how does ros schedule nodes

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Yes, ROS 1 uses the linux scheduler. By default, roscpp has a background thread to handle socket activity, and processes callbacks serially on the main thread when you can ros::spin() or ros::spinOnce(). You can change how many threads are used by using an async spinner or multithreaded spinner.

You are free to use the linux tools for scheduling to change how linux schedules your processes or threads. (I am not sure about ROS 2 but I think it is the same).