How do I run different ros nodes on different cpu cores simulatenously?

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ROS is not special in terms of cpu usage. It does not control which core does what job, your OS will manage that via a scheduler. If the scheduler was the problem, you would notice it on all other applications. See stackoverflow.

However, you can allow ROS to use multiple threads. This is done via specifying the executor for your nodes, see documentation on executors, and in turn their callback groups. There was a recent writeup on how it works, I recommend reading it if you are interested in it.

Use htop to view your per-core load.