`datasets.map` multi processing much slower than single processing 
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.
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)

`datasets.map` multi processing much slower than single processing  
 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)
 
 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.