with Pool(num_proc) as pool:
                with pbar:
                    for job_id, done, content in iflatmap_unordered(
                        pool, self._prepare_split_single, kwargs_iterable=kwargs_per_job
                    ):
                        if done:
                            # the content is the result of the job
                            (
                                examples_per_job[job_id],
                                bytes_per_job[job_id],
                                features_per_job[job_id],
                                shards_per_job[job_id],
                                shard_lengths_per_job[job_id],
                            ) = content
                        else:
                            # the content is the number of examples progress update
                            pbar.update(content)