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There is a theorical problem in the wait queue, which is that with many threads, one could spend a lot of time looping on the newly expired tasks, causing a lot of contention on the global wq_lock and on the global rq_lock. This initially sounds bening, but if another thread does just a task_schedule() or task_queue(), it might end up waiting for a long time on this lock, and this wait time will count on its execution budget, degrading the end user's experience and possibly risking to trigger the watchdog if that lasts too long. The simplest (and backportable) solution here consists in bounding the number of expired tasks that may be picked from the global wait queue at once by a thread, given that all other ones will do it as well anyway. We don't need to pick more than global.tune.runqueue_depth tasks at once as we won't process more, so this counter is updated for both the local and the global queues: threads with more local expired tasks will pick less global tasks and conversely, keeping the load balanced between all threads. This will guarantee a much lower latency if/when wakeup storms happen (e.g. hundreds of thousands of synchronized health checks). Note that some crashes have been witnessed with 1/4 of the threads in wake_expired_tasks() and, while the issue might or might not be related, not having reasonable bounds here definitely justifies why we can spend so much time there. This patch should be backported, probably as far as 2.0 (maybe with some adaptations). |
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The HAProxy documentation has been split into a number of different files for ease of use. Please refer to the following files depending on what you're looking for : - INSTALL for instructions on how to build and install HAProxy - BRANCHES to understand the project's life cycle and what version to use - LICENSE for the project's license - CONTRIBUTING for the process to follow to submit contributions The more detailed documentation is located into the doc/ directory : - doc/intro.txt for a quick introduction on HAProxy - doc/configuration.txt for the configuration's reference manual - doc/lua.txt for the Lua's reference manual - doc/SPOE.txt for how to use the SPOE engine - doc/network-namespaces.txt for how to use network namespaces under Linux - doc/management.txt for the management guide - doc/regression-testing.txt for how to use the regression testing suite - doc/peers.txt for the peers protocol reference - doc/coding-style.txt for how to adopt HAProxy's coding style - doc/internals for developer-specific documentation (not all up to date)