Threading model =============== All OpenStack services use *green thread* model of threading, implemented through using the Python `eventlet `_ and `greenlet `_ libraries. Green threads use a cooperative model of threading: thread context switches can only occur when specific eventlet or greenlet library calls are made (e.g., sleep, certain I/O calls). From the operating system's point of view, each OpenStack service runs in a single thread. The use of green threads reduces the likelihood of race conditions, but does not completely eliminate them. In some cases, you may need to use the ``@utils.synchronized(...)`` decorator to avoid races. In addition, since there is only one operating system thread, a call that blocks that main thread will block the entire process. Yielding the thread in long-running tasks ----------------------------------------- If a code path takes a long time to execute and does not contain any methods that trigger an eventlet context switch, the long-running thread will block any pending threads. This scenario can be avoided by adding calls to the eventlet sleep method in the long-running code path. The sleep call will trigger a context switch if there are pending threads, and using an argument of 0 will avoid introducing delays in the case that there is only a single green thread:: from eventlet import greenthread ... greenthread.sleep(0) In current code, time.sleep(0)does the same thing as greenthread.sleep(0) if time module is patched through eventlet.monkey_patch(). To be explicit, we recommend contributors use ``greenthread.sleep()`` instead of ``time.sleep()``. MySQL access and eventlet ------------------------- There are some MySQL DB API drivers for oslo.db, like `PyMySQL`_, MySQL-python etc. PyMySQL is the default MySQL DB API driver for oslo.db, and it works well with eventlet. MySQL-python uses an external C library for accessing the MySQL database. Since eventlet cannot use monkey-patching to intercept blocking calls in a C library, queries to the MySQL database using libraries like MySQL-python will block the main thread of a service. The Diablo release contained a thread-pooling implementation that did not block, but this implementation resulted in a `bug`_ and was removed. See this `mailing list thread`_ for a discussion of this issue, including a discussion of the `impact on performance`_. .. _bug: https://bugs.launchpad.net/cinder/+bug/838581 .. _mailing list thread: https://lists.launchpad.net/openstack/msg08118.html .. _impact on performance: https://lists.launchpad.net/openstack/msg08217.html .. _PyMySQL: https://wiki.openstack.org/wiki/PyMySQL_evaluation