1# Copyright 2009 Brian Quinlan. All Rights Reserved. 2# Licensed to PSF under a Contributor Agreement. 3 4"""Implements ProcessPoolExecutor. 5 6The follow diagram and text describe the data-flow through the system: 7 8|======================= In-process =====================|== Out-of-process ==| 9 10+----------+ +----------+ +--------+ +-----------+ +---------+ 11| | => | Work Ids | => | | => | Call Q | => | | 12| | +----------+ | | +-----------+ | | 13| | | ... | | | | ... | | | 14| | | 6 | | | | 5, call() | | | 15| | | 7 | | | | ... | | | 16| Process | | ... | | Local | +-----------+ | Process | 17| Pool | +----------+ | Worker | | #1..n | 18| Executor | | Thread | | | 19| | +----------- + | | +-----------+ | | 20| | <=> | Work Items | <=> | | <= | Result Q | <= | | 21| | +------------+ | | +-----------+ | | 22| | | 6: call() | | | | ... | | | 23| | | future | | | | 4, result | | | 24| | | ... | | | | 3, except | | | 25+----------+ +------------+ +--------+ +-----------+ +---------+ 26 27Executor.submit() called: 28- creates a uniquely numbered _WorkItem and adds it to the "Work Items" dict 29- adds the id of the _WorkItem to the "Work Ids" queue 30 31Local worker thread: 32- reads work ids from the "Work Ids" queue and looks up the corresponding 33 WorkItem from the "Work Items" dict: if the work item has been cancelled then 34 it is simply removed from the dict, otherwise it is repackaged as a 35 _CallItem and put in the "Call Q". New _CallItems are put in the "Call Q" 36 until "Call Q" is full. NOTE: the size of the "Call Q" is kept small because 37 calls placed in the "Call Q" can no longer be cancelled with Future.cancel(). 38- reads _ResultItems from "Result Q", updates the future stored in the 39 "Work Items" dict and deletes the dict entry 40 41Process #1..n: 42- reads _CallItems from "Call Q", executes the calls, and puts the resulting 43 _ResultItems in "Request Q" 44""" 45 46import atexit 47from concurrent.futures import _base 48import Queue as queue 49import multiprocessing 50import threading 51import weakref 52import sys 53 54__author__ = 'Brian Quinlan (brian@sweetapp.com)' 55 56# Workers are created as daemon threads and processes. This is done to allow the 57# interpreter to exit when there are still idle processes in a 58# ProcessPoolExecutor's process pool (i.e. shutdown() was not called). However, 59# allowing workers to die with the interpreter has two undesirable properties: 60# - The workers would still be running during interpretor shutdown, 61# meaning that they would fail in unpredictable ways. 62# - The workers could be killed while evaluating a work item, which could 63# be bad if the callable being evaluated has external side-effects e.g. 64# writing to a file. 65# 66# To work around this problem, an exit handler is installed which tells the 67# workers to exit when their work queues are empty and then waits until the 68# threads/processes finish. 69 70_threads_queues = weakref.WeakKeyDictionary() 71_shutdown = False 72 73def _python_exit(): 74 global _shutdown 75 _shutdown = True 76 items = list(_threads_queues.items()) if _threads_queues else () 77 for t, q in items: 78 q.put(None) 79 for t, q in items: 80 t.join(sys.maxint) 81 82# Controls how many more calls than processes will be queued in the call queue. 83# A smaller number will mean that processes spend more time idle waiting for 84# work while a larger number will make Future.cancel() succeed less frequently 85# (Futures in the call queue cannot be cancelled). 86EXTRA_QUEUED_CALLS = 1 87 88class _WorkItem(object): 89 def __init__(self, future, fn, args, kwargs): 90 self.future = future 91 self.fn = fn 92 self.args = args 93 self.kwargs = kwargs 94 95class _ResultItem(object): 96 def __init__(self, work_id, exception=None, result=None): 97 self.work_id = work_id 98 self.exception = exception 99 self.result = result 100 101class _CallItem(object): 102 def __init__(self, work_id, fn, args, kwargs): 103 self.work_id = work_id 104 self.fn = fn 105 self.args = args 106 self.kwargs = kwargs 107 108def _process_worker(call_queue, result_queue): 109 """Evaluates calls from call_queue and places the results in result_queue. 110 111 This worker is run in a separate process. 112 113 Args: 114 call_queue: A multiprocessing.Queue of _CallItems that will be read and 115 evaluated by the worker. 116 result_queue: A multiprocessing.Queue of _ResultItems that will written 117 to by the worker. 118 shutdown: A multiprocessing.Event that will be set as a signal to the 119 worker that it should exit when call_queue is empty. 120 """ 121 while True: 122 call_item = call_queue.get(block=True) 123 if call_item is None: 124 # Wake up queue management thread 125 result_queue.put(None) 126 return 127 try: 128 r = call_item.fn(*call_item.args, **call_item.kwargs) 129 except: 130 e = sys.exc_info()[1] 131 result_queue.put(_ResultItem(call_item.work_id, 132 exception=e)) 133 else: 134 result_queue.put(_ResultItem(call_item.work_id, 135 result=r)) 136 137def _add_call_item_to_queue(pending_work_items, 138 work_ids, 139 call_queue): 140 """Fills call_queue with _WorkItems from pending_work_items. 141 142 This function never blocks. 143 144 Args: 145 pending_work_items: A dict mapping work ids to _WorkItems e.g. 146 {5: <_WorkItem...>, 6: <_WorkItem...>, ...} 147 work_ids: A queue.Queue of work ids e.g. Queue([5, 6, ...]). Work ids 148 are consumed and the corresponding _WorkItems from 149 pending_work_items are transformed into _CallItems and put in 150 call_queue. 151 call_queue: A multiprocessing.Queue that will be filled with _CallItems 152 derived from _WorkItems. 153 """ 154 while True: 155 if call_queue.full(): 156 return 157 try: 158 work_id = work_ids.get(block=False) 159 except queue.Empty: 160 return 161 else: 162 work_item = pending_work_items[work_id] 163 164 if work_item.future.set_running_or_notify_cancel(): 165 call_queue.put(_CallItem(work_id, 166 work_item.fn, 167 work_item.args, 168 work_item.kwargs), 169 block=True) 170 else: 171 del pending_work_items[work_id] 172 continue 173 174def _queue_management_worker(executor_reference, 175 processes, 176 pending_work_items, 177 work_ids_queue, 178 call_queue, 179 result_queue): 180 """Manages the communication between this process and the worker processes. 181 182 This function is run in a local thread. 183 184 Args: 185 executor_reference: A weakref.ref to the ProcessPoolExecutor that owns 186 this thread. Used to determine if the ProcessPoolExecutor has been 187 garbage collected and that this function can exit. 188 process: A list of the multiprocessing.Process instances used as 189 workers. 190 pending_work_items: A dict mapping work ids to _WorkItems e.g. 191 {5: <_WorkItem...>, 6: <_WorkItem...>, ...} 192 work_ids_queue: A queue.Queue of work ids e.g. Queue([5, 6, ...]). 193 call_queue: A multiprocessing.Queue that will be filled with _CallItems 194 derived from _WorkItems for processing by the process workers. 195 result_queue: A multiprocessing.Queue of _ResultItems generated by the 196 process workers. 197 """ 198 nb_shutdown_processes = [0] 199 def shutdown_one_process(): 200 """Tell a worker to terminate, which will in turn wake us again""" 201 call_queue.put(None) 202 nb_shutdown_processes[0] += 1 203 while True: 204 _add_call_item_to_queue(pending_work_items, 205 work_ids_queue, 206 call_queue) 207 208 result_item = result_queue.get(block=True) 209 if result_item is not None: 210 work_item = pending_work_items[result_item.work_id] 211 del pending_work_items[result_item.work_id] 212 213 if result_item.exception: 214 work_item.future.set_exception(result_item.exception) 215 else: 216 work_item.future.set_result(result_item.result) 217 # Delete references to object. See issue16284 218 del work_item 219 # Check whether we should start shutting down. 220 executor = executor_reference() 221 # No more work items can be added if: 222 # - The interpreter is shutting down OR 223 # - The executor that owns this worker has been collected OR 224 # - The executor that owns this worker has been shutdown. 225 if _shutdown or executor is None or executor._shutdown_thread: 226 # Since no new work items can be added, it is safe to shutdown 227 # this thread if there are no pending work items. 228 if not pending_work_items: 229 while nb_shutdown_processes[0] < len(processes): 230 shutdown_one_process() 231 # If .join() is not called on the created processes then 232 # some multiprocessing.Queue methods may deadlock on Mac OS 233 # X. 234 for p in processes: 235 p.join() 236 call_queue.close() 237 return 238 del executor 239 240_system_limits_checked = False 241_system_limited = None 242def _check_system_limits(): 243 global _system_limits_checked, _system_limited 244 if _system_limits_checked: 245 if _system_limited: 246 raise NotImplementedError(_system_limited) 247 _system_limits_checked = True 248 try: 249 import os 250 nsems_max = os.sysconf("SC_SEM_NSEMS_MAX") 251 except (AttributeError, ValueError): 252 # sysconf not available or setting not available 253 return 254 if nsems_max == -1: 255 # indetermine limit, assume that limit is determined 256 # by available memory only 257 return 258 if nsems_max >= 256: 259 # minimum number of semaphores available 260 # according to POSIX 261 return 262 _system_limited = "system provides too few semaphores (%d available, 256 necessary)" % nsems_max 263 raise NotImplementedError(_system_limited) 264 265 266class ProcessPoolExecutor(_base.Executor): 267 def __init__(self, max_workers=None): 268 """Initializes a new ProcessPoolExecutor instance. 269 270 Args: 271 max_workers: The maximum number of processes that can be used to 272 execute the given calls. If None or not given then as many 273 worker processes will be created as the machine has processors. 274 """ 275 _check_system_limits() 276 277 if max_workers is None: 278 self._max_workers = multiprocessing.cpu_count() 279 else: 280 if max_workers <= 0: 281 raise ValueError("max_workers must be greater than 0") 282 283 self._max_workers = max_workers 284 285 # Make the call queue slightly larger than the number of processes to 286 # prevent the worker processes from idling. But don't make it too big 287 # because futures in the call queue cannot be cancelled. 288 self._call_queue = multiprocessing.Queue(self._max_workers + 289 EXTRA_QUEUED_CALLS) 290 self._result_queue = multiprocessing.Queue() 291 self._work_ids = queue.Queue() 292 self._queue_management_thread = None 293 self._processes = set() 294 295 # Shutdown is a two-step process. 296 self._shutdown_thread = False 297 self._shutdown_lock = threading.Lock() 298 self._queue_count = 0 299 self._pending_work_items = {} 300 301 def _start_queue_management_thread(self): 302 # When the executor gets lost, the weakref callback will wake up 303 # the queue management thread. 304 def weakref_cb(_, q=self._result_queue): 305 q.put(None) 306 if self._queue_management_thread is None: 307 self._queue_management_thread = threading.Thread( 308 target=_queue_management_worker, 309 args=(weakref.ref(self, weakref_cb), 310 self._processes, 311 self._pending_work_items, 312 self._work_ids, 313 self._call_queue, 314 self._result_queue)) 315 self._queue_management_thread.daemon = True 316 self._queue_management_thread.start() 317 _threads_queues[self._queue_management_thread] = self._result_queue 318 319 def _adjust_process_count(self): 320 for _ in range(len(self._processes), self._max_workers): 321 p = multiprocessing.Process( 322 target=_process_worker, 323 args=(self._call_queue, 324 self._result_queue)) 325 p.start() 326 self._processes.add(p) 327 328 def submit(self, fn, *args, **kwargs): 329 with self._shutdown_lock: 330 if self._shutdown_thread: 331 raise RuntimeError('cannot schedule new futures after shutdown') 332 333 f = _base.Future() 334 w = _WorkItem(f, fn, args, kwargs) 335 336 self._pending_work_items[self._queue_count] = w 337 self._work_ids.put(self._queue_count) 338 self._queue_count += 1 339 # Wake up queue management thread 340 self._result_queue.put(None) 341 342 self._start_queue_management_thread() 343 self._adjust_process_count() 344 return f 345 submit.__doc__ = _base.Executor.submit.__doc__ 346 347 def shutdown(self, wait=True): 348 with self._shutdown_lock: 349 self._shutdown_thread = True 350 if self._queue_management_thread: 351 # Wake up queue management thread 352 self._result_queue.put(None) 353 if wait: 354 self._queue_management_thread.join(sys.maxint) 355 # To reduce the risk of openning too many files, remove references to 356 # objects that use file descriptors. 357 self._queue_management_thread = None 358 self._call_queue = None 359 self._result_queue = None 360 self._processes = None 361 shutdown.__doc__ = _base.Executor.shutdown.__doc__ 362 363atexit.register(_python_exit) 364