OMOPSO with multi-process failed

[jacktang]

Hello,

I want run OMOPSO in parallel using multi-process, and the code looks like

problem = ZDT1()

mutation_probability = 1.0 / problem.number_of_variables
max_evaluations = 25000
swarm_size = 100

algorithm = OMOPSO(
    problem=problem,
    swarm_size=swarm_size,
    epsilon=0.0075,
    uniform_mutation=UniformMutation(probability=mutation_probability, perturbation=0.5),
    non_uniform_mutation=NonUniformMutation(mutation_probability, perturbation=0.5,
                                            max_iterations=int(max_evaluations / swarm_size)),
    leaders=CrowdingDistanceArchive(100),
    termination_criterion=StoppingByEvaluations(max_evaluations=max_evaluations),
    swarm_evaluator = MultiprocessEvaluator(processes=4),
)

and the error:

File ~/opt/miniconda3/lib/python3.9/site-packages/jmetal/core/algorithm.py:84, in Algorithm.run(self)
     82 LOGGER.debug('Running main loop until termination criteria is met')
     83 while not self.stopping_condition_is_met():
---> 84     self.step()
     85     self.update_progress()
     87 self.total_computing_time = time.time() - self.start_computing_time

File ~/opt/miniconda3/lib/python3.9/site-packages/jmetal/core/algorithm.py:222, in ParticleSwarmOptimization.step(self)
    220 self.update_position(self.solutions)
    221 self.perturbation(self.solutions)
--> 222 self.solutions = self.evaluate(self.solutions)
    223 self.update_global_best(self.solutions)
    224 self.update_particle_best(self.solutions)

File ~/opt/miniconda3/lib/python3.9/site-packages/jmetal/algorithm/multiobjective/omopso.py:93, in OMOPSO.evaluate(self, solution_list)
     92 def evaluate(self, solution_list: List[FloatSolution]):
---> 93     return self.swarm_evaluator.evaluate(solution_list, self.problem)
...
    769     return self._value
    770 else:
--> 771     raise self._value

MaybeEncodingError: Error sending result: '[<jmetal.core.solution.FloatSolution object at 0x7ff59f4abc40>, <jmetal.core.solution.FloatSolution object at 0x7ff5a2248a30>, <jmetal.core.solution.FloatSolution object at 0x7ff5a22d5820>, <jmetal.core.solution.FloatSolution object at 0x7ff5a2303ee0>, <jmetal.core.solution.FloatSolution object at 0x7ff5a20cb8b0>, <jmetal.core.solution.FloatSolution object at 0x7ff5a2012610>, <jmetal.core.solution.FloatSolution object at 0x7ff5a22b6340>]'. Reason: 'RecursionError('maximum recursion depth exceeded while calling a Python object')'

Is it a bug?