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* Added tests for shared_memory submodule. * Added tests for ShareableList. * Fix bug in allocationn size during creation of empty ShareableList illuminated by existing test run on Linux. * Initial set of docs for shared_memory module. * Added docs for ShareableList, added doctree entry for shared_memory submodule, name refactoring for greater clarity. * Added examples to SharedMemoryManager docs, for ease of documentation switched away from exclusively registered functions to some explicit methods on SharedMemoryManager. * Wording tweaks to docs. * Fix test failures on Windows. * Added tests around SharedMemoryManager. * Documentation tweaks. * Fix inappropriate test on Windows. * Further documentation tweaks. * Fix bare exception. * Removed __copyright__. * Fixed typo in doc, removed comment. * Updated SharedMemoryManager preliminary tests to reflect change of not supporting all registered functions on SyncManager. * Added Sphinx doctest run controls. * CloseHandle should be in a finally block in case MapViewOfFile fails. * Missed opportunity to use with statement. * Switch to self.addCleanup to spare long try/finally blocks and save one indentation, change to use decorator to skip test instead. * Simplify the posixshmem extension module. Provide shm_open() and shm_unlink() functions. Move other functionality into the shared_memory.py module. * Added to doc around size parameter of SharedMemory. * Changed PosixSharedMemory.size to use os.fstat. * Change SharedMemory.buf to a read-only property as well as NamedSharedMemory.size. * Marked as provisional per PEP411 in docstring. * Changed SharedMemoryTracker to be private. * Removed registered Proxy Objects from SharedMemoryManager. * Removed shareable_wrap(). * Removed shareable_wrap() and dangling references to it. * For consistency added __reduce__ to key classes. * Fix for potential race condition on Windows for O_CREX. * Remove unused imports. * Update access to kernel32 on Windows per feedback from eryksun. * Moved kernel32 calls to _winapi. * Removed ShareableList.copy as redundant. * Changes to _winapi use from eryksun feedback. * Adopt simpler SharedMemory API, collapsing PosixSharedMemory and WindowsNamedSharedMemory into one. * Fix missing docstring on class, add test for ignoring size when attaching. * Moved SharedMemoryManager to managers module, tweak to fragile test. * Tweak to exception in OpenFileMapping suggested by eryksun. * Mark a few dangling bits as private as suggested by Giampaolo.
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:mod:`multiprocessing.shared_memory` --- Provides shared memory for direct access across processes | ||
=================================================================================================== | ||
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.. module:: multiprocessing.shared_memory | ||
:synopsis: Provides shared memory for direct access across processes. | ||
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**Source code:** :source:`Lib/multiprocessing/shared_memory.py` | ||
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.. versionadded:: 3.8 | ||
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.. index:: | ||
single: Shared Memory | ||
single: POSIX Shared Memory | ||
single: Named Shared Memory | ||
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-------------- | ||
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This module provides a class, :class:`SharedMemory`, for the allocation | ||
and management of shared memory to be accessed by one or more processes | ||
on a multicore or symmetric multiprocessor (SMP) machine. To assist with | ||
the life-cycle management of shared memory especially across distinct | ||
processes, a :class:`~multiprocessing.managers.BaseManager` subclass, | ||
:class:`SharedMemoryManager`, is also provided in the | ||
``multiprocessing.managers`` module. | ||
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In this module, shared memory refers to "System V style" shared memory blocks | ||
(though is not necessarily implemented explicitly as such) and does not refer | ||
to "distributed shared memory". This style of shared memory permits distinct | ||
processes to potentially read and write to a common (or shared) region of | ||
volatile memory. Processes are conventionally limited to only have access to | ||
their own process memory space but shared memory permits the sharing | ||
of data between processes, avoiding the need to instead send messages between | ||
processes containing that data. Sharing data directly via memory can provide | ||
significant performance benefits compared to sharing data via disk or socket | ||
or other communications requiring the serialization/deserialization and | ||
copying of data. | ||
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.. class:: SharedMemory(name=None, create=False, size=0) | ||
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Creates a new shared memory block or attaches to an existing shared | ||
memory block. Each shared memory block is assigned a unique name. | ||
In this way, one process can create a shared memory block with a | ||
particular name and a different process can attach to that same shared | ||
memory block using that same name. | ||
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As a resource for sharing data across processes, shared memory blocks | ||
may outlive the original process that created them. When one process | ||
no longer needs access to a shared memory block that might still be | ||
needed by other processes, the :meth:`close()` method should be called. | ||
When a shared memory block is no longer needed by any process, the | ||
:meth:`unlink()` method should be called to ensure proper cleanup. | ||
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*name* is the unique name for the requested shared memory, specified as | ||
a string. When creating a new shared memory block, if ``None`` (the | ||
default) is supplied for the name, a novel name will be generated. | ||
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*create* controls whether a new shared memory block is created (``True``) | ||
or an existing shared memory block is attached (``False``). | ||
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*size* specifies the requested number of bytes when creating a new shared | ||
memory block. Because some platforms choose to allocate chunks of memory | ||
based upon that platform's memory page size, the exact size of the shared | ||
memory block may be larger or equal to the size requested. When attaching | ||
to an existing shared memory block, the ``size`` parameter is ignored. | ||
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.. method:: close() | ||
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Closes access to the shared memory from this instance. In order to | ||
ensure proper cleanup of resources, all instances should call | ||
``close()`` once the instance is no longer needed. Note that calling | ||
``close()`` does not cause the shared memory block itself to be | ||
destroyed. | ||
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.. method:: unlink() | ||
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Requests that the underlying shared memory block be destroyed. In | ||
order to ensure proper cleanup of resources, ``unlink()`` should be | ||
called once (and only once) across all processes which have need | ||
for the shared memory block. After requesting its destruction, a | ||
shared memory block may or may not be immediately destroyed and | ||
this behavior may differ across platforms. Attempts to access data | ||
inside the shared memory block after ``unlink()`` has been called may | ||
result in memory access errors. Note: the last process relinquishing | ||
its hold on a shared memory block may call ``unlink()`` and | ||
:meth:`close()` in either order. | ||
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.. attribute:: buf | ||
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A memoryview of contents of the shared memory block. | ||
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.. attribute:: name | ||
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Read-only access to the unique name of the shared memory block. | ||
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.. attribute:: size | ||
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Read-only access to size in bytes of the shared memory block. | ||
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The following example demonstrates low-level use of :class:`SharedMemory` | ||
instances:: | ||
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>>> from multiprocessing import shared_memory | ||
>>> shm_a = shared_memory.SharedMemory(create=True, size=10) | ||
>>> type(shm_a.buf) | ||
<class 'memoryview'> | ||
>>> buffer = shm_a.buf | ||
>>> len(buffer) | ||
10 | ||
>>> buffer[:4] = bytearray([22, 33, 44, 55]) # Modify multiple at once | ||
>>> buffer[4] = 100 # Modify single byte at a time | ||
>>> # Attach to an existing shared memory block | ||
>>> shm_b = shared_memory.SharedMemory(shm_a.name) | ||
>>> import array | ||
>>> array.array('b', shm_b.buf[:5]) # Copy the data into a new array.array | ||
array('b', [22, 33, 44, 55, 100]) | ||
>>> shm_b.buf[:5] = b'howdy' # Modify via shm_b using bytes | ||
>>> bytes(shm_a.buf[:5]) # Access via shm_a | ||
b'howdy' | ||
>>> shm_b.close() # Close each SharedMemory instance | ||
>>> shm_a.close() | ||
>>> shm_a.unlink() # Call unlink only once to release the shared memory | ||
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The following example demonstrates a practical use of the :class:`SharedMemory` | ||
class with `NumPy arrays <https://www.numpy.org/>`_, accessing the | ||
same ``numpy.ndarray`` from two distinct Python shells: | ||
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.. doctest:: | ||
:options: +SKIP | ||
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>>> # In the first Python interactive shell | ||
>>> import numpy as np | ||
>>> a = np.array([1, 1, 2, 3, 5, 8]) # Start with an existing NumPy array | ||
>>> from multiprocessing import shared_memory | ||
>>> shm = shared_memory.SharedMemory(create=True, size=a.nbytes) | ||
>>> # Now create a NumPy array backed by shared memory | ||
>>> b = np.ndarray(a.shape, dtype=a.dtype, buffer=shm.buf) | ||
>>> b[:] = a[:] # Copy the original data into shared memory | ||
>>> b | ||
array([1, 1, 2, 3, 5, 8]) | ||
>>> type(b) | ||
<class 'numpy.ndarray'> | ||
>>> type(a) | ||
<class 'numpy.ndarray'> | ||
>>> shm.name # We did not specify a name so one was chosen for us | ||
'psm_21467_46075' | ||
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>>> # In either the same shell or a new Python shell on the same machine | ||
>>> import numpy as np | ||
>>> from multiprocessing import shared_memory | ||
>>> # Attach to the existing shared memory block | ||
>>> existing_shm = shared_memory.SharedMemory(name='psm_21467_46075') | ||
>>> # Note that a.shape is (6,) and a.dtype is np.int64 in this example | ||
>>> c = np.ndarray((6,), dtype=np.int64, buffer=existing_shm.buf) | ||
>>> c | ||
array([1, 1, 2, 3, 5, 8]) | ||
>>> c[-1] = 888 | ||
>>> c | ||
array([ 1, 1, 2, 3, 5, 888]) | ||
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>>> # Back in the first Python interactive shell, b reflects this change | ||
>>> b | ||
array([ 1, 1, 2, 3, 5, 888]) | ||
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>>> # Clean up from within the second Python shell | ||
>>> del c # Unnecessary; merely emphasizing the array is no longer used | ||
>>> existing_shm.close() | ||
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>>> # Clean up from within the first Python shell | ||
>>> del b # Unnecessary; merely emphasizing the array is no longer used | ||
>>> shm.close() | ||
>>> shm.unlink() # Free and release the shared memory block at the very end | ||
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.. class:: SharedMemoryManager([address[, authkey]]) | ||
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A subclass of :class:`~multiprocessing.managers.BaseManager` which can be | ||
used for the management of shared memory blocks across processes. | ||
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A call to :meth:`~multiprocessing.managers.BaseManager.start` on a | ||
:class:`SharedMemoryManager` instance causes a new process to be started. | ||
This new process's sole purpose is to manage the life cycle | ||
of all shared memory blocks created through it. To trigger the release | ||
of all shared memory blocks managed by that process, call | ||
:meth:`~multiprocessing.managers.BaseManager.shutdown()` on the instance. | ||
This triggers a :meth:`SharedMemory.unlink()` call on all of the | ||
:class:`SharedMemory` objects managed by that process and then | ||
stops the process itself. By creating ``SharedMemory`` instances | ||
through a ``SharedMemoryManager``, we avoid the need to manually track | ||
and trigger the freeing of shared memory resources. | ||
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This class provides methods for creating and returning :class:`SharedMemory` | ||
instances and for creating a list-like object (:class:`ShareableList`) | ||
backed by shared memory. | ||
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Refer to :class:`multiprocessing.managers.BaseManager` for a description | ||
of the inherited *address* and *authkey* optional input arguments and how | ||
they may be used to connect to an existing ``SharedMemoryManager`` service | ||
from other processes. | ||
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.. method:: SharedMemory(size) | ||
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Create and return a new :class:`SharedMemory` object with the | ||
specified ``size`` in bytes. | ||
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.. method:: ShareableList(sequence) | ||
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Create and return a new :class:`ShareableList` object, initialized | ||
by the values from the input ``sequence``. | ||
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The following example demonstrates the basic mechanisms of a | ||
:class:`SharedMemoryManager`: | ||
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.. doctest:: | ||
:options: +SKIP | ||
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>>> from multiprocessing import shared_memory | ||
>>> smm = shared_memory.SharedMemoryManager() | ||
>>> smm.start() # Start the process that manages the shared memory blocks | ||
>>> sl = smm.ShareableList(range(4)) | ||
>>> sl | ||
ShareableList([0, 1, 2, 3], name='psm_6572_7512') | ||
>>> raw_shm = smm.SharedMemory(size=128) | ||
>>> another_sl = smm.ShareableList('alpha') | ||
>>> another_sl | ||
ShareableList(['a', 'l', 'p', 'h', 'a'], name='psm_6572_12221') | ||
>>> smm.shutdown() # Calls unlink() on sl, raw_shm, and another_sl | ||
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The following example depicts a potentially more convenient pattern for using | ||
:class:`SharedMemoryManager` objects via the :keyword:`with` statement to | ||
ensure that all shared memory blocks are released after they are no longer | ||
needed: | ||
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.. doctest:: | ||
:options: +SKIP | ||
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>>> with shared_memory.SharedMemoryManager() as smm: | ||
... sl = smm.ShareableList(range(2000)) | ||
... # Divide the work among two processes, storing partial results in sl | ||
... p1 = Process(target=do_work, args=(sl, 0, 1000)) | ||
... p2 = Process(target=do_work, args=(sl, 1000, 2000)) | ||
... p1.start() | ||
... p2.start() # A multiprocessing.Pool might be more efficient | ||
... p1.join() | ||
... p2.join() # Wait for all work to complete in both processes | ||
... total_result = sum(sl) # Consolidate the partial results now in sl | ||
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When using a :class:`SharedMemoryManager` in a :keyword:`with` statement, the | ||
shared memory blocks created using that manager are all released when the | ||
:keyword:`with` statement's code block finishes execution. | ||
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.. class:: ShareableList(sequence=None, *, name=None) | ||
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Provides a mutable list-like object where all values stored within are | ||
stored in a shared memory block. This constrains storable values to | ||
only the ``int``, ``float``, ``bool``, ``str`` (less than 10M bytes each), | ||
``bytes`` (less than 10M bytes each), and ``None`` built-in data types. | ||
It also notably differs from the built-in ``list`` type in that these | ||
lists can not change their overall length (i.e. no append, insert, etc.) | ||
and do not support the dynamic creation of new :class:`ShareableList` | ||
instances via slicing. | ||
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*sequence* is used in populating a new ``ShareableList`` full of values. | ||
Set to ``None`` to instead attach to an already existing | ||
``ShareableList`` by its unique shared memory name. | ||
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*name* is the unique name for the requested shared memory, as described | ||
in the definition for :class:`SharedMemory`. When attaching to an | ||
existing ``ShareableList``, specify its shared memory block's unique | ||
name while leaving ``sequence`` set to ``None``. | ||
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.. method:: count(value) | ||
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Returns the number of occurrences of ``value``. | ||
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.. method:: index(value) | ||
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Returns first index position of ``value``. Raises :exc:`ValueError` if | ||
``value`` is not present. | ||
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.. attribute:: format | ||
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Read-only attribute containing the :mod:`struct` packing format used by | ||
all currently stored values. | ||
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.. attribute:: shm | ||
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The :class:`SharedMemory` instance where the values are stored. | ||
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The following example demonstrates basic use of a :class:`ShareableList` | ||
instance: | ||
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>>> from multiprocessing import shared_memory | ||
>>> a = shared_memory.ShareableList(['howdy', b'HoWdY', -273.154, 100, None, True, 42]) | ||
>>> [ type(entry) for entry in a ] | ||
[<class 'str'>, <class 'bytes'>, <class 'float'>, <class 'int'>, <class 'NoneType'>, <class 'bool'>, <class 'int'>] | ||
>>> a[2] | ||
-273.154 | ||
>>> a[2] = -78.5 | ||
>>> a[2] | ||
-78.5 | ||
>>> a[2] = 'dry ice' # Changing data types is supported as well | ||
>>> a[2] | ||
'dry ice' | ||
>>> a[2] = 'larger than previously allocated storage space' | ||
Traceback (most recent call last): | ||
... | ||
ValueError: exceeds available storage for existing str | ||
>>> a[2] | ||
'dry ice' | ||
>>> len(a) | ||
7 | ||
>>> a.index(42) | ||
6 | ||
>>> a.count(b'howdy') | ||
0 | ||
>>> a.count(b'HoWdY') | ||
1 | ||
>>> a.shm.close() | ||
>>> a.shm.unlink() | ||
>>> del a # Use of a ShareableList after call to unlink() is unsupported | ||
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The following example depicts how one, two, or many processes may access the | ||
same :class:`ShareableList` by supplying the name of the shared memory block | ||
behind it: | ||
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>>> b = shared_memory.ShareableList(range(5)) # In a first process | ||
>>> c = shared_memory.ShareableList(name=b.shm.name) # In a second process | ||
>>> c | ||
ShareableList([0, 1, 2, 3, 4], name='...') | ||
>>> c[-1] = -999 | ||
>>> b[-1] | ||
-999 | ||
>>> b.shm.close() | ||
>>> c.shm.close() | ||
>>> c.shm.unlink() | ||
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