90060722a9
This patch introduces a new directory layout in doc/source in conformance with the OpenStack manuals project migration spec [1], moves the existing content in manila/doc/source into the new directories, and adjusts index files accordingly. This is the first step in the migration process as outlined in the spec. [1] https://specs.openstack.org/openstack/docs-specs/specs/pike/os-manuals-migration.html Partial-Bug: #1706181 Needed-By: I7924d94b82e7c8d9716bad7a219fc38c57970773 Depends-On: Ifc80fc56648cef74c85464321d1850e8c68449a0 Depends-On: Ia750cb049c0f53a234ea70ce1f2bbbb7a2aa9454 Change-Id: Ieea33262101a1d2459492c1c8aaac5fe042279f6
59 lines
2.6 KiB
ReStructuredText
59 lines
2.6 KiB
ReStructuredText
Threading model
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===============
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All OpenStack services use *green thread* model of threading, implemented
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through using the Python `eventlet <http://eventlet.net/>`_ and
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`greenlet <http://packages.python.org/greenlet/>`_ libraries.
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Green threads use a cooperative model of threading: thread context
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switches can only occur when specific eventlet or greenlet library calls are
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made (e.g., sleep, certain I/O calls). From the operating system's point of
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view, each OpenStack service runs in a single thread.
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The use of green threads reduces the likelihood of race conditions, but does
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not completely eliminate them. In some cases, you may need to use the
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``@utils.synchronized(...)`` decorator to avoid races.
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In addition, since there is only one operating system thread, a call that
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blocks that main thread will block the entire process.
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Yielding the thread in long-running tasks
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-----------------------------------------
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If a code path takes a long time to execute and does not contain any methods
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that trigger an eventlet context switch, the long-running thread will block
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any pending threads.
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This scenario can be avoided by adding calls to the eventlet sleep method
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in the long-running code path. The sleep call will trigger a context switch
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if there are pending threads, and using an argument of 0 will avoid introducing
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delays in the case that there is only a single green thread::
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from eventlet import greenthread
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...
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greenthread.sleep(0)
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In current code, time.sleep(0) does the same thing as greenthread.sleep(0)
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if time module is patched through eventlet.monkey_patch(). To be explicit,
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we recommend contributors to use ``greenthread.sleep()`` instead of
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``time.sleep()``.
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MySQL access and eventlet
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-------------------------
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There are some MySQL DB API drivers for oslo.db, like `PyMySQL`_, MySQL-python,
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etc. PyMySQL is the default MySQL DB API driver for oslo.db, and it works well
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with eventlet. MySQL-python uses an external C library for accessing the MySQL
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database. Since eventlet cannot use monkey-patching to intercept blocking calls
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in a C library, queries to the MySQL database will block the main thread of a
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service.
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The Diablo release contained a thread-pooling implementation that did not
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block, but this implementation resulted in a `bug`_ and was removed.
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See this `mailing list thread`_ for a discussion of this issue, including
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a discussion of the `impact on performance`_.
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.. _bug: https://bugs.launchpad.net/manila/+bug/838581
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.. _mailing list thread: https://lists.launchpad.net/openstack/msg08118.html
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.. _impact on performance: https://lists.launchpad.net/openstack/msg08217.html
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.. _PyMySQL: https://wiki.openstack.org/wiki/PyMySQL_evaluation
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