bpo-29842: Make Executor.map less eager so it handles large/unbounded…

Doc/library/concurrent.futures.rst

@@ -38,7 +38,7 @@ Executor Objects
               future = executor.submit(pow, 323, 1235)
               print(future.result())
 
-    .. method:: map(func, *iterables, timeout=None, chunksize=1)
+    .. method:: map(func, *iterables, timeout=None, chunksize=1, prefetch=None)
 
        Similar to :func:`map(func, *iterables) <map>` except:
 
@@ -64,8 +64,15 @@ Executor Objects
        performance compared to the default size of 1.  With
        :class:`ThreadPoolExecutor`, *chunksize* has no effect.
 
+       By default, a reasonable number of tasks are
+       queued beyond the number of workers, an explicit *prefetch* count may be
+       provided to specify how many extra tasks should be queued.
+
        .. versionchanged:: 3.5
           Added the *chunksize* argument.
+
+       .. versionchanged:: 3.12
+          Added the *prefetch* argument.
 
     .. method:: shutdown(wait=True, *, cancel_futures=False)
 

Lib/concurrent/futures/_base.py

@@ -4,6 +4,7 @@
 __author__ = 'Brian Quinlan ([email protected])'
 
 import collections
+import itertools
 import logging
 import threading
 import time
@@ -568,7 +569,7 @@ def submit(self, fn, /, *args, **kwargs):
         """
         raise NotImplementedError()
 
-    def map(self, fn, *iterables, timeout=None, chunksize=1):
+    def map(self, fn, *iterables, timeout=None, chunksize=1, prefetch=None):
         """Returns an iterator equivalent to map(fn, iter).
 
         Args:
@@ -580,6 +581,8 @@ def map(self, fn, *iterables, timeout=None, chunksize=1):
                 before being passed to a child process. This argument is only
                 used by ProcessPoolExecutor; it is ignored by
                 ThreadPoolExecutor.
+            prefetch: The number of chunks to queue beyond the number of
+                workers on the executor. If None, a reasonable default is used.
 
         Returns:
             An iterator equivalent to: map(func, *iterables) but the calls may
@@ -592,21 +595,38 @@ def map(self, fn, *iterables, timeout=None, chunksize=1):
         """
         if timeout is not None:
             end_time = timeout + time.monotonic()
+        if prefetch is None:
+            prefetch = self._max_workers
+        if prefetch < 0:
+            raise ValueError("prefetch count may not be negative")
 
-        fs = [self.submit(fn, *args) for args in zip(*iterables)]
+        argsiter = zip(*iterables)
+        initialargs = itertools.islice(argsiter, self._max_workers + prefetch)
+
+        fs = collections.deque(self.submit(fn, *args) for args in initialargs)
 
         # Yield must be hidden in closure so that the futures are submitted
         # before the first iterator value is required.
         def result_iterator():
+            nonlocal argsiter
             try:
-                # reverse to keep finishing order
-                fs.reverse()
                 while fs:
-                    # Careful not to keep a reference to the popped future
                     if timeout is None:
-                        yield fs.pop().result()
+                        res = fs.popleft().result()
                     else:
-                        yield fs.pop().result(end_time - time.monotonic())
+                        res = fs.popleft().result(end_time - time.monotonic())
+
+                    # Dispatch next task before yielding to keep
+                    # pipeline full
+                    if argsiter:
+                        try:
+                            args = next(argsiter)
+                        except StopIteration:
+                            argsiter = None
+                        else:
+                            fs.append(self.submit(fn, *args))
+
+                    yield res
             finally:
                 for future in fs:
                     future.cancel()

Lib/concurrent/futures/process.py

@@ -784,7 +784,7 @@ def submit(self, fn, /, *args, **kwargs):
             return f
     submit.__doc__ = _base.Executor.submit.__doc__
 
-    def map(self, fn, *iterables, timeout=None, chunksize=1):
+    def map(self, fn, *iterables, timeout=None, chunksize=1, prefetch=None):
         """Returns an iterator equivalent to map(fn, iter).
 
         Args:
@@ -795,6 +795,8 @@ def map(self, fn, *iterables, timeout=None, chunksize=1):
             chunksize: If greater than one, the iterables will be chopped into
                 chunks of size chunksize and submitted to the process pool.
                 If set to one, the items in the list will be sent one at a time.
+            prefetch: The number of chunks to queue beyond the number of
+                workers on the executor. If None, a reasonable default is used.
 
         Returns:
             An iterator equivalent to: map(func, *iterables) but the calls may
@@ -810,7 +812,7 @@ def map(self, fn, *iterables, timeout=None, chunksize=1):
 
         results = super().map(partial(_process_chunk, fn),
                               _get_chunks(*iterables, chunksize=chunksize),
-                              timeout=timeout)
+                              timeout=timeout, prefetch=prefetch)
         return _chain_from_iterable_of_lists(results)
 
     def shutdown(self, wait=True, *, cancel_futures=False):

Lib/test/test_concurrent_futures.py

@@ -890,7 +890,22 @@ def record_finished(n):
 
         self.executor.map(record_finished, range(10))
         self.executor.shutdown(wait=True)
-        self.assertCountEqual(finished, range(10))
+        # No guarantees on how many tasks dispatched,
+        # but at least one should have been dispatched
+        self.assertGreater(len(finished), 0)
+
+    def test_infinite_map_input_completes_work(self):
+        import itertools
+        def identity(x):
+            return x
+
+        mapobj = self.executor.map(identity, itertools.count(0))
+        # Get one result, which shows we handle infinite inputs
+        # without waiting for all work to be dispatched
+        res = next(mapobj)
+        mapobj.close()  # Make sure futures cancelled
+
+        self.assertEqual(res, 0)
 
     def test_default_workers(self):
         executor = self.executor_type()

Misc/NEWS.d/next/Library/2019-05-06-12-49-21.bpo-29842.Tw-ETh.rst

@@ -0,0 +1,4 @@
+:meth:`concurrent.futures.Executor.map` no longer eagerly creates all futures prior to yielding any
+results. This allows it to work with huge or infinite :term:`iterable` without
+consuming excessive resources or crashing, making it more suitable as a drop
+in replacement for the built-in :func:`map`. Patch by Josh Rosenberg.