----------------------------------------------------------------------------- -- | -- Module : Control.Concurrent -- Copyright : (c) The University of Glasgow 2001 -- License : BSD-style (see the file libraries/base/LICENSE) -- -- Maintainer : libraries@haskell.org -- Stability : experimental -- Portability : non-portable (concurrency) -- -- A common interface to a collection of useful concurrency -- abstractions. -- ----------------------------------------------------------------------------- module Control.Concurrent ( -- * Concurrent Haskell -- $conc_intro -- * Basic concurrency operations ThreadId, forkIO, -- * Scheduling -- $conc_scheduling yield, -- :: IO () -- ** Blocking -- $blocking -- * 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 -- $merge -- * GHC's implementation of concurrency -- |This section describes features specific to GHC's -- implementation of Concurrent Haskell. -- ** Haskell threads and Operating System threads -- $osthreads -- ** Terminating the program -- $termination -- ** Pre-emption -- $preemption ) where import Prelude import Control.Exception as Exception import Hugs.ConcBase import Control.Concurrent.MVar import Control.Concurrent.Chan import Control.Concurrent.QSem import Control.Concurrent.QSemN import Control.Concurrent.SampleVar type ThreadId = () {- $conc_intro The concurrency extension for Haskell is described in the paper /Concurrent Haskell/ . 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. -} {- $conc_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. -} {- $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. -} -- --------------------------------------------------------------------------- -- 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 reqwests 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 above). 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 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. -} {- $preemption 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\@ 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 'System.IO.Handle'. Only one thread may hold the lock on a 'System.IO.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 :-) -}