5abd344818
Fix errors identified by doc8 other than D001 for lines too long. Change-Id: If351030cd5add101a19be4b5b759f56ca49b92d0
52 lines
2.3 KiB
ReStructuredText
52 lines
2.3 KiB
ReStructuredText
Threading model
|
|
===============
|
|
|
|
All OpenStack services use *green thread* 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 (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)
|
|
|
|
|
|
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
|
|
.. _impact on performance: https://lists.launchpad.net/openstack/msg08217.html
|