation passed as the first argument. If the calling thread is /bound/, an unbound thread is created temporarily using 'forkIO'. @runInBoundThread@ doesn't finish until the 'IO' computation finishes. Use this function /only/ in the rare case that you have actually observed a performance loss due to the use of bound threads. A program that doesn't need its main thread to be bound and makes /heavy/ use of concurrency (e.g. a web server), might want to wrap its @main@ action in @runInUnboundThread@. Note that exceptions which are thrown to the current thread are thrown in turn to the thread that is executing the given computation. This ensures there's always a way of killing the forked thread. -} runInUnboundThread :: IO a -> IO a runInUnboundThread action = do bound <- isCurrentThreadBound if bound then do mv <- newEmptyMVar mask $ \restore -> do tid <- forkIO $ Exception.try (restore action) >>= putMVar mv let wait = takeMVar mv `catchException` \(e :: SomeException) -> Exception.throwTo tid e >> wait wait >>= unsafeResult else action unsafeResult :: Either SomeException a -> IO a unsafeResult = either Exception.throwIO return -- --------------------------------------------------------------------------- -- threadWaitRead/threadWaitWrite -- | Block the current thread until data is available to read on the -- given file descriptor (GHC only). -- -- This will throw an 'IOError' if the file descriptor was closed -- while this thread was blocked. To safely close a file descriptor -- that has been used with 'threadWaitRead', use -- 'GHC.Conc.closeFdWith'. threadWaitRead :: Fd -> IO () threadWaitRead fd #if defined(mingw32_HOST_OS) -- we have no IO manager implementing threadWaitRead on Windows. -- fdReady does the right thing, but we have to call it in a -- separate thread, otherwise threadWaitRead won't be interruptible, -- and this only works with -threaded. | threaded = withThread (waitFd fd 0) | otherwise = case fd of 0 -> do _ <- hWaitForInput stdin (-1) return () -- hWaitForInput does work properly, but we can only -- do this for stdin since we know its FD. _ -> errorWithoutStackTrace "threadWaitRead requires -threaded on Windows, or use System.IO.hWaitForInput" #else = GHC.Conc.threadWaitRead fd #endif -- | Block the current thread until data can be written to the -- given file descriptor (GHC only). -- -- This will throw an 'IOError' if the file descriptor was closed -- while this thread was blocked. To safely close a file descriptor -- that has been used with 'threadWaitWrite', use -- 'GHC.Conc.closeFdWith'. threadWaitWrite :: Fd -> IO () threadWaitWrite fd #if defined(mingw32_HOST_OS) | threaded = withThread (waitFd fd 1) | otherwise = errorWithoutStackTrace "threadWaitWrite requires -threaded on Windows" #else = GHC.Conc.threadWaitWrite fd #endif -- | Returns an STM action that can be used to wait for data -- to read from a file descriptor. The second returned value -- is an IO action that can be used to deregister interest -- in the file descriptor. -- -- @since 4.7.0.0 threadWaitReadSTM :: Fd -> IO (STM (), IO ()) threadWaitReadSTM fd #if defined(mingw32_HOST_OS) | threaded = do v <- newTVarIO Nothing mask_ $ void $ forkIO $ do result <- try (waitFd fd 0) atomically (writeTVar v $ Just result) let waitAction = do result <- readTVar v case result of Nothing -> retry Just (Right ()) -> return () Just (Left e) -> throwSTM (e :: IOException) let killAction = return () return (waitAction, killAction) | otherwise = errorWithoutStackTrace "threadWaitReadSTM requires -threaded on Windows" #else = GHC.Conc.threadWaitReadSTM fd #endif -- | Returns an STM action that can be used to wait until data -- can be written to a file descriptor. The second returned value -- is an IO action that can be used to deregister interest -- in the file descriptor. -- -- @since 4.7.0.0 threadWaitWriteSTM :: Fd -> IO (STM (), IO ()) threadWaitWriteSTM fd #if defined(mingw32_HOST_OS) | threaded = do v <- newTVarIO Nothing mask_ $ void $ forkIO $ do result <- try (waitFd fd 1) atomically (writeTVar v $ Just result) let waitAction = do result <- readTVar v case result of Nothing -> retry Just (Right ()) -> return () Just (Left e) -> throwSTM (e :: IOException) let killAction = return () return (waitAction, killAction) | otherwise = errorWithoutStackTrace "threadWaitWriteSTM requires -threaded on Windows" #else = GHC.Conc.threadWaitWriteSTM fd #endif #if defined(mingw32_HOST_OS) foreign import ccall unsafe "rtsSupportsBoundThreads" threaded :: Bool withThread :: IO a -> IO a withThread io = do m <- newEmptyMVar _ <- mask_ $ forkIO $ try io >>= putMVar m x <- takeMVar m case x of Right a -> return a Left e -> throwIO (e :: IOException) waitFd :: Fd -> CInt -> IO () waitFd fd write = do throwErrnoIfMinus1_ "fdReady" $ fdReady (fromIntegral fd) write (-1) 0 foreign import ccall safe "fdReady" fdReady :: CInt -> CInt -> Int64 -> CInt -> IO CInt #endif -- --------------------------------------------------------------------------- -- More docs {- $osthreads #osthreads# In GHC, threads created by 'forkIO' are lightweight threads, and are managed entirely by the GHC runtime. Typically Haskell threads are an order of magnitude or two more efficient (in terms of both time and space) than operating system threads. The downside of having lightweight threads is that only one can run at a time, so if one thread blocks in a foreign call, for example, the other threads cannot continue. The GHC runtime works around this by making use of full OS threads where necessary. When the program is built with the @-threaded@ option (to link against the multithreaded version of the runtime), a thread making a @safe@ foreign call will not block the other threads in the system; another OS thread will take over running Haskell threads until the original call returns. The runtime maintains a pool of these /worker/ threads so that multiple Haskell threads can be involved in external calls simultaneously. The "System.IO" library manages multiplexing in its own way. On Windows systems it uses @safe@ foreign calls to ensure that threads doing I\/O operations don't block the whole runtime, whereas on Unix systems all the currently blocked I\/O requests are managed by a single thread (the /IO manager thread/) using a mechanism such as @epoll@ or @kqueue@, depending on what is provided by the host operating system. The runtime will run a Haskell thread using any of the available worker OS threads. If you need control over which particular OS thread is used to run a given Haskell thread, perhaps because you need to call a foreign library that uses OS-thread-local state, then you need bound threads (see "Control.Concurrent#boundthreads"). If you don't use the @-threaded@ option, then the runtime does not make use of multiple OS threads. Foreign calls will block all other running Haskell threads until the call returns. The "System.IO" library still does multiplexing, so there can be multiple threads doing I\/O, and this is handled internally by the runtime using @select@. -} {- $termination In a standalone GHC program, only the main thread is required to terminate in order for the process to terminate. Thus all other forked threads will simply terminate at the same time as the main thread (the terminology for this kind of behaviour is \"daemonic threads\"). If you want the program to wait for child threads to finish before exiting, you need to program this yourself. A simple mechanism is to have each child thread write to an 'MVar' when it completes, and have the main thread wait on all the 'MVar's before exiting: > myForkIO :: IO () -> IO (MVar ()) > myForkIO io = do > mvar <- newEmptyMVar > forkFinally io (\_ -> putMVar mvar ()) > return mvar Note that we use 'forkFinally' to make sure that the 'MVar' is written to even if the thread dies or is killed for some reason. A better method is to keep a global list of all child threads which we should wait for at the end of the program: > children :: MVar [MVar ()] > children = unsafePerformIO (newMVar []) > > waitForChildren :: IO () > waitForChildren = do > cs <- takeMVar children > case cs of > [] -> return () > m:ms -> do > putMVar children ms > takeMVar m > waitForChildren > > forkChild :: IO () -> IO ThreadId > forkChild io = do > mvar <- newEmptyMVar > childs <- takeMVar children > putMVar children (mvar:childs) > forkFinally io (\_ -> putMVar mvar ()) > > main = > later waitForChildren $ > ... The main thread principle also applies to calls to Haskell from outside, using @foreign export@. When the @foreign export@ed function is invoked, it starts a new main thread, and it returns when this main thread terminates. If the call causes new threads to be forked, they may remain in the system after the @foreign export@ed function has returned. -} {- $preemption GHC implements pre-emptive multitasking: the execution of threads are interleaved in a random fashion. More specifically, a thread may be pre-empted whenever it allocates some memory, which unfortunately means that tight loops which do no allocation tend to lock out other threads (this only seems to happen with pathological benchmark-style code, however). The rescheduling timer runs on a 20ms granularity by default, but this may be altered using the @-i\<n\>@ RTS option. After a rescheduling \"tick\" the running thread is pre-empted as soon as possible. One final note: the @aaaa@ @bbbb@ example may not work too well on GHC (see Scheduling, above), due to the locking on a 'System.IO.Handle'. Only one thread may hold the lock on a 'System.IO.Handle' at any one time, so if a reschedule happens while a thread is holding the lock, the other thread won't be able to run. The upshot is that the switch from @aaaa@ to @bbbbb@ happens infrequently. It can be improved by lowering the reschedule tick period. We also have a patch that causes a reschedule whenever a thread waiting on a lock is woken up, but haven't found it to be useful for anything other than this example :-) -} {- $deadlock GHC attempts to detect when threads are deadlocked using the garbage collector. A thread that is not reachable (cannot be found by following pointers from live objects) must be deadlocked, and in this case the thread is sent an exception. The exception is either 'BlockedIndefinitelyOnMVar', 'BlockedIndefinitelyOnSTM', 'NonTermination', or 'Deadlock', depending on the way in which the thread is deadlocked. Note that this feature is intended for debugging, and should not be relied on for the correct operation of your program. There is no guarantee that the garbage collector will be accurate enough to detect your deadlock, and no guarantee that the garbage collector will run in a timely enough manner. Basically, the same caveats as for finalizers apply to deadlock detection. There is a subtle interaction between deadlock detection and finalizers (as created by 'Foreign.Concurrent.newForeignPtr' or the functions in "System.Mem.Weak"): if a thread is blocked waiting for a finalizer to run, then the thread will be considered deadlocked and sent an exception. So preferably don't do this, but if you have no alternative then it is possible to prevent the thread from being considered deadlocked by making a 'StablePtr' pointing to it. Don't forget to release the 'StablePtr' later with 'freeStablePtr'. -}