+{-# 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,
#ifdef __GLASGOW_HASKELL__
- myThreadId,
+ myThreadId,
#endif
- forkIO,
+ forkIO,
#ifdef __GLASGOW_HASKELL__
- killThread,
- throwTo,
+ 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
- -- * Bound Threads
- -- $boundthreads
#ifdef __GLASGOW_HASKELL__
- rtsSupportsBoundThreads,
- forkOS,
- isCurrentThreadBound,
- runInBoundThread
+ -- * Bound Threads
+ -- $boundthreads
+ rtsSupportsBoundThreads,
+ forkOS,
+ isCurrentThreadBound,
+ runInBoundThread,
+ runInUnboundThread,
#endif
- -- * GHC's implementation of concurrency
+ -- * GHC's implementation of concurrency
+
+ -- |This section describes features specific to GHC's
+ -- implementation of Concurrent Haskell.
+
+ -- ** Haskell threads and Operating System threads
+
+ -- $osthreads
- -- |This section describes features specific to GHC's
- -- implementation of Concurrent Haskell.
-
- -- ** Terminating the program
+ -- ** Terminating the program
- -- $termination
+ -- $termination
- -- ** Pre-emption
+ -- ** Pre-emption
+
+ -- $preemption
+
+ -- * Deprecated functions
+ forkIOUnmasked
- -- $preemption
) 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.IOBase ( newIORef, readIORef, writeIORef )
+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__
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:
-}
{- $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.
-
-The new thread will be a lightweight thread; if you want to use a foreign
-library that uses thread-local storage, use 'forkOS' instead.
+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
-- 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 when the runtime system
-has been compiled using a special option.
-
-When recompiling GHC, use ./configure --enable-threaded-rts to enable this.
-To find your GHC has already been compiled that way, use
-'rtsSupportsBoundThreads' from GHCi.
+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.
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@.
--}
-#ifdef __GLASGOW_HASKELL__
+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'
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@ uses operating system-supplied multithreading support to create
-a new operating system thread. The new thread is /bound/, which means that
-all foreign calls made by the 'IO' computation are guaranteed to be executed
-in this new operating system thread; also, the operating system thread is not
-used for any other foreign calls.
-
-This means that you can use all kinds of foreign libraries from this thread
-(even those that rely on thread-local state), without the limitations of 'forkIO'.
+{- |
+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
foreign import ccall "forkOS_entry" forkOS_entry_reimported
:: StablePtr (IO ()) -> IO ()
+forkOS_entry :: StablePtr (IO ()) -> IO ()
forkOS_entry stableAction = do
- action <- deRefStablePtr stableAction
- action
+ action <- deRefStablePtr stableAction
+ action
foreign import ccall forkOS_createThread
:: StablePtr (IO ()) -> IO CInt
-forkOS action
+failNonThreaded :: IO a
+failNonThreaded = fail $ "RTS doesn't support multiple OS threads "
+ ++"(use ghc -threaded when linking)"
+
+forkOS action0
| rtsSupportsBoundThreads = do
- mv <- newEmptyMVar
- let action_plus = Exception.catch action 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 = fail "RTS not built to support multiple OS threads."
+ 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# ->
+isCurrentThreadBound = IO $ \ s# ->
case isCurrentThreadBound# s# of
(# s2#, flg #) -> (# s2#, not (flg ==# 0#) #)
runInBoundThread action
| rtsSupportsBoundThreads = do
- bound <- isCurrentThreadBound
- if bound
- then action
- else do
- ref <- newIORef undefined
- let action_plus = Exception.try action >>= writeIORef ref
- resultOrException <-
- bracket (newStablePtr action_plus)
- freeStablePtr
- (\cEntry -> forkOS_entry_reimported cEntry >> readIORef ref)
- case resultOrException of
- Left exception -> Exception.throw exception
- Right result -> return result
- | otherwise = fail "RTS not built to support multiple OS threads."
+ 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
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
- forkIO (Exception.try action >>= putMVar mv)
- takeMVar mv >>= \either -> case either of
- Left exception -> Exception.throw exception
- Right result -> return result
- else action
-
+ 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
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
lock is woken up, but haven't found it to be useful for anything
other than this example :-)
-}
+#endif /* __GLASGOW_HASKELL__ */