mirror of
https://github.com/python/cpython.git
synced 2025-10-27 19:54:38 +00:00
371 lines
14 KiB
ReStructuredText
371 lines
14 KiB
ReStructuredText
:mod:`multiprocessing.shared_memory` --- Shared memory for direct access across processes
|
|
=========================================================================================
|
|
|
|
.. module:: multiprocessing.shared_memory
|
|
:synopsis: Provides shared memory for direct access across processes.
|
|
|
|
**Source code:** :source:`Lib/multiprocessing/shared_memory.py`
|
|
|
|
.. versionadded:: 3.8
|
|
|
|
.. index::
|
|
single: Shared Memory
|
|
single: POSIX Shared Memory
|
|
single: Named Shared Memory
|
|
|
|
--------------
|
|
|
|
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.
|
|
|
|
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.
|
|
|
|
|
|
.. class:: SharedMemory(name=None, create=False, size=0)
|
|
|
|
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.
|
|
|
|
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.
|
|
|
|
*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.
|
|
|
|
*create* controls whether a new shared memory block is created (``True``)
|
|
or an existing shared memory block is attached (``False``).
|
|
|
|
*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.
|
|
|
|
.. method:: close()
|
|
|
|
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.
|
|
|
|
.. method:: unlink()
|
|
|
|
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.
|
|
|
|
.. attribute:: buf
|
|
|
|
A memoryview of contents of the shared memory block.
|
|
|
|
.. attribute:: name
|
|
|
|
Read-only access to the unique name of the shared memory block.
|
|
|
|
.. attribute:: size
|
|
|
|
Read-only access to size in bytes of the shared memory block.
|
|
|
|
|
|
The following example demonstrates low-level use of :class:`SharedMemory`
|
|
instances::
|
|
|
|
>>> 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
|
|
|
|
|
|
|
|
The following example demonstrates a practical use of the :class:`SharedMemory`
|
|
class with `NumPy arrays <https://numpy.org/>`_, accessing the
|
|
same ``numpy.ndarray`` from two distinct Python shells:
|
|
|
|
.. doctest::
|
|
:options: +SKIP
|
|
|
|
>>> # 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'
|
|
|
|
>>> # 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])
|
|
|
|
>>> # Back in the first Python interactive shell, b reflects this change
|
|
>>> b
|
|
array([ 1, 1, 2, 3, 5, 888])
|
|
|
|
>>> # Clean up from within the second Python shell
|
|
>>> del c # Unnecessary; merely emphasizing the array is no longer used
|
|
>>> existing_shm.close()
|
|
|
|
>>> # 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
|
|
|
|
|
|
.. class:: SharedMemoryManager([address[, authkey]])
|
|
:module: multiprocessing.managers
|
|
|
|
A subclass of :class:`~multiprocessing.managers.BaseManager` which can be
|
|
used for the management of shared memory blocks across processes.
|
|
|
|
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.
|
|
|
|
This class provides methods for creating and returning :class:`SharedMemory`
|
|
instances and for creating a list-like object (:class:`ShareableList`)
|
|
backed by shared memory.
|
|
|
|
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.
|
|
|
|
.. method:: SharedMemory(size)
|
|
|
|
Create and return a new :class:`SharedMemory` object with the
|
|
specified ``size`` in bytes.
|
|
|
|
.. method:: ShareableList(sequence)
|
|
|
|
Create and return a new :class:`ShareableList` object, initialized
|
|
by the values from the input ``sequence``.
|
|
|
|
|
|
The following example demonstrates the basic mechanisms of a
|
|
:class:`SharedMemoryManager`:
|
|
|
|
.. doctest::
|
|
:options: +SKIP
|
|
|
|
>>> from multiprocessing.managers import SharedMemoryManager
|
|
>>> smm = 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
|
|
|
|
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:
|
|
|
|
.. doctest::
|
|
:options: +SKIP
|
|
|
|
>>> with 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
|
|
|
|
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.
|
|
|
|
|
|
.. class:: ShareableList(sequence=None, *, name=None)
|
|
|
|
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.
|
|
|
|
*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.
|
|
|
|
*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``.
|
|
|
|
.. method:: count(value)
|
|
|
|
Returns the number of occurrences of ``value``.
|
|
|
|
.. method:: index(value)
|
|
|
|
Returns first index position of ``value``. Raises :exc:`ValueError` if
|
|
``value`` is not present.
|
|
|
|
.. attribute:: format
|
|
|
|
Read-only attribute containing the :mod:`struct` packing format used by
|
|
all currently stored values.
|
|
|
|
.. attribute:: shm
|
|
|
|
The :class:`SharedMemory` instance where the values are stored.
|
|
|
|
|
|
The following example demonstrates basic use of a :class:`ShareableList`
|
|
instance:
|
|
|
|
>>> 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
|
|
|
|
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:
|
|
|
|
>>> 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()
|
|
|
|
The following examples demonstrates that ``ShareableList``
|
|
(and underlying ``SharedMemory``) objects
|
|
can be pickled and unpickled if needed.
|
|
Note, that it will still be the same shared object.
|
|
This happens, because the deserialized object has
|
|
the same unique name and is just attached to an existing
|
|
object with the same name (if the object is still alive):
|
|
|
|
>>> import pickle
|
|
>>> from multiprocessing import shared_memory
|
|
>>> sl = shared_memory.ShareableList(range(10))
|
|
>>> list(sl)
|
|
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
|
|
|
|
>>> deserialized_sl = pickle.loads(pickle.dumps(sl))
|
|
>>> list(deserialized_sl)
|
|
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
|
|
|
|
>>> sl[0] = -1
|
|
>>> deserialized_sl[1] = -2
|
|
>>> list(sl)
|
|
[-1, -2, 2, 3, 4, 5, 6, 7, 8, 9]
|
|
>>> list(deserialized_sl)
|
|
[-1, -2, 2, 3, 4, 5, 6, 7, 8, 9]
|
|
|
|
>>> sl.shm.close()
|
|
>>> sl.shm.unlink()
|