52 lines
2.2 KiB
ReStructuredText
52 lines
2.2 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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``@lockutils.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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MySQL access and eventlet
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-------------------------
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Queries to the MySQL database will block the main thread of a service. This is
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because OpenStack services use an external C library for accessing the MySQL
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database. Since eventlet cannot use monkey-patching to intercept blocking
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calls in a C library, the resulting database query blocks the thread.
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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/nova/+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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