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* Explain these docs are for trunk (copied from ironic) * All the docs in this repo are meant to be developer docs, so having a devref inside of the docs is redundant and just makes the docs more complicated to navigate. Move everything out of the devref folder and link to everything from main index. * Move man pages into separate section. The man pages are pretty sparse * right now, we should either make them useful or just delete them * Remove dead docs from unused_docs list in doc/source/conf.py * Shuffle docs landing page, move common referees to the top (API, hypervisor support matrix), Add a introduction section and more. The hope is the updated layout makes this document easier to navigate. * Use maxdepth of 1 * Rename a few sections with what are hopefully better names The next step is to prune out outdated documents and further cleanup this page. Change-Id: Iff453e47ccc902a0e72b1a5f6ce1ee939ff3a1a0
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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)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.