data_args.preprocessing_num_workers almost freezes 
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Hi.
I cannot always reproduce this issue, and on later runs I did not see it so far. Sometimes also I set 8 processes but I see less being showed, is this normal, here only 5 are shown for 8 being set, thanks

```
#3:  11%|███████████████▊                                                                                                                                  | 172/1583 [00:46<06:21,  3.70ba/s]
#4:   9%|█████████████▏                                                                                                                                    | 143/1583 [00:46<07:46,  3.09ba/s]
#7:   6%|█████████                                                                                                                                          | 98/1583 [00:45<11:34,  2.14ba/s]
#5:   8%|███████████▍                                                                                                                                      | 124/1583 [00:46<09:03,  2.68ba/s]
#6:   7%|██████████▏  
``` closing since I cannot reproduce it again, thanks 
Hi @lhoestq 

I am running this code from huggingface transformers https://github.com/huggingface/transformers/blob/master/examples/language-modeling/run_mlm.py 

to speed up tokenization, since I am running on multiple datasets, I am using data_args.preprocessing_num_workers = 4 with opus100 corpus but this moves on till a point and then this freezes almost for sometime during  tokenization steps and then this is back again, overall to me taking more time than normal case, I appreciate your advice on how I can use this option properly to speed up.

thanks
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