Control.Concurrent

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 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 ThreadIds. 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 tid = throwTo tid (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 as and bs. 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 MVars 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 exported 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.

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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