`datasets.map` multi processing much slower than single processing 
Hi @hwijeen, you might want to look at issues #1796 and #1949. I think it could be something related to the I/O operations being performed. I see that many people are experiencing the same issue. Is this problem considered an "official" bug that is worth a closer look? @lhoestq Yes this is an official bug. On my side I haven't managed to reproduce it but @theo-m has. We'll investigate this ! Thank you for the reply! I would be happy to follow the discussions related to the issue.
If you do not mind, could you also give a little more explanation on my p.s.2? I am having a hard time figuring out why the single processing `map` uses all of my cores.
@lhoestq @theo-m  Regarding your ps2: It depends what function you pass to `map`.
For example, fast tokenizers from `transformers` in Rust tokenize texts and parallelize the tokenization over all the cores. I am still experiencing this issue with datasets 1.9.0..
Has there been a further investigation? 
<img width="442" alt="image" src="https://user-images.githubusercontent.com/29157715/126143387-8b5ddca2-a896-4e18-abf7-4fbf62a48b41.png">

Hi, thank you for the great library.

I've been using datasets to pretrain language models, and it often involves datasets as large as ~70G.
My data preparation step is roughly two steps: `load_dataset` which splits corpora into a table of sentences, and `map` converts a sentence into a list of integers, using a tokenizer.

I noticed that `map` function with `num_proc=mp.cpu_count() //2` takes more than 20 hours to finish the job where as `num_proc=1` gets the job done in about 5 hours. The machine I used has 40 cores, with 126G of RAM. There were no other jobs when `map` function was running.

What could be the reason? I would be happy to provide information necessary to spot the reason.

p.s. I was experiencing the imbalance issue mentioned in [here](https://github.com/huggingface/datasets/issues/610#issuecomment-705177036) when I was using multi processing.
p.s.2 When I run `map` with `num_proc=1`, I see one tqdm bar but all the cores are working. When `num_proc=20`, only 20 cores work. 
![Screen Shot 2021-03-05 at 11 04 59](https://user-images.githubusercontent.com/29157715/110056895-ef6cf000-7da2-11eb-8307-6698e9fb1ad4.png)
