add doc link to bound threads section
[packages/old-locale.git] / Control / Concurrent.hs
1 -----------------------------------------------------------------------------
2 -- |
3 -- Module : Control.Concurrent
4 -- Copyright : (c) The University of Glasgow 2001
5 -- License : BSD-style (see the file libraries/base/LICENSE)
6 --
7 -- Maintainer : libraries@haskell.org
8 -- Stability : experimental
9 -- Portability : non-portable (concurrency)
10 --
11 -- A common interface to a collection of useful concurrency
12 -- abstractions.
13 --
14 -----------------------------------------------------------------------------
15
16 module Control.Concurrent (
17 -- * Concurrent Haskell
18
19 -- $conc_intro
20
21 -- * Basic concurrency operations
22
23 ThreadId,
24 #ifdef __GLASGOW_HASKELL__
25 myThreadId,
26 #endif
27
28 forkIO,
29 #ifdef __GLASGOW_HASKELL__
30 killThread,
31 throwTo,
32 #endif
33
34 -- * Scheduling
35
36 -- $conc_scheduling
37 yield, -- :: IO ()
38
39 -- ** Blocking
40
41 -- $blocking
42
43 #ifdef __GLASGOW_HASKELL__
44 -- ** Waiting
45 threadDelay, -- :: Int -> IO ()
46 threadWaitRead, -- :: Int -> IO ()
47 threadWaitWrite, -- :: Int -> IO ()
48 #endif
49
50 -- * Communication abstractions
51
52 module Control.Concurrent.MVar,
53 module Control.Concurrent.Chan,
54 module Control.Concurrent.QSem,
55 module Control.Concurrent.QSemN,
56 module Control.Concurrent.SampleVar,
57
58 -- * Merging of streams
59 #ifndef __HUGS__
60 mergeIO, -- :: [a] -> [a] -> IO [a]
61 nmergeIO, -- :: [[a]] -> IO [a]
62 #endif
63 -- $merge
64
65 #ifdef __GLASGOW_HASKELL__
66 -- * Bound Threads
67 -- $boundthreads
68 rtsSupportsBoundThreads,
69 forkOS,
70 isCurrentThreadBound,
71 runInBoundThread,
72 runInUnboundThread
73 #endif
74
75 -- * GHC's implementation of concurrency
76
77 -- |This section describes features specific to GHC's
78 -- implementation of Concurrent Haskell.
79
80 -- ** Haskell threads and Operating System threads
81
82 -- $osthreads
83
84 -- ** Terminating the program
85
86 -- $termination
87
88 -- ** Pre-emption
89
90 -- $preemption
91 ) where
92
93 import Prelude
94
95 import Control.Exception as Exception
96
97 #ifdef __GLASGOW_HASKELL__
98 import GHC.Conc ( ThreadId(..), myThreadId, killThread, yield,
99 threadDelay, threadWaitRead, threadWaitWrite,
100 forkIO, childHandler )
101 import GHC.TopHandler ( reportStackOverflow, reportError )
102 import GHC.IOBase ( IO(..) )
103 import GHC.IOBase ( unsafeInterleaveIO )
104 import GHC.IOBase ( newIORef, readIORef, writeIORef )
105 import GHC.Base
106
107 import Foreign.StablePtr
108 import Foreign.C.Types ( CInt )
109 import Control.Monad ( when )
110 #endif
111
112 #ifdef __HUGS__
113 import Hugs.ConcBase
114 #endif
115
116 import Control.Concurrent.MVar
117 import Control.Concurrent.Chan
118 import Control.Concurrent.QSem
119 import Control.Concurrent.QSemN
120 import Control.Concurrent.SampleVar
121
122 #ifdef __HUGS__
123 type ThreadId = ()
124 #endif
125
126 {- $conc_intro
127
128 The concurrency extension for Haskell is described in the paper
129 /Concurrent Haskell/
130 <http://www.haskell.org/ghc/docs/papers/concurrent-haskell.ps.gz>.
131
132 Concurrency is \"lightweight\", which means that both thread creation
133 and context switching overheads are extremely low. Scheduling of
134 Haskell threads is done internally in the Haskell runtime system, and
135 doesn't make use of any operating system-supplied thread packages.
136
137 However, if you want to interact with a foreign library that expects your
138 program to use the operating system-supplied thread package, you can do so
139 by using 'forkOS' instead of 'forkIO'.
140
141 Haskell threads can communicate via 'MVar's, a kind of synchronised
142 mutable variable (see "Control.Concurrent.MVar"). Several common
143 concurrency abstractions can be built from 'MVar's, and these are
144 provided by the "Control.Concurrent" library.
145 In GHC, threads may also communicate via exceptions.
146 -}
147
148 {- $conc_scheduling
149
150 Scheduling may be either pre-emptive or co-operative,
151 depending on the implementation of Concurrent Haskell (see below
152 for information related to specific compilers). In a co-operative
153 system, context switches only occur when you use one of the
154 primitives defined in this module. This means that programs such
155 as:
156
157
158 > main = forkIO (write 'a') >> write 'b'
159 > where write c = putChar c >> write c
160
161 will print either @aaaaaaaaaaaaaa...@ or @bbbbbbbbbbbb...@,
162 instead of some random interleaving of @a@s and @b@s. In
163 practice, cooperative multitasking is sufficient for writing
164 simple graphical user interfaces.
165 -}
166
167 {- $blocking
168 Calling a foreign C procedure (such as @getchar@) that blocks waiting
169 for input will block /all/ threads, unless the @threadsafe@ attribute
170 is used on the foreign call (and your compiler \/ operating system
171 supports it). GHC's I\/O system uses non-blocking I\/O internally to
172 implement thread-friendly I\/O, so calling standard Haskell I\/O
173 functions blocks only the thread making the call.
174 -}
175
176 #ifndef __HUGS__
177 max_buff_size :: Int
178 max_buff_size = 1
179
180 mergeIO :: [a] -> [a] -> IO [a]
181 nmergeIO :: [[a]] -> IO [a]
182
183 -- $merge
184 -- The 'mergeIO' and 'nmergeIO' functions fork one thread for each
185 -- input list that concurrently evaluates that list; the results are
186 -- merged into a single output list.
187 --
188 -- Note: Hugs does not provide these functions, since they require
189 -- preemptive multitasking.
190
191 mergeIO ls rs
192 = newEmptyMVar >>= \ tail_node ->
193 newMVar tail_node >>= \ tail_list ->
194 newQSem max_buff_size >>= \ e ->
195 newMVar 2 >>= \ branches_running ->
196 let
197 buff = (tail_list,e)
198 in
199 forkIO (suckIO branches_running buff ls) >>
200 forkIO (suckIO branches_running buff rs) >>
201 takeMVar tail_node >>= \ val ->
202 signalQSem e >>
203 return val
204
205 type Buffer a
206 = (MVar (MVar [a]), QSem)
207
208 suckIO :: MVar Int -> Buffer a -> [a] -> IO ()
209
210 suckIO branches_running buff@(tail_list,e) vs
211 = case vs of
212 [] -> takeMVar branches_running >>= \ val ->
213 if val == 1 then
214 takeMVar tail_list >>= \ node ->
215 putMVar node [] >>
216 putMVar tail_list node
217 else
218 putMVar branches_running (val-1)
219 (x:xs) ->
220 waitQSem e >>
221 takeMVar tail_list >>= \ node ->
222 newEmptyMVar >>= \ next_node ->
223 unsafeInterleaveIO (
224 takeMVar next_node >>= \ y ->
225 signalQSem e >>
226 return y) >>= \ next_node_val ->
227 putMVar node (x:next_node_val) >>
228 putMVar tail_list next_node >>
229 suckIO branches_running buff xs
230
231 nmergeIO lss
232 = let
233 len = length lss
234 in
235 newEmptyMVar >>= \ tail_node ->
236 newMVar tail_node >>= \ tail_list ->
237 newQSem max_buff_size >>= \ e ->
238 newMVar len >>= \ branches_running ->
239 let
240 buff = (tail_list,e)
241 in
242 mapIO (\ x -> forkIO (suckIO branches_running buff x)) lss >>
243 takeMVar tail_node >>= \ val ->
244 signalQSem e >>
245 return val
246 where
247 mapIO f xs = sequence (map f xs)
248 #endif /* __HUGS__ */
249
250 #ifdef __GLASGOW_HASKELL__
251 -- ---------------------------------------------------------------------------
252 -- Bound Threads
253
254 {- $boundthreads
255 #boundthreads#
256
257 Support for multiple operating system threads and bound threads as described
258 below is currently only available in the GHC runtime system if you use the
259 /-threaded/ option when linking.
260
261 Other Haskell systems do not currently support multiple operating system threads.
262
263 A bound thread is a haskell thread that is /bound/ to an operating system
264 thread. While the bound thread is still scheduled by the Haskell run-time
265 system, the operating system thread takes care of all the foreign calls made
266 by the bound thread.
267
268 To a foreign library, the bound thread will look exactly like an ordinary
269 operating system thread created using OS functions like @pthread_create@
270 or @CreateThread@.
271
272 Bound threads can be created using the 'forkOS' function below. All foreign
273 exported functions are run in a bound thread (bound to the OS thread that
274 called the function). Also, the @main@ action of every Haskell program is
275 run in a bound thread.
276
277 Why do we need this? Because if a foreign library is called from a thread
278 created using 'forkIO', it won't have access to any /thread-local state/ -
279 state variables that have specific values for each OS thread
280 (see POSIX's @pthread_key_create@ or Win32's @TlsAlloc@). Therefore, some
281 libraries (OpenGL, for example) will not work from a thread created using
282 'forkIO'. They work fine in threads created using 'forkOS' or when called
283 from @main@ or from a @foreign export@.
284 -}
285
286 -- | 'True' if bound threads are supported.
287 -- If @rtsSupportsBoundThreads@ is 'False', 'isCurrentThreadBound'
288 -- will always return 'False' and both 'forkOS' and 'runInBoundThread' will
289 -- fail.
290 foreign import ccall rtsSupportsBoundThreads :: Bool
291
292
293 {- |
294 Like 'forkIO', this sparks off a new thread to run the 'IO' computation passed as the
295 first argument, and returns the 'ThreadId' of the newly created
296 thread.
297
298 However, @forkOS@ uses operating system-supplied multithreading support to create
299 a new operating system thread. The new thread is /bound/, which means that
300 all foreign calls made by the 'IO' computation are guaranteed to be executed
301 in this new operating system thread; also, the operating system thread is not
302 used for any other foreign calls.
303
304 This means that you can use all kinds of foreign libraries from this thread
305 (even those that rely on thread-local state), without the limitations of 'forkIO'.
306
307 Just to clarify, 'forkOS' is /only/ necessary if you need to associate
308 a Haskell thread with a particular OS thread. It is not necessary if
309 you only need to make non-blocking foreign calls (see "Control.Concurrent#osthreads").
310
311 -}
312 forkOS :: IO () -> IO ThreadId
313
314 foreign export ccall forkOS_entry
315 :: StablePtr (IO ()) -> IO ()
316
317 foreign import ccall "forkOS_entry" forkOS_entry_reimported
318 :: StablePtr (IO ()) -> IO ()
319
320 forkOS_entry stableAction = do
321 action <- deRefStablePtr stableAction
322 action
323
324 foreign import ccall forkOS_createThread
325 :: StablePtr (IO ()) -> IO CInt
326
327 failNonThreaded = fail $ "RTS doesn't support multiple OS threads "
328 ++"(use ghc -threaded when linking)"
329
330 forkOS action
331 | rtsSupportsBoundThreads = do
332 mv <- newEmptyMVar
333 let action_plus = Exception.catch action childHandler
334 entry <- newStablePtr (myThreadId >>= putMVar mv >> action_plus)
335 err <- forkOS_createThread entry
336 when (err /= 0) $ fail "Cannot create OS thread."
337 tid <- takeMVar mv
338 freeStablePtr entry
339 return tid
340 | otherwise = failNonThreaded
341
342 -- | Returns 'True' if the calling thread is /bound/, that is, if it is
343 -- safe to use foreign libraries that rely on thread-local state from the
344 -- calling thread.
345 isCurrentThreadBound :: IO Bool
346 isCurrentThreadBound = IO $ \ s# ->
347 case isCurrentThreadBound# s# of
348 (# s2#, flg #) -> (# s2#, not (flg ==# 0#) #)
349
350
351 {- |
352 Run the 'IO' computation passed as the first argument. If the calling thread
353 is not /bound/, a bound thread is created temporarily. @runInBoundThread@
354 doesn't finish until the 'IO' computation finishes.
355
356 You can wrap a series of foreign function calls that rely on thread-local state
357 with @runInBoundThread@ so that you can use them without knowing whether the
358 current thread is /bound/.
359 -}
360 runInBoundThread :: IO a -> IO a
361
362 runInBoundThread action
363 | rtsSupportsBoundThreads = do
364 bound <- isCurrentThreadBound
365 if bound
366 then action
367 else do
368 ref <- newIORef undefined
369 let action_plus = Exception.try action >>= writeIORef ref
370 resultOrException <-
371 bracket (newStablePtr action_plus)
372 freeStablePtr
373 (\cEntry -> forkOS_entry_reimported cEntry >> readIORef ref)
374 case resultOrException of
375 Left exception -> Exception.throw exception
376 Right result -> return result
377 | otherwise = failNonThreaded
378
379 {- |
380 Run the 'IO' computation passed as the first argument. If the calling thread
381 is /bound/, an unbound thread is created temporarily using 'forkIO'.
382 @runInBoundThread@ doesn't finish until the 'IO' computation finishes.
383
384 Use this function /only/ in the rare case that you have actually observed a
385 performance loss due to the use of bound threads. A program that
386 doesn't need it's main thread to be bound and makes /heavy/ use of concurrency
387 (e.g. a web server), might want to wrap it's @main@ action in
388 @runInUnboundThread@.
389 -}
390 runInUnboundThread :: IO a -> IO a
391
392 runInUnboundThread action = do
393 bound <- isCurrentThreadBound
394 if bound
395 then do
396 mv <- newEmptyMVar
397 forkIO (Exception.try action >>= putMVar mv)
398 takeMVar mv >>= \either -> case either of
399 Left exception -> Exception.throw exception
400 Right result -> return result
401 else action
402
403 #endif /* __GLASGOW_HASKELL__ */
404
405 -- ---------------------------------------------------------------------------
406 -- More docs
407
408 {- $osthreads
409
410 #osthreads# In GHC, threads created by 'forkIO' are lightweight threads, and
411 are managed entirely by the GHC runtime. Typically Haskell
412 threads are an order of magnitude or two more efficient (in
413 terms of both time and space) than operating system threads.
414
415 The downside of having lightweight threads is that only one can
416 run at a time, so if one thread blocks in a foreign call, for
417 example, the other threads cannot continue. The GHC runtime
418 works around this by making use of full OS threads where
419 necessary. When the program is built with the @-threaded@
420 option (to link against the multithreaded version of the
421 runtime), a thread making a @safe@ foreign call will not block
422 the other threads in the system; another OS thread will take
423 over running Haskell threads until the original call returns.
424 The runtime maintains a pool of these /worker/ threads so that
425 multiple Haskell threads can be involved in external calls
426 simultaneously.
427
428 The "System.IO" library manages multiplexing in its own way. On
429 Windows systems it uses @safe@ foreign calls to ensure that
430 threads doing I\/O operations don't block the whole runtime,
431 whereas on Unix systems all the currently blocked I\/O reqwests
432 are managed by a single thread (the /IO manager thread/) using
433 @select@.
434
435 The runtime will run a Haskell thread using any of the available
436 worker OS threads. If you need control over which particular OS
437 thread is used to run a given Haskell thread, perhaps because
438 you need to call a foreign library that uses OS-thread-local
439 state, then you need bound threads (see "Control.Concurrent#boundthreads").
440
441 If you don't use the @-threaded@ option, then the runtime does
442 not make use of multiple OS threads. Foreign calls will block
443 all other running Haskell threads until the call returns. The
444 "System.IO" library still does multiplexing, so there can be multiple
445 threads doing I\/O, and this is handled internally by the runtime using
446 @select@.
447 -}
448
449 {- $termination
450
451 In a standalone GHC program, only the main thread is
452 required to terminate in order for the process to terminate.
453 Thus all other forked threads will simply terminate at the same
454 time as the main thread (the terminology for this kind of
455 behaviour is \"daemonic threads\").
456
457 If you want the program to wait for child threads to
458 finish before exiting, you need to program this yourself. A
459 simple mechanism is to have each child thread write to an
460 'MVar' when it completes, and have the main
461 thread wait on all the 'MVar's before
462 exiting:
463
464 > myForkIO :: IO () -> IO (MVar ())
465 > myForkIO io = do
466 > mvar <- newEmptyMVar
467 > forkIO (io `finally` putMVar mvar ())
468 > return mvar
469
470 Note that we use 'finally' from the
471 "Control.Exception" module to make sure that the
472 'MVar' is written to even if the thread dies or
473 is killed for some reason.
474
475 A better method is to keep a global list of all child
476 threads which we should wait for at the end of the program:
477
478 > children :: MVar [MVar ()]
479 > children = unsafePerformIO (newMVar [])
480 >
481 > waitForChildren :: IO ()
482 > waitForChildren = do
483 > cs <- takeMVar children
484 > case cs of
485 > [] -> return ()
486 > m:ms -> do
487 > putMVar children ms
488 > takeMVar m
489 > waitForChildren
490 >
491 > forkChild :: IO () -> IO ()
492 > forkChild io = do
493 > mvar <- newEmptyMVar
494 > childs <- takeMVar children
495 > putMVar children (mvar:childs)
496 > forkIO (io `finally` putMVar mvar ())
497 >
498 > main =
499 > later waitForChildren $
500 > ...
501
502 The main thread principle also applies to calls to Haskell from
503 outside, using @foreign export@. When the @foreign export@ed
504 function is invoked, it starts a new main thread, and it returns
505 when this main thread terminates. If the call causes new
506 threads to be forked, they may remain in the system after the
507 @foreign export@ed function has returned.
508 -}
509
510 {- $preemption
511
512 GHC implements pre-emptive multitasking: the execution of
513 threads are interleaved in a random fashion. More specifically,
514 a thread may be pre-empted whenever it allocates some memory,
515 which unfortunately means that tight loops which do no
516 allocation tend to lock out other threads (this only seems to
517 happen with pathological benchmark-style code, however).
518
519 The rescheduling timer runs on a 20ms granularity by
520 default, but this may be altered using the
521 @-i\<n\>@ RTS option. After a rescheduling
522 \"tick\" the running thread is pre-empted as soon as
523 possible.
524
525 One final note: the
526 @aaaa@ @bbbb@ example may not
527 work too well on GHC (see Scheduling, above), due
528 to the locking on a 'System.IO.Handle'. Only one thread
529 may hold the lock on a 'System.IO.Handle' at any one
530 time, so if a reschedule happens while a thread is holding the
531 lock, the other thread won't be able to run. The upshot is that
532 the switch from @aaaa@ to
533 @bbbbb@ happens infrequently. It can be
534 improved by lowering the reschedule tick period. We also have a
535 patch that causes a reschedule whenever a thread waiting on a
536 lock is woken up, but haven't found it to be useful for anything
537 other than this example :-)
538 -}