X-Git-Url: http://git.megacz.com/?a=blobdiff_plain;f=Control%2FConcurrent.hs;h=62a30b434889b0cdd1e410d2c10275d740d85b65;hb=7a97ec4b12e1fbec5505f82032cf4dc435b5a60c;hp=965adeb972834b4092765c52f3450786b6ba403a;hpb=9b49610310bf2f332b5d8f3643f36c5239557f90;p=ghc-base.git diff --git a/Control/Concurrent.hs b/Control/Concurrent.hs index 965adeb..62a30b4 100644 --- a/Control/Concurrent.hs +++ b/Control/Concurrent.hs @@ -1,3 +1,11 @@ +{-# LANGUAGE CPP + , ForeignFunctionInterface + , MagicHash + , UnboxedTuples + , ScopedTypeVariables + #-} +{-# OPTIONS_GHC -fno-warn-unused-imports #-} + ----------------------------------------------------------------------------- -- | -- Module : Control.Concurrent @@ -14,79 +22,114 @@ ----------------------------------------------------------------------------- module Control.Concurrent ( - -- * Concurrent Haskell + -- * Concurrent Haskell - -- $conc_intro + -- $conc_intro - -- * Basic concurrency operations + -- * Basic concurrency operations -#ifndef __HUGS__ ThreadId, - myThreadId, +#ifdef __GLASGOW_HASKELL__ + myThreadId, #endif - forkIO, -#ifndef __HUGS__ - killThread, - throwTo, + forkIO, +#ifdef __GLASGOW_HASKELL__ + forkIOWithUnmask, + killThread, + throwTo, #endif - -- * Scheduling + -- ** Threads with affinity + forkOn, + forkOnWithUnmask, + getNumCapabilities, + threadCapability, + + -- * Scheduling + + -- $conc_scheduling + yield, -- :: IO () - -- $conc_scheduling - yield, -- :: IO () + -- ** Blocking - -- ** Blocking - - -- $blocking + -- $blocking #ifdef __GLASGOW_HASKELL__ - -- ** Waiting - threadDelay, -- :: Int -> IO () - threadWaitRead, -- :: Int -> IO () - threadWaitWrite, -- :: Int -> IO () + -- ** Waiting + threadDelay, -- :: Int -> IO () + threadWaitRead, -- :: Int -> IO () + threadWaitWrite, -- :: Int -> IO () #endif - -- * Communication abstractions + -- * Communication abstractions - module Control.Concurrent.MVar, - module Control.Concurrent.Chan, - module Control.Concurrent.QSem, - module Control.Concurrent.QSemN, - module Control.Concurrent.SampleVar, + module Control.Concurrent.MVar, + module Control.Concurrent.Chan, + module Control.Concurrent.QSem, + module Control.Concurrent.QSemN, + module Control.Concurrent.SampleVar, - -- * Merging of streams + -- * Merging of streams #ifndef __HUGS__ - mergeIO, -- :: [a] -> [a] -> IO [a] - nmergeIO, -- :: [[a]] -> IO [a] + mergeIO, -- :: [a] -> [a] -> IO [a] + nmergeIO, -- :: [[a]] -> IO [a] #endif - -- $merge + -- $merge + +#ifdef __GLASGOW_HASKELL__ + -- * Bound Threads + -- $boundthreads + rtsSupportsBoundThreads, + forkOS, + isCurrentThreadBound, + runInBoundThread, + runInUnboundThread, +#endif + + -- * GHC's implementation of concurrency + + -- |This section describes features specific to GHC's + -- implementation of Concurrent Haskell. + + -- ** Haskell threads and Operating System threads + + -- $osthreads - -- * GHC's implementation of concurrency + -- ** Terminating the program - -- |This section describes features specific to GHC's - -- implementation of Concurrent Haskell. - - -- ** Terminating the program + -- $termination - -- $termination + -- ** Pre-emption - -- ** Pre-emption + -- $preemption - -- $preemption + -- * Deprecated functions + forkIOUnmasked ) where import Prelude -import Control.Exception as Exception +import Control.Exception.Base as Exception #ifdef __GLASGOW_HASKELL__ -import GHC.Conc -import GHC.TopHandler ( reportStackOverflow, reportError ) -import GHC.IOBase ( IO(..) ) -import GHC.IOBase ( unsafeInterleaveIO ) +import GHC.Exception +import GHC.Conc hiding (threadWaitRead, threadWaitWrite) +import qualified GHC.Conc +import GHC.IO ( IO(..), unsafeInterleaveIO, unsafeUnmask ) +import GHC.IORef ( newIORef, readIORef, writeIORef ) import GHC.Base + +import System.Posix.Types ( Fd ) +import Foreign.StablePtr +import Foreign.C.Types ( CInt ) +import Control.Monad ( when ) + +#ifdef mingw32_HOST_OS +import Foreign.C +import System.IO +#endif #endif #ifdef __HUGS__ @@ -99,6 +142,10 @@ import Control.Concurrent.QSem import Control.Concurrent.QSemN import Control.Concurrent.SampleVar +#ifdef __HUGS__ +type ThreadId = () +#endif + {- $conc_intro The concurrency extension for Haskell is described in the paper @@ -110,18 +157,22 @@ 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 "Concurrent" library. Threads may also communicate -via exceptions. +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 imformation related to specific compilers). In a co-operative + 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: @@ -137,77 +188,17 @@ via exceptions. -} {- $blocking -Calling a foreign C procedure (such as @getchar@) that blocks waiting -for input will block /all/ threads, unless the @threadsafe@ attribute -is used on the foreign call (and your compiler \/ operating system -supports it). GHC's I\/O system uses non-blocking I\/O internally to -implement thread-friendly I\/O, so calling standard Haskell I\/O -functions blocks only the thread making the call. --} - --- Thread Ids, specifically the instances of Eq and Ord for these things. --- The ThreadId type itself is defined in std/PrelConc.lhs. +Different Haskell implementations have different characteristics with +regard to which operations block /all/ threads. --- Rather than define a new primitve, we use a little helper function --- cmp_thread in the RTS. +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. -#ifdef __GLASGOW_HASKELL__ -id2TSO :: ThreadId -> ThreadId# -id2TSO (ThreadId t) = t - -foreign import ccall unsafe "cmp_thread" cmp_thread :: ThreadId# -> ThreadId# -> Int --- Returns -1, 0, 1 - -cmpThread :: ThreadId -> ThreadId -> Ordering -cmpThread t1 t2 = - case cmp_thread (id2TSO t1) (id2TSO t2) of - -1 -> LT - 0 -> EQ - _ -> GT -- must be 1 - -instance Eq ThreadId where - t1 == t2 = - case t1 `cmpThread` t2 of - EQ -> True - _ -> False - -instance Ord ThreadId where - compare = cmpThread - -foreign import ccall unsafe "rts_getThreadId" getThreadId :: ThreadId# -> Int - -instance Show ThreadId where - showsPrec d t = - showString "ThreadId " . - showsPrec d (getThreadId (id2TSO t)) - -{- | -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. +Using Hugs, all I\/O operations and foreign calls will block all other +Haskell threads. -} -forkIO :: IO () -> IO ThreadId -forkIO action = IO $ \ s -> - case (fork# action_plus s) of (# s1, id #) -> (# s1, ThreadId id #) - where - action_plus = Exception.catch action childHandler - -childHandler :: Exception -> IO () -childHandler err = Exception.catch (real_handler err) childHandler - -real_handler :: Exception -> IO () -real_handler ex = - case ex of - -- ignore thread GC and killThread exceptions: - BlockedOnDeadMVar -> return () - AsyncException ThreadKilled -> return () - - -- report all others: - AsyncException StackOverflow -> reportStackOverflow False - ErrorCall s -> reportError False s - other -> reportError False (showsPrec 0 other "\n") - -#endif /* __GLASGOW_HASKELL__ */ #ifndef __HUGS__ max_buff_size :: Int @@ -225,8 +216,8 @@ nmergeIO :: [[a]] -> IO [a] -- preemptive multitasking. mergeIO ls rs - = newEmptyMVar >>= \ tail_node -> - newMVar tail_node >>= \ tail_list -> + = newEmptyMVar >>= \ tail_node -> + newMVar tail_node >>= \ tail_list -> newQSem max_buff_size >>= \ e -> newMVar 2 >>= \ branches_running -> let @@ -234,58 +225,355 @@ mergeIO ls rs in forkIO (suckIO branches_running buff ls) >> forkIO (suckIO branches_running buff rs) >> - takeMVar tail_node >>= \ val -> - signalQSem e >> + takeMVar tail_node >>= \ val -> + signalQSem e >> return val -type Buffer a +type Buffer a = (MVar (MVar [a]), QSem) suckIO :: MVar Int -> Buffer a -> [a] -> IO () suckIO branches_running buff@(tail_list,e) vs = case vs of - [] -> takeMVar branches_running >>= \ val -> - if val == 1 then - takeMVar tail_list >>= \ node -> - putMVar node [] >> - putMVar tail_list node - else - putMVar branches_running (val-1) - (x:xs) -> - waitQSem e >> - takeMVar tail_list >>= \ node -> - newEmptyMVar >>= \ next_node -> - unsafeInterleaveIO ( - takeMVar next_node >>= \ y -> - signalQSem e >> - return y) >>= \ next_node_val -> - putMVar node (x:next_node_val) >> - putMVar tail_list next_node >> - suckIO branches_running buff xs + [] -> takeMVar branches_running >>= \ val -> + if val == 1 then + takeMVar tail_list >>= \ node -> + putMVar node [] >> + putMVar tail_list node + else + putMVar branches_running (val-1) + (x:xs) -> + waitQSem e >> + takeMVar tail_list >>= \ node -> + newEmptyMVar >>= \ next_node -> + unsafeInterleaveIO ( + takeMVar next_node >>= \ y -> + signalQSem e >> + return y) >>= \ next_node_val -> + putMVar node (x:next_node_val) >> + putMVar tail_list next_node >> + suckIO branches_running buff xs nmergeIO lss = let len = length lss in - newEmptyMVar >>= \ tail_node -> - newMVar tail_node >>= \ tail_list -> + newEmptyMVar >>= \ tail_node -> + newMVar tail_node >>= \ tail_list -> newQSem max_buff_size >>= \ e -> - newMVar len >>= \ branches_running -> + newMVar len >>= \ branches_running -> let buff = (tail_list,e) in mapIO (\ x -> forkIO (suckIO branches_running buff x)) lss >> - takeMVar tail_node >>= \ val -> - signalQSem e >> + takeMVar tail_node >>= \ val -> + signalQSem e >> return val where mapIO f xs = sequence (map f xs) #endif /* __HUGS__ */ +#ifdef __GLASGOW_HASKELL__ +-- --------------------------------------------------------------------------- +-- 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 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 = Exception.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 + +-- | 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#, not (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 it's main thread to be bound and makes /heavy/ use of concurrency +(e.g. a web server), might want to wrap it's @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 `Exception.catch` \(e :: SomeException) -> + Exception.throwTo tid e >> wait + wait >>= unsafeResult + else action + +unsafeResult :: Either SomeException a -> IO a +unsafeResult = either Exception.throwIO return +#endif /* __GLASGOW_HASKELL__ */ + +#ifdef __GLASGOW_HASKELL__ +-- --------------------------------------------------------------------------- +-- 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 +#ifdef 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. + _ -> error "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 +#ifdef mingw32_HOST_OS + | threaded = withThread (waitFd fd 1) + | otherwise = error "threadWaitWrite requires -threaded on Windows" +#else + = GHC.Conc.threadWaitWrite fd +#endif + +#ifdef 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 iNFINITE 0 + +iNFINITE :: CInt +iNFINITE = 0xFFFFFFFF -- urgh + +foreign import ccall safe "fdReady" + fdReady :: CInt -> CInt -> CInt -> 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 + @select@. + + 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 @@ -303,40 +591,41 @@ nmergeIO lss > myForkIO :: IO () -> IO (MVar ()) > myForkIO io = do -> mvar \<- newEmptyMVar -> forkIO (io \`finally\` putMVar mvar ()) +> mvar <- newEmptyMVar +> forkIO (io `finally` putMVar mvar ()) > return mvar Note that we use 'finally' from the - "Exception" module to make sure that the + "Control.Exception" module 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 -> (mvar:mvars) \<- takeMVar children -> putMVar children mvars -> takeMVar mvar -> waitForChildren -> -> forkChild :: IO () -> IO () -> forkChild io = do -> mvar \<- newEmptyMVar -> forkIO (p \`finally\` putMVar mvar ()) -> childs \<- takeMVar children -> putMVar children (mvar:childs) -> -> later = flip finally -> +> 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) +> forkIO (io `finally` putMVar mvar ()) +> > main = -> later waitForChildren $ -> ... +> later waitForChildren $ +> ... The main thread principle also applies to calls to Haskell from outside, using @foreign export@. When the @foreign export@ed @@ -353,19 +642,19 @@ nmergeIO lss 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 pathalogical benchmark-style code, however). + 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@ RTS option. After a rescheduling + @-i\@ 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 'Handle'. Only one thread - may hold the lock on a 'Handle' at any one + 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 @@ -375,3 +664,4 @@ nmergeIO lss lock is woken up, but haven't found it to be useful for anything other than this example :-) -} +#endif /* __GLASGOW_HASKELL__ */