Control.Concurrent

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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 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 :: Int -> IO ()
threadWaitWrite :: Int -> 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]
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. 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.
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.
Instances
Eq ThreadId
Ord ThreadId
Show ThreadId
myThreadId :: IO ThreadId
Returns the `ThreadId` of the calling thread.
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.
killThread :: ThreadId -> IO ()
`killThread` terminates the given thread (Note: `killThread` is not implemented in Hugs). 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 may be defined in terms of `throwTo`:
       killThread = throwTo (AsyncException ThreadKilled)
throwTo :: ThreadId -> Exception -> IO ()
`throwTo` raises an arbitrary exception in the target thread. `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.
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 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 thead making the call.
Waiting
threadDelay :: Int -> IO ()
The `threadDelay` operation will cause the current thread to suspend for a given number of microseconds. Note that the resolution used by the Haskell runtime system's internal timer together with the fact that the thread may take some time to be rescheduled after the time has expired, means that the accuracy is more like 1/50 second.
threadWaitRead :: Int -> IO ()
Block the current thread until data is available to read on the given file descriptor.
threadWaitWrite :: Int -> IO ()
Block the current thread until data can be written to the given file descriptor.
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.
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 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

         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 pathalogical benchmark-style code, however). The rescheduling timer runs on a 20ms granularity by default, but this may be altered using the `-in` 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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