openstack
/
nova
Code Issues Proposed changes

Added docs on MySQL queries blocking main thread. 

Added to the RST docs to describe how queries to MySQL will block a nova
service.

Change-Id: I4ae209e2827978cc5469162d386197f2e46b342d
This commit is contained in:
Lorin Hochstein 2012-03-02 09:39:36 -05:00
parent f3a7149a85
commit d304d828d0
3 changed files with 42 additions and 7 deletions
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1
.mailmap
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@ -38,6 +38,7 @@
<josh@jk0.org> <josh.kearney@rackspace.com>
<justin@fathomdb.com> <justinsb@justinsb-desktop>
<justin@fathomdb.com> <superstack@superstack.org>
<lorin@nimbisservices.com> <lorin@isi.edu>
<masumotok@nttdata.co.jp> Masumoto<masumotok@nttdata.co.jp>
<masumotok@nttdata.co.jp> <root@openstack2-api>
<matt.dietz@rackspace.com> <matthewdietz@Matthew-Dietzs-MacBook-Pro.local>

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Authors
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@ -112,7 +112,7 @@ Koji Iida <iida.koji@lab.ntt.co.jp>
Liam Kelleher <liam.kelleher@hp.com>
Likitha Shetty <likitha.shetty@citrix.com>
Loganathan Parthipan <parthipan@hp.com>
Lorin Hochstein <lorin@isi.edu>
Lorin Hochstein <lorin@nimbisservices.com>
Lvov Maxim <usrleon@gmail.com>
Mandell Degerness <mdegerne@gmail.com>
Mark McLoughlin <markmc@redhat.com>

46
doc/source/devref/threading.rst
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@ -1,17 +1,51 @@
Threading model
===============
All OpenStack services use *green thread* model of threading, implemented
through using the Python `eventlet <http://eventlet.net/>`_ and
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
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
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/nova/+bug/838581
.. _mailing list thread: https://lists.launchpad.net/openstack/msg08118.html
.. _impact on performance: https://lists.launchpad.net/openstack/msg08217.html