envs = [gym.make(self.env_name) for _ in range(multiprocessing.cpu_count())]

amount_per_thread = int(np.floor(self.population_size / multiprocessing.cpu_count()))
left_over = self.population_size - amount_per_thread * multiprocessing.cpu_count()

fitnesses = np.zeros(len(self.population))

def get_weights_reward(begin, size, env):
    for i in range(begin, begin + size):
        fitnesses[i] = -self.get_reward(self.population[i],
                                        self.sess,
                                        env,
                                        self.meta)
threads = []
idx = 0
for i in range(multiprocessing.cpu_count()):
    amt = (amount_per_thread + 1) if i < left_over else amount_per_thread
    thread = threading.Thread(target=get_weights_reward,
                              args=[idx, amt, envs[i]])
    threads.append(thread)
    idx += amt

assert idx == len(self.population)

for t in threads:
    t.start()
for t in threads:
    t.join()