- Concurrent Haskell
- Basic concurrency operations
- Communication abstractions
- Merging of streams
- Bound Threads
- Weak references to ThreadIds
- GHC's implementation of concurrency
- Deprecated functions
A common interface to a collection of useful concurrency abstractions.
- data ThreadId
- myThreadId :: IO ThreadId
- forkIO :: IO () -> IO ThreadId
- forkFinally :: IO a -> (Either SomeException a -> IO ()) -> IO ThreadId
- forkIOWithUnmask :: ((forall a. IO a -> IO a) -> IO ()) -> IO ThreadId
- killThread :: ThreadId -> IO ()
- throwTo :: Exception e => ThreadId -> e -> IO ()
- forkOn :: Int -> IO () -> IO ThreadId
- forkOnWithUnmask :: Int -> ((forall a. IO a -> IO a) -> IO ()) -> IO ThreadId
- getNumCapabilities :: IO Int
- setNumCapabilities :: Int -> IO ()
- threadCapability :: ThreadId -> IO (Int, Bool)
- yield :: IO ()
- threadDelay :: Int -> IO ()
- threadWaitRead :: Fd -> IO ()
- threadWaitWrite :: Fd -> IO ()
- module Control.Concurrent.MVar
- module Control.Concurrent.Chan
- module Control.Concurrent.QSem
- module Control.Concurrent.QSemN
- module Control.Concurrent.SampleVar
- mergeIO :: [a] -> [a] -> IO [a]
- nmergeIO :: [[a]] -> IO [a]
- rtsSupportsBoundThreads :: Bool
- forkOS :: IO () -> IO ThreadId
- isCurrentThreadBound :: IO Bool
- runInBoundThread :: IO a -> IO a
- runInUnboundThread :: IO a -> IO a
- mkWeakThreadId :: ThreadId -> IO (Weak ThreadId)
- forkIOUnmasked :: IO () -> IO ThreadId
The concurrency extension for Haskell is described in the paper Concurrent Haskell http://www.haskell.org/ghc/docs/papers/concurrent-haskell.ps.gz.
Concurrency is "lightweight", which means that both thread creation and context switching overheads are extremely low. Scheduling of Haskell threads is done internally in the Haskell runtime system, and doesn't make use of any operating system-supplied thread packages.
Haskell threads can communicate via
MVars, a kind of synchronised
mutable variable (see Control.Concurrent.MVar). Several common
concurrency abstractions can be built from
MVars, and these are
provided by the Control.Concurrent library.
In GHC, threads may also communicate via exceptions.
Basic concurrency operations
ThreadId is an abstract type representing a handle to a thread.
ThreadId is an instance of
Ord instance implements an arbitrary total ordering over
Show instance lets you convert an arbitrary-valued
ThreadId to string form; showing a
ThreadId value is occasionally
useful when debugging or diagnosing the behaviour of a concurrent
Note: in GHC, if you have a
ThreadId, you essentially have
a pointer to the thread itself. This means the thread itself can't be
garbage collected until you drop the
This misfeature will hopefully be corrected at a later date.
Note: Hugs does not provide any operations on other threads;
ThreadId as a synonym for ().
The new thread will be a lightweight thread; if you want to use a foreign
library that uses thread-local storage, use
GHC note: the new thread inherits the masked state of the parent
The newly created thread has an exception handler that discards the
ThreadKilled, and passes all other exceptions to the uncaught
fork a thread and call the supplied function when the thread is about to terminate, with an exception or a returned value. The function is called with asynchronous exceptions masked.
forkFinally action and_then = mask $ \restore -> forkIO $ try (restore action) >>= and_then
This function is useful for informing the parent when a child terminates, for example.
forkIO, but the child thread is passed a function that can
be used to unmask asynchronous exceptions. This function is
typically used in the following way
... mask_ $ forkIOWithUnmask $ \unmask -> catch (unmask ...) handler
so that the exception handler in the child thread is established with asynchronous exceptions masked, meanwhile the main body of the child thread is executed in the unmasked state.
Note that the unmask function passed to the child thread should only be used in that thread; the behaviour is undefined if it is invoked in a different thread.
throwTo raises an arbitrary exception in the target thread (GHC only).
throwTo does not return until the exception has been raised in the
The calling thread can thus be certain that the target
thread has received the exception. This is a useful property to know
when dealing with race conditions: eg. if there are two threads that
can kill each other, it is guaranteed that only one of the threads
will get to kill the other.
Whatever work the target thread was doing when the exception was raised is not lost: the computation is suspended until required by another thread.
If the target thread is currently making a foreign call, then the
exception will not be raised (and hence
throwTo will not return)
until the call has completed. This is the case regardless of whether
the call is inside a
mask or not. However, in GHC a foreign call
can be annotated as
interruptible, in which case a
cause the RTS to attempt to cause the call to return; see the GHC
documentation for more details.
Important note: the behaviour of
throwTo differs from that described in
the paper "Asynchronous exceptions in Haskell"
In the paper,
throwTo is non-blocking; but the library implementation adopts
a more synchronous design in which
throwTo does not return until the exception
is received by the target thread. The trade-off is discussed in Section 9 of the paper.
Like any blocking operation,
throwTo is therefore interruptible (see Section 5.3 of
the paper). Unlike other interruptible operations, however,
is always interruptible, even if it does not actually block.
There is no guarantee that the exception will be delivered promptly,
although the runtime will endeavour to ensure that arbitrary
delays don't occur. In GHC, an exception can only be raised when a
thread reaches a safe point, where a safe point is where memory
allocation occurs. Some loops do not perform any memory allocation
inside the loop and therefore cannot be interrupted by a
If the target of
throwTo is the calling thread, then the behaviour
is the same as
throwIO, except that the exception
is thrown as an asynchronous exception. This means that if there is
an enclosing pure computation, which would be the case if the current
IO operation is inside
computation is not permanently replaced by the exception, but is
suspended as if it had received an asynchronous exception.
Threads with affinity
forkIO, but lets you specify on which processor the thread
should run. Unlike a
forkIO thread, a thread created by
will stay on the same processor for its entire lifetime (
threads can migrate between processors according to the scheduling
forkOn is useful for overriding the scheduling policy when
you know in advance how best to distribute the threads.
Int argument specifies a capability number (see
getNumCapabilities). Typically capabilities correspond to physical
processors, but the exact behaviour is implementation-dependent. The
value passed to
forkOn is interpreted modulo the total number of
capabilities as returned by
GHC note: the number of capabilities is specified by the
option when the program is started. Capabilities can be fixed to
actual processor cores with
+RTS -qa if the underlying operating
system supports that, although in practice this is usually unnecessary
(and may actually degrade perforamnce in some cases - experimentation
Returns the number of Haskell threads that can run truly
simultaneously (on separate physical processors) at any given time. To change
this value, use
Set the number of Haskell threads that can run truly simultaneously
(on separate physical processors) at any given time. The number
forkOn is interpreted modulo this value. The initial
value is given by the
+RTS -N runtime flag.
This is also the number of threads that will participate in parallel garbage collection. It is strongly recommended that the number of capabilities is not set larger than the number of physical processor cores, and it may often be beneficial to leave one or more cores free to avoid contention with other processes in the machine.
returns the number of the capability on which the thread is currently
running, and a boolean indicating whether the thread is locked to
that capability or not. A thread is locked to a capability if it
was created with
Scheduling may be either pre-emptive or co-operative, depending on the implementation of Concurrent Haskell (see below for information related to specific compilers). In a co-operative system, context switches only occur when you use one of the primitives defined in this module. This means that programs such as:
main = forkIO (write 'a') >> write 'b' where write c = putChar c >> write c
will print either
instead of some random interleaving of
practice, cooperative multitasking is sufficient for writing
simple graphical user interfaces.
yield action allows (forces, in a co-operative multitasking
implementation) a context-switch to any other currently runnable
threads (if any), and is occasionally useful when implementing
Different Haskell implementations have different characteristics with regard to which operations block all threads.
Using GHC without the
-threaded option, all foreign calls will block
all other Haskell threads in the system, although I/O operations will
not. With the
-threaded option, only foreign calls with the
attribute will block all other threads.
Using Hugs, all I/O operations and foreign calls will block all other Haskell threads.
Suspends the current thread for a given number of microseconds (GHC only).
There is no guarantee that the thread will be rescheduled promptly when the delay has expired, but the thread will never continue to run earlier than specified.
Merging of streams
Deprecated: Control.Concurrent.mergeIO will be removed in GHC 7.8. Please use an alternative, e.g. the SafeSemaphore package, instead.
Deprecated: Control.Concurrent.nmergeIO will be removed in GHC 7.8. Please use an alternative, e.g. the SafeSemaphore package, instead.
Note: Hugs does not provide these functions, since they require preemptive multitasking.
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
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
forkIO, it won't have access to any thread-local state -
state variables that have specific values for each OS thread
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
main or from a
In terms of performance,
forkOS (aka bound) threads are much more
forkIO (aka unbound) threads, because a
thread is tied to a particular OS thread, whereas a
can be run by any OS thread. Context-switching between a
thread and a
forkIO thread is many times more expensive than between
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
waiting for the results in the main thread.
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
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
-threaded option when linking your program, and to make sure
the foreign import is not marked
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
You can wrap a series of foreign function calls that rely on thread-local state
runInBoundThread so that you can use them without knowing whether the
current thread is bound.
IO computation passed as the first argument. If the calling thread
is bound, an unbound thread is created temporarily using
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
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.
Weak references to ThreadIds
make a weak pointer to a
ThreadId. It can be important to do
this if you want to hold a reference to a
ThreadId while still
allowing the thread to receive the
BlockedIndefinitely family of
BlockedIndefinitelyOnMVar). Holding a normal
ThreadId reference will prevent the delivery of
BlockedIndefinitely exceptions because the reference could be
used as the target of
throwTo at any time, which would unblock
Weak ThreadId, on the other hand, will not prevent the
thread from receiving
BlockedIndefinitely exceptions. It is
still possible to throw an exception to a
Weak ThreadId, but the
caller must use
deRefWeak first to determine whether the thread
GHC's implementation of concurrency
This section describes features specific to GHC's implementation of Concurrent Haskell.
Haskell threads and Operating System threads
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
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
The System.IO library manages multiplexing in its own way. On
Windows systems it uses
safe foreign calls to ensure that
threads doing I/O operations don't block the whole runtime,
whereas on Unix systems all the currently blocked I/O requests
are managed by a single thread (the IO manager thread) using
a mechanism such as
kqueue, depending on what is
provided by the host operating system.
The runtime will run a Haskell thread using any of the available worker OS threads. If you need control over which particular OS thread is used to run a given Haskell thread, perhaps because you need to call a foreign library that uses OS-thread-local state, then you need bound threads (see Control.Concurrent).
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
Terminating the program
In a standalone GHC program, only the main thread is required to terminate in order for the process to terminate. Thus all other forked threads will simply terminate at the same time as the main thread (the terminology for this kind of behaviour is "daemonic threads").
If you want the program to wait for child threads to
finish before exiting, you need to program this yourself. A
simple mechanism is to have each child thread write to an
MVar when it completes, and have the main
thread wait on all the
myForkIO :: IO () -> IO (MVar ()) myForkIO io = do mvar <- newEmptyMVar forkFinally io (\_ -> putMVar mvar ()) return mvar
A better method is to keep a global list of all child threads which we should wait for at the end of the program:
children :: MVar [MVar ()] children = unsafePerformIO (newMVar ) waitForChildren :: IO () waitForChildren = do cs <- takeMVar children case cs of  -> return () m:ms -> do putMVar children ms takeMVar m waitForChildren forkChild :: IO () -> IO ThreadId forkChild io = do mvar <- newEmptyMVar childs <- takeMVar children putMVar children (mvar:childs) forkFinally io (\_ -> putMVar mvar ()) main = later waitForChildren $ ...
The main thread principle also applies to calls to Haskell from
foreign export. When the
function is invoked, it starts a new main thread, and it returns
when this main thread terminates. If the call causes new
threads to be forked, they may remain in the system after the
foreign exported function has returned.
GHC implements pre-emptive multitasking: the execution of threads are interleaved in a random fashion. More specifically, a thread may be pre-empted whenever it allocates some memory, which unfortunately means that tight loops which do no allocation tend to lock out other threads (this only seems to happen with pathological benchmark-style code, however).
The rescheduling timer runs on a 20ms granularity by
default, but this may be altered using the
-i<n> RTS option. After a rescheduling
"tick" the running thread is pre-empted as soon as
One final note: the
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
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
bbbbb happens infrequently. It can be
improved by lowering the reschedule tick period. We also have a
patch that causes a reschedule whenever a thread waiting on a
lock is woken up, but haven't found it to be useful for anything
other than this example :-)