Description
When debugging a performance problem with my node, I created an MWE in C++ which shows interesting behavior with serious performance hit. In case you use MultiThreadedSpinner or AsyncSpinner with >1 threads and one of your callbacks takes too long (messages arrive quicker), the spinner eats up a lot of CPU.
I'll post the MWE on Monday, so for now just a textual description. I have a normal node with the MT/Async spinner, I subscribe to one 50 Hz PointCloud2 topic with a single subscriber. The callback contains just ros::Duration::sleep(time).
When running the code with a single thread (either with SingleThreadedSpinner, or even MT spinner with 1 thread), the node takes up about 5% of CPU and just throws away the messages it did not have time to process.
As soon as there are at least 2 threads, CPU consumption goes high (40-400% of CPU) and scales (lineraly?) with the duration I wait in the callback and the number of threads.
My research ended in spinner.cpp, where I see that in case of 1 thread, callAvailable() is used, whereas with more threads, callOne() is used. However, I can't see through all the multi-threading magic in there. I ran a profiler on the node, and most of the time is spent with mutexes and other sync primitives.
I haven't tried yet with SubscribeOptions.allow_concurrent_callbacks .
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