a8ec3317be
Make use of doc8 to verify all rst files which are not autogenerated for errors and fail if there are any issues found. The doc8 checks are now part of the tox 'docs' environment and ran automatically. Checks can also be called direcly via 'tox -e docs'. Fix all issues found by doc8. Closes-Bug: #1664841 Change-Id: I9215524d35646de7485504e4c5ff86fd91a1d09f Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>
2.6 KiB
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 to
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 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.