Threading
Threading-Model
===============
model of threading, implemented
through using the Python `eventlet [http://eventlet.net/]`_ and
`greenlet [http://packages.python.org/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. For example, sleep and 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)
Mysql-Access-And-Eventlet
-------------------------
Queries to the MySQL database will block the main thread of a service. This is
because OpenStack services use an external C library for accessing the MySQL
database. Since eventlet cannot use monkey-patching to intercept blocking
calls in a C library, the resulting database query blocks the thread.
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