openstack/cinder
Code Issues Proposed changes

Adjust doc about threading

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There are two ways to give other greenthread chance to run:
greenthread.sleep(0) or time.sleep(0). Add the second way in
threading.rst and recommend the first way for contributors.

Also, PyMySQL works well with eventlet [1], and it's the default MySQL
DB API driver for oslo.db now[2].
So adjust 'threading model' doc with such info.

[1] https://wiki.openstack.org/wiki/PyMySQL_evaluation
[2] http://docs.openstack.org/developer/oslo.db/installation.html

Change-Id: I1fcf7242a6794d74da16aa5f326887cfa7257a1b
This commit is contained in:
ChangBo Guo(gcb) 2016-09-09 14:42:46 +08:00
parent 533ea09a0d
commit dd0ab650c8
1 changed files with 13 additions and 7 deletions
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20
doc/source/devref/threading.rst
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@ -28,17 +28,22 @@ 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)
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
-------------------------
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.
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.
@ -49,3 +54,4 @@ 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