def running_instances(self, context, process_name):
        """Get a list of running instances.

        Args:
            context (`ResolvedContext`): Context the process is running in.
            process_name (str): Name of the process.

        Returns:
            List of (`subprocess.Popen`, start-time) 2-tuples, where start_time
            is the epoch time the process was added.
        """
        handle = (id(context), process_name)
        it = self.processes.get(handle, {}).itervalues()
        entries = [x for x in it if x[0].poll() is None]
        return entries