Slow #0 when using map to tokenize.
Hi, I have found the reason of it. Before using the map function to tokenize the data, I concatenate the wikipedia and bookcorpus first, like this:
```if args.dataset_name1 is not None:
        dataset1 = load_dataset(args.dataset_name1, args.dataset_config_name1, split="train")
        dataset1 = dataset1.remove_columns('title')
        if args.dataset_name2 is not None:
            dataset2 = load_dataset(args.dataset_name2, args.dataset_config_name2,split="train")
            assert dataset1.features.type == dataset2.features.type, str(dataset1.features.type)+';'+str(dataset2.features.type)
           datasets12 = concatenate_datasets([dataset1, dataset2], split='train')
```
When I just use one datasets, e.g. wikipedia, the problem seems no longer exist:
![image](https://user-images.githubusercontent.com/31714566/116967059-13d24380-ace4-11eb-8d14-b7b9c9a275cc.png)

Bookcorpus has more row numbers than Wikipedia, however, it takes much more time to process each batch of wiki than that of bookcorpus. When we first concatenate two datasets and then use _map_ to process the concatenated datasets,  e.g. `num_proc=5`, process 0 has to process all of the wikipedia data, causing the problem that #0 takes a longer time to finish the job. 

The problem is caused by the different characteristic of different datasets. One solution might be using _map_ first to process two datasets seperately, then concatenate the tokenized and processed datasets before input to the `Dataloader`.


Hi, _datasets_ is really amazing! I am following [run_mlm_no_trainer.py](url) to pre-train BERT, and it uses `tokenized_datasets = raw_datasets.map(
            tokenize_function,
            batched=True,
            num_proc=args.preprocessing_num_workers,
            remove_columns=column_names,
            load_from_cache_file=not args.overwrite_cache,
        )` to tokenize by multiprocessing. However, I have found that when `num_proc`>1，the process _#0_ is much slower than others.
It looks like this:
![image](https://user-images.githubusercontent.com/31714566/116665555-81246280-a9cc-11eb-8a37-6e608ab310d0.png)
It takes more than 12 hours for #0, while others just about half an hour. Could anyone tell me it is normal or not, and is there any methods to speed up it?

Slow #0 when using map to tokenize. 
 Hi, _datasets_ is really amazing! I am following [run_mlm_no_trainer.py](url) to pre-train BERT, and it uses `tokenized_datasets = raw_datasets.map(
            tokenize_function,
            batched=True,
            num_proc=args.preprocessing_num_workers,
            remove_columns=column_names,
            load_from_cache_file=not args.overwrite_cache,
        )` to tokenize by multiprocessing. However, I have found that when `num_proc`>1，the process _#0_ is much slower than others.
It looks like this:
![image](https://user-images.githubusercontent.com/31714566/116665555-81246280-a9cc-11eb-8a37-6e608ab310d0.png)
It takes more than 12 hours for #0, while others just about half an hour. Could anyone tell me it is normal or not, and is there any methods to speed up it?
 
 Hi, I have found the reason of it. Before using the map function to tokenize the data, I concatenate the wikipedia and bookcorpus first, like this:
```if args.dataset_name1 is not None:
        dataset1 = load_dataset(args.dataset_name1, args.dataset_config_name1, split="train")
        dataset1 = dataset1.remove_columns('title')
        if args.dataset_name2 is not None:
            dataset2 = load_dataset(args.dataset_name2, args.dataset_config_name2,split="train")
            assert dataset1.features.type == dataset2.features.type, str(dataset1.features.type)+';'+str(dataset2.features.type)
           datasets12 = concatenate_datasets([dataset1, dataset2], split='train')
```
When I just use one datasets, e.g. wikipedia, the problem seems no longer exist:
![image](https://user-images.githubusercontent.com/31714566/116967059-13d24380-ace4-11eb-8d14-b7b9c9a275cc.png)

Bookcorpus has more row numbers than Wikipedia, however, it takes much more time to process each batch of wiki than that of bookcorpus. When we first concatenate two datasets and then use _map_ to process the concatenated datasets,  e.g. `num_proc=5`, process 0 has to process all of the wikipedia data, causing the problem that #0 takes a longer time to finish the job. 

The problem is caused by the different characteristic of different datasets. One solution might be using _map_ first to process two datasets seperately, then concatenate the tokenized and processed datasets before input to the `Dataloader`.

