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{-# OPTIONS_GHC -fno-warn-unused-imports #-}
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-----------------------------------------------------------------------------
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-- Module : Control.Concurrent
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-- Copyright : (c) The University of Glasgow 2001
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-- License : BSD-style (see the file libraries/base/LICENSE)
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-- Maintainer : libraries@haskell.org
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-- Stability : experimental
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-- Portability : non-portable (concurrency)
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-- A common interface to a collection of useful concurrency
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-----------------------------------------------------------------------------
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module Control.Concurrent (
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-- * Concurrent Haskell
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-- * Basic concurrency operations
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#ifdef __GLASGOW_HASKELL__
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#ifdef __GLASGOW_HASKELL__
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#ifdef __GLASGOW_HASKELL__
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threadDelay, -- :: Int -> IO ()
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threadWaitRead, -- :: Int -> IO ()
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threadWaitWrite, -- :: Int -> IO ()
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-- * Communication abstractions
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module Control.Concurrent.MVar,
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module Control.Concurrent.Chan,
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module Control.Concurrent.QSem,
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module Control.Concurrent.QSemN,
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module Control.Concurrent.SampleVar,
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-- * Merging of streams
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mergeIO, -- :: [a] -> [a] -> IO [a]
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nmergeIO, -- :: [[a]] -> IO [a]
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#ifdef __GLASGOW_HASKELL__
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rtsSupportsBoundThreads,
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-- * GHC's implementation of concurrency
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-- |This section describes features specific to GHC's
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-- implementation of Concurrent Haskell.
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-- ** Haskell threads and Operating System threads
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-- ** Terminating the program
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import Control.Exception.Base as Exception
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#ifdef __GLASGOW_HASKELL__
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import GHC.Conc ( ThreadId(..), myThreadId, killThread, yield,
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threadDelay, forkIO, forkIOUnmasked, childHandler )
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import qualified GHC.Conc
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import GHC.IO ( IO(..), unsafeInterleaveIO, unsafeUnmask )
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import GHC.IORef ( newIORef, readIORef, writeIORef )
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import System.Posix.Types ( Fd )
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import Foreign.StablePtr
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import Foreign.C.Types ( CInt )
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import Control.Monad ( when )
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#ifdef mingw32_HOST_OS
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import Control.Concurrent.MVar
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import Control.Concurrent.Chan
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import Control.Concurrent.QSem
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import Control.Concurrent.QSemN
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import Control.Concurrent.SampleVar
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The concurrency extension for Haskell is described in the paper
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<http://www.haskell.org/ghc/docs/papers/concurrent-haskell.ps.gz>.
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Concurrency is \"lightweight\", which means that both thread creation
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and context switching overheads are extremely low. Scheduling of
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Haskell threads is done internally in the Haskell runtime system, and
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doesn't make use of any operating system-supplied thread packages.
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However, if you want to interact with a foreign library that expects your
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program to use the operating system-supplied thread package, you can do so
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by using 'forkOS' instead of 'forkIO'.
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Haskell threads can communicate via 'MVar's, a kind of synchronised
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mutable variable (see "Control.Concurrent.MVar"). Several common
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concurrency abstractions can be built from 'MVar's, and these are
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provided by the "Control.Concurrent" library.
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In GHC, threads may also communicate via exceptions.
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Scheduling may be either pre-emptive or co-operative,
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depending on the implementation of Concurrent Haskell (see below
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for information related to specific compilers). In a co-operative
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system, context switches only occur when you use one of the
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primitives defined in this module. This means that programs such
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> main = forkIO (write 'a') >> write 'b'
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> where write c = putChar c >> write c
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will print either @aaaaaaaaaaaaaa...@ or @bbbbbbbbbbbb...@,
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instead of some random interleaving of @a@s and @b@s. In
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practice, cooperative multitasking is sufficient for writing
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simple graphical user interfaces.
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Different Haskell implementations have different characteristics with
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regard to which operations block /all/ threads.
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Using GHC without the @-threaded@ option, all foreign calls will block
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all other Haskell threads in the system, although I\/O operations will
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not. With the @-threaded@ option, only foreign calls with the @unsafe@
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attribute will block all other threads.
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Using Hugs, all I\/O operations and foreign calls will block all other
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mergeIO :: [a] -> [a] -> IO [a]
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nmergeIO :: [[a]] -> IO [a]
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-- The 'mergeIO' and 'nmergeIO' functions fork one thread for each
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-- input list that concurrently evaluates that list; the results are
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-- merged into a single output list.
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-- Note: Hugs does not provide these functions, since they require
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-- preemptive multitasking.
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= newEmptyMVar >>= \ tail_node ->
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newMVar tail_node >>= \ tail_list ->
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newQSem max_buff_size >>= \ e ->
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newMVar 2 >>= \ branches_running ->
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forkIO (suckIO branches_running buff ls) >>
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forkIO (suckIO branches_running buff rs) >>
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takeMVar tail_node >>= \ val ->
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= (MVar (MVar [a]), QSem)
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suckIO :: MVar Int -> Buffer a -> [a] -> IO ()
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suckIO branches_running buff@(tail_list,e) vs
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[] -> takeMVar branches_running >>= \ val ->
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takeMVar tail_list >>= \ node ->
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putMVar tail_list node
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putMVar branches_running (val-1)
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takeMVar tail_list >>= \ node ->
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newEmptyMVar >>= \ next_node ->
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takeMVar next_node >>= \ y ->
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return y) >>= \ next_node_val ->
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putMVar node (x:next_node_val) >>
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putMVar tail_list next_node >>
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suckIO branches_running buff xs
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newEmptyMVar >>= \ tail_node ->
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newMVar tail_node >>= \ tail_list ->
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newQSem max_buff_size >>= \ e ->
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newMVar len >>= \ branches_running ->
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mapIO (\ x -> forkIO (suckIO branches_running buff x)) lss >>
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takeMVar tail_node >>= \ val ->
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mapIO f xs = sequence (map f xs)
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#endif /* __HUGS__ */
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#ifdef __GLASGOW_HASKELL__
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-- ---------------------------------------------------------------------------
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Support for multiple operating system threads and bound threads as described
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below is currently only available in the GHC runtime system if you use the
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/-threaded/ option when linking.
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Other Haskell systems do not currently support multiple operating system threads.
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A bound thread is a haskell thread that is /bound/ to an operating system
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thread. While the bound thread is still scheduled by the Haskell run-time
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system, the operating system thread takes care of all the foreign calls made
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To a foreign library, the bound thread will look exactly like an ordinary
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operating system thread created using OS functions like @pthread_create@
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Bound threads can be created using the 'forkOS' function below. All foreign
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exported functions are run in a bound thread (bound to the OS thread that
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called the function). Also, the @main@ action of every Haskell program is
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run in a bound thread.
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Why do we need this? Because if a foreign library is called from a thread
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created using 'forkIO', it won't have access to any /thread-local state/ -
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state variables that have specific values for each OS thread
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(see POSIX's @pthread_key_create@ or Win32's @TlsAlloc@). Therefore, some
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libraries (OpenGL, for example) will not work from a thread created using
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'forkIO'. They work fine in threads created using 'forkOS' or when called
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from @main@ or from a @foreign export@.
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In terms of performance, 'forkOS' (aka bound) threads are much more
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expensive than 'forkIO' (aka unbound) threads, because a 'forkOS'
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thread is tied to a particular OS thread, whereas a 'forkIO' thread
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can be run by any OS thread. Context-switching between a 'forkOS'
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thread and a 'forkIO' thread is many times more expensive than between
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two 'forkIO' threads.
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Note in particular that the main program thread (the thread running
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@Main.main@) is always a bound thread, so for good concurrency
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performance you should ensure that the main thread is not doing
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repeated communication with other threads in the system. Typically
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this means forking subthreads to do the work using 'forkIO', and
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waiting for the results in the main thread.
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-- | 'True' if bound threads are supported.
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-- If @rtsSupportsBoundThreads@ is 'False', 'isCurrentThreadBound'
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-- will always return 'False' and both 'forkOS' and 'runInBoundThread' will
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foreign import ccall rtsSupportsBoundThreads :: Bool
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Like 'forkIO', this sparks off a new thread to run the 'IO'
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computation passed as the first argument, and returns the 'ThreadId'
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of the newly created thread.
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However, 'forkOS' creates a /bound/ thread, which is necessary if you
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need to call foreign (non-Haskell) libraries that make use of
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thread-local state, such as OpenGL (see "Control.Concurrent#boundthreads").
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Using 'forkOS' instead of 'forkIO' makes no difference at all to the
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scheduling behaviour of the Haskell runtime system. It is a common
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misconception that you need to use 'forkOS' instead of 'forkIO' to
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avoid blocking all the Haskell threads when making a foreign call;
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this isn't the case. To allow foreign calls to be made without
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blocking all the Haskell threads (with GHC), it is only necessary to
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use the @-threaded@ option when linking your program, and to make sure
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the foreign import is not marked @unsafe@.
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forkOS :: IO () -> IO ThreadId
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foreign export ccall forkOS_entry
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:: StablePtr (IO ()) -> IO ()
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foreign import ccall "forkOS_entry" forkOS_entry_reimported
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:: StablePtr (IO ()) -> IO ()
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forkOS_entry :: StablePtr (IO ()) -> IO ()
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forkOS_entry stableAction = do
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action <- deRefStablePtr stableAction
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foreign import ccall forkOS_createThread
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:: StablePtr (IO ()) -> IO CInt
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failNonThreaded :: IO a
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failNonThreaded = fail $ "RTS doesn't support multiple OS threads "
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++"(use ghc -threaded when linking)"
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| rtsSupportsBoundThreads = do
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b <- Exception.getMaskingState
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-- async exceptions are masked in the child if they are masked
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-- in the parent, as for forkIO (see #1048). forkOS_createThread
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-- creates a thread with exceptions masked by default.
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Unmasked -> unsafeUnmask action0
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MaskedInterruptible -> action0
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MaskedUninterruptible -> uninterruptibleMask_ action0
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action_plus = Exception.catch action1 childHandler
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entry <- newStablePtr (myThreadId >>= putMVar mv >> action_plus)
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err <- forkOS_createThread entry
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when (err /= 0) $ fail "Cannot create OS thread."
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| otherwise = failNonThreaded
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-- | Returns 'True' if the calling thread is /bound/, that is, if it is
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-- safe to use foreign libraries that rely on thread-local state from the
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isCurrentThreadBound :: IO Bool
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isCurrentThreadBound = IO $ \ s# ->
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case isCurrentThreadBound# s# of
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(# s2#, flg #) -> (# s2#, not (flg ==# 0#) #)
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Run the 'IO' computation passed as the first argument. If the calling thread
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is not /bound/, a bound thread is created temporarily. @runInBoundThread@
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doesn't finish until the 'IO' computation finishes.
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You can wrap a series of foreign function calls that rely on thread-local state
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with @runInBoundThread@ so that you can use them without knowing whether the
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current thread is /bound/.
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runInBoundThread :: IO a -> IO a
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runInBoundThread action
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| rtsSupportsBoundThreads = do
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bound <- isCurrentThreadBound
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ref <- newIORef undefined
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let action_plus = Exception.try action >>= writeIORef ref
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bracket (newStablePtr action_plus)
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(\cEntry -> forkOS_entry_reimported cEntry >> readIORef ref)
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case resultOrException of
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Left exception -> Exception.throw (exception :: SomeException)
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Right result -> return result
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| otherwise = failNonThreaded
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Run the 'IO' computation passed as the first argument. If the calling thread
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is /bound/, an unbound thread is created temporarily using 'forkIO'.
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@runInBoundThread@ doesn't finish until the 'IO' computation finishes.
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Use this function /only/ in the rare case that you have actually observed a
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performance loss due to the use of bound threads. A program that
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doesn't need it's main thread to be bound and makes /heavy/ use of concurrency
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(e.g. a web server), might want to wrap it's @main@ action in
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@runInUnboundThread@.
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runInUnboundThread :: IO a -> IO a
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runInUnboundThread action = do
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bound <- isCurrentThreadBound
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_ <- mask $ \restore -> forkIO $
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Exception.try (if b then action else restore action) >>=
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takeMVar mv >>= \ei -> case ei of
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Left exception -> Exception.throw (exception :: SomeException)
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Right result -> return result
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#endif /* __GLASGOW_HASKELL__ */
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#ifdef __GLASGOW_HASKELL__
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-- ---------------------------------------------------------------------------
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-- threadWaitRead/threadWaitWrite
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-- | Block the current thread until data is available to read on the
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-- given file descriptor (GHC only).
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threadWaitRead :: Fd -> IO ()
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#ifdef mingw32_HOST_OS
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-- we have no IO manager implementing threadWaitRead on Windows.
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-- fdReady does the right thing, but we have to call it in a
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-- separate thread, otherwise threadWaitRead won't be interruptible,
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-- and this only works with -threaded.
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| threaded = withThread (waitFd fd 0)
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| otherwise = case fd of
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0 -> do _ <- hWaitForInput stdin (-1)
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-- hWaitForInput does work properly, but we can only
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-- do this for stdin since we know its FD.
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_ -> error "threadWaitRead requires -threaded on Windows, or use System.IO.hWaitForInput"
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= GHC.Conc.threadWaitRead fd
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-- | Block the current thread until data can be written to the
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-- given file descriptor (GHC only).
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threadWaitWrite :: Fd -> IO ()
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#ifdef mingw32_HOST_OS
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| threaded = withThread (waitFd fd 1)
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| otherwise = error "threadWaitWrite requires -threaded on Windows"
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= GHC.Conc.threadWaitWrite fd
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#ifdef mingw32_HOST_OS
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foreign import ccall unsafe "rtsSupportsBoundThreads" threaded :: Bool
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withThread :: IO a -> IO a
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_ <- mask_ $ forkIO $ try io >>= putMVar m
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Left e -> throwIO (e :: IOException)
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waitFd :: Fd -> CInt -> IO ()
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throwErrnoIfMinus1_ "fdReady" $
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fdReady (fromIntegral fd) write (fromIntegral iNFINITE) 0
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iNFINITE = 0xFFFFFFFF -- urgh
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foreign import ccall safe "fdReady"
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fdReady :: CInt -> CInt -> CInt -> CInt -> IO CInt
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-- ---------------------------------------------------------------------------
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#osthreads# In GHC, threads created by 'forkIO' are lightweight threads, and
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are managed entirely by the GHC runtime. Typically Haskell
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threads are an order of magnitude or two more efficient (in
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terms of both time and space) than operating system threads.
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The downside of having lightweight threads is that only one can
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run at a time, so if one thread blocks in a foreign call, for
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example, the other threads cannot continue. The GHC runtime
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works around this by making use of full OS threads where
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necessary. When the program is built with the @-threaded@
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option (to link against the multithreaded version of the
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runtime), a thread making a @safe@ foreign call will not block
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the other threads in the system; another OS thread will take
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over running Haskell threads until the original call returns.
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The runtime maintains a pool of these /worker/ threads so that
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multiple Haskell threads can be involved in external calls
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The "System.IO" library manages multiplexing in its own way. On
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Windows systems it uses @safe@ foreign calls to ensure that
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threads doing I\/O operations don't block the whole runtime,
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whereas on Unix systems all the currently blocked I\/O requests
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are managed by a single thread (the /IO manager thread/) using
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The runtime will run a Haskell thread using any of the available
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worker OS threads. If you need control over which particular OS
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thread is used to run a given Haskell thread, perhaps because
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you need to call a foreign library that uses OS-thread-local
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state, then you need bound threads (see "Control.Concurrent#boundthreads").
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If you don't use the @-threaded@ option, then the runtime does
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not make use of multiple OS threads. Foreign calls will block
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all other running Haskell threads until the call returns. The
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"System.IO" library still does multiplexing, so there can be multiple
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threads doing I\/O, and this is handled internally by the runtime using
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In a standalone GHC program, only the main thread is
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required to terminate in order for the process to terminate.
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Thus all other forked threads will simply terminate at the same
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time as the main thread (the terminology for this kind of
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behaviour is \"daemonic threads\").
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If you want the program to wait for child threads to
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finish before exiting, you need to program this yourself. A
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simple mechanism is to have each child thread write to an
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'MVar' when it completes, and have the main
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thread wait on all the 'MVar's before
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> myForkIO :: IO () -> IO (MVar ())
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> mvar <- newEmptyMVar
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> forkIO (io `finally` putMVar mvar ())
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Note that we use 'finally' from the
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"Control.Exception" module to make sure that the
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'MVar' is written to even if the thread dies or
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is killed for some reason.
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A better method is to keep a global list of all child
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threads which we should wait for at the end of the program:
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> children :: MVar [MVar ()]
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> children = unsafePerformIO (newMVar [])
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> waitForChildren :: IO ()
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> waitForChildren = do
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> cs <- takeMVar children
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> putMVar children ms
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> forkChild :: IO () -> IO ThreadId
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> mvar <- newEmptyMVar
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> childs <- takeMVar children
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> putMVar children (mvar:childs)
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> forkIO (io `finally` putMVar mvar ())
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> later waitForChildren $
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The main thread principle also applies to calls to Haskell from
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outside, using @foreign export@. When the @foreign export@ed
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function is invoked, it starts a new main thread, and it returns
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when this main thread terminates. If the call causes new
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threads to be forked, they may remain in the system after the
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@foreign export@ed function has returned.
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GHC implements pre-emptive multitasking: the execution of
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threads are interleaved in a random fashion. More specifically,
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a thread may be pre-empted whenever it allocates some memory,
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which unfortunately means that tight loops which do no
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allocation tend to lock out other threads (this only seems to
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happen with pathological benchmark-style code, however).
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The rescheduling timer runs on a 20ms granularity by
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default, but this may be altered using the
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@-i\<n\>@ RTS option. After a rescheduling
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\"tick\" the running thread is pre-empted as soon as
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@aaaa@ @bbbb@ example may not
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work too well on GHC (see Scheduling, above), due
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to the locking on a 'System.IO.Handle'. Only one thread
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may hold the lock on a 'System.IO.Handle' at any one
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time, so if a reschedule happens while a thread is holding the
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lock, the other thread won't be able to run. The upshot is that
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the switch from @aaaa@ to
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@bbbbb@ happens infrequently. It can be
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improved by lowering the reschedule tick period. We also have a
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patch that causes a reschedule whenever a thread waiting on a
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lock is woken up, but haven't found it to be useful for anything
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other than this example :-)
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#endif /* __GLASGOW_HASKELL__ */