Process 0 very slow when using num_procs with map to tokenizer
Ok weird, I don't manage to reproduce this issue on my side.
Does it happen even with `num_proc=2` for example ?
Also could you provide more details about your OS and the versions of tokenizers/datasets/multiprocess that you're using ?
<img width="721" alt="image" src="https://user-images.githubusercontent.com/17930170/97066109-776d0d00-15ed-11eb-8bba-bb4d2e0fcc33.png">
The code I am using is
```

        dataset = load_dataset("text", data_files=[file_path], split='train')
        dataset = dataset.map(lambda ex: tokenizer(ex["text"], add_special_tokens=True,
                                                truncation=True, max_length=args.block_size), num_proc=8)
        dataset.set_format(type='torch', columns=['input_ids'])
        dataset.save_to_disk(file_path+'.arrow')
```

Process 0 very slow when using num_procs with map to tokenizer 
 <img width="721" alt="image" src="https://user-images.githubusercontent.com/17930170/97066109-776d0d00-15ed-11eb-8bba-bb4d2e0fcc33.png">
The code I am using is
```

        dataset = load_dataset("text", data_files=[file_path], split='train')
        dataset = dataset.map(lambda ex: tokenizer(ex["text"], add_special_tokens=True,
                                                truncation=True, max_length=args.block_size), num_proc=8)
        dataset.set_format(type='torch', columns=['input_ids'])
        dataset.save_to_disk(file_path+'.arrow')
```
 
 Ok weird, I don't manage to reproduce this issue on my side.
Does it happen even with `num_proc=2` for example ?
Also could you provide more details about your OS and the versions of tokenizers/datasets/multiprocess that you're using ?