Tornado ProcessPoolExecutor: how to limit the number of processes

Question

I am writing a REST server in Tornado. I use a ProcessPoolExecutor with a max_workers configurable parameter. However, the problem is that it seems to not effectively limit the number of processes the way I want. The code is

    def post(self):
        ...
        self.process_pool_executor.submit(_execute_scenario_optimization, self.project_name,
                                      self.scenario_name)

        self._generate_output_json_from_dict({"execution_status": "RET_OK"})
        return

I need submit to generate an exception when the maximum processes active in pool at the same time is more than, for example, 4. Do you have any idea?

Answer 1

The pool executors limit the number of processes that can run at once, but if you give them more tasks than processes, the extra tasks just go onto a queue instead of raising an exception. There doesn't appear to be a way to limit the size of this queue, so you should probably use a semaphore to limit the number of items you add to the queue.