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Control.Concurrent

Portability | non-portable (concurrency)
Stability | experimental
Maintainer | libraries@haskell.org

Contents

Concurrent Haskell
Basic concurrency operations
Scheduling
  Blocking
  Waiting
Communication abstractions
Merging of streams
Bound Threads
GHC's implementation of concurrency
  Terminating the program
  Pre-emption

Description
A common interface to a collection of useful concurrency abstractions.
Synopsis

data ThreadId
myThreadId :: IO ThreadId
forkIO :: IO () -> IO ThreadId
killThread :: ThreadId -> IO ()
throwTo :: ThreadId -> Exception -> IO ()
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

Concurrent Haskell

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.

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 Control.Concurrent library. In GHC, threads may also communicate via exceptions.

Basic concurrency operations
data ThreadId

A `ThreadId` is an abstract type representing a handle to a thread. `ThreadId` is an instance of `Eq`, `Ord` and Show, where the `Ord` instance implements an arbitrary total ordering over `ThreadId`s. The 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 program.

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 `ThreadId`. This misfeature will hopefully be corrected at a later date.

Note: Hugs does not provide any operations on other threads; it defines `ThreadId` as a synonym for ().

Instances

Data ThreadId
Eq ThreadId
Ord ThreadId
Show ThreadId
Typeable ThreadId

myThreadId :: IO ThreadId
Returns the `ThreadId` of the calling thread (GHC only).
forkIO :: IO () -> IO ThreadId
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.
killThread :: ThreadId -> IO ()

`killThread` terminates the given thread (GHC only). Any work already done by the thread isn't lost: the computation is suspended until required by another thread. The memory used by the thread will be garbage collected if it isn't referenced from anywhere. The `killThread` function is defined in terms of `throwTo`:

     killThread tid = throwTo tid (AsyncException ThreadKilled)

throwTo :: ThreadId -> Exception -> IO ()

`throwTo` raises an arbitrary exception in the target thread (GHC only).

`throwTo` does not return until the exception has been raised in the target thread. 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.

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 block or not.

Scheduling

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 `aaaaaaaaaaaaaa...` or `bbbbbbbbbbbb...`, instead of some random interleaving of `a`s and `b`s. In practice, cooperative multitasking is sufficient for writing simple graphical user interfaces.

yield :: IO ()
The `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 concurrency abstractions.
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.
Waiting
threadDelay :: Int -> IO ()

Suspends the current thread for a given number of microseconds (GHC only).

Note that the resolution used by the Haskell runtime system's internal timer is 1/50 second, and `threadDelay` will round its argument up to the nearest multiple of this resolution.

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.

threadWaitRead :: Fd -> IO ()
Block the current thread until data is available to read on the given file descriptor (GHC only).
threadWaitWrite :: Fd -> IO ()
Block the current thread until data can be written to the given file descriptor (GHC only).
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
mergeIO :: [a] -> [a] -> IO [a]
nmergeIO :: [[a]] -> IO [a]
The `mergeIO` and `nmergeIO` functions fork one thread for each input list that concurrently evaluates that list; the results are merged into a single output list. Note: Hugs does not provide these functions, since they require preemptive multitasking.
Bound Threads

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`.

rtsSupportsBoundThreads :: Bool
`True` if bound threads are supported. If `rtsSupportsBoundThreads` is `False`, `isCurrentThreadBound` will always return `False` and both `forkOS` and `runInBoundThread` will fail.
forkOS :: IO () -> IO ThreadId

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`.

isCurrentThreadBound :: IO Bool
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.
runInBoundThread :: IO a -> IO a
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.
runInUnboundThread :: IO a -> IO a

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`.

GHC's implementation of concurrency
This section describes features specific to GHC's implementation of Concurrent Haskell.
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 `MVar`s before exiting:

       myForkIO :: IO () -> IO (MVar ())
       myForkIO io = do
         mvar <- newEmptyMVar
         forkIO (io `finally` putMVar mvar ())
         return mvar

Note that we use `finally` from 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
          cs <- takeMVar children
          case cs of
            []   -> return ()
            m:ms -> do
        	    putMVar children ms
        	    takeMVar m
        	    waitForChildren

        forkChild :: IO () -> IO ()
        forkChild io = do
        	 mvar <- newEmptyMVar
        	 childs <- takeMVar children
        	 putMVar children (mvar:childs)
        	 forkIO (io `finally` putMVar mvar ())

         main =
         	later waitForChildren $
         	...

The main thread principle also applies to calls to Haskell from outside, using `foreign export`. When the `foreign export`ed 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 export`ed function has returned.

Pre-emption

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 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 time, so if a reschedule happens while a thread is holding the lock, the other thread won't be able to run. The upshot is that the switch from `aaaa` to `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 :-)

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