+{-# LANGUAGE CPP
+ , ForeignFunctionInterface
+ , MagicHash
+ , UnboxedTuples
+ , ScopedTypeVariables
+ #-}
+{-# OPTIONS_GHC -fno-warn-unused-imports #-}
+
-----------------------------------------------------------------------------
-- |
-- Module : Control.Concurrent
-----------------------------------------------------------------------------
module Control.Concurrent (
- -- * Concurrent Haskell
+ -- * Concurrent Haskell
- -- $conc_intro
+ -- $conc_intro
- -- * Basic concurrency operations
+ -- * Basic concurrency operations
ThreadId,
- myThreadId,
+#ifdef __GLASGOW_HASKELL__
+ myThreadId,
+#endif
+
+ forkIO,
+#ifdef __GLASGOW_HASKELL__
+ forkIOWithUnmask,
+ killThread,
+ throwTo,
+#endif
- forkIO,
- killThread,
- throwTo,
+ -- ** Threads with affinity
+ forkOn,
+ forkOnWithUnmask,
+ getNumCapabilities,
+ threadCapability,
- -- * Scheduling
+ -- * 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,
+
+ -- * Merging of streams
+#ifndef __HUGS__
+ mergeIO, -- :: [a] -> [a] -> IO [a]
+ nmergeIO, -- :: [[a]] -> IO [a]
+#endif
+ -- $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.
- module Control.Concurrent.MVar,
- module Control.Concurrent.Chan,
- module Control.Concurrent.QSem,
- module Control.Concurrent.QSemN,
- module Control.Concurrent.SampleVar,
+ -- ** Haskell threads and Operating System threads
- -- * Merging of streams
- mergeIO, -- :: [a] -> [a] -> IO [a]
- nmergeIO, -- :: [[a]] -> IO [a]
- -- $merge
+ -- $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__
-import IOExts ( unsafeInterleaveIO )
-import ConcBase
+import Hugs.ConcBase
#endif
import Control.Concurrent.MVar
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
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.
+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:
-> main = forkIO (write \'a\') >> write \'b\'
+> main = forkIO (write 'a') >> write 'b'
> where write c = putChar c >> write c
will print either @aaaaaaaaaaaaaa...@ or @bbbbbbbbbbbb...@,
-}
{- $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 thead 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__
-foreign import ccall unsafe "cmp_thread" cmp_thread :: Addr# -> Addr# -> Int
--- Returns -1, 0, 1
-
-cmpThread :: ThreadId -> ThreadId -> Ordering
-cmpThread (ThreadId t1) (ThreadId t2) =
- case cmp_thread (unsafeCoerce# t1) (unsafeCoerce# 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 :: Addr# -> Int
-
-instance Show ThreadId where
- showsPrec d (ThreadId t) =
- showString "ThreadId " .
- showsPrec d (getThreadId (unsafeCoerce# 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
max_buff_size = 1
-- 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
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
> 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
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<n>@ RTS option. After a rescheduling
+ @-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 '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
+ 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
+ lock is woken up, but haven't found it to be useful for anything
other than this example :-)
-}
+#endif /* __GLASGOW_HASKELL__ */
projects / ghc-base.git / blobdiff