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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 second way for contributors. Change-Id: I33dc73a0edfa5fde0e5fa8051a594874ab166970
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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 @lockutils.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 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.
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.