Issue: OSError: Memory mapping file failed: Cannot allocate memory

Hi,
I am trying to run a code with a wikipedia dataset, here is the command to reproduce the error. You can find the codes for run_mlm.py in huggingface repo here: https://github.com/huggingface/transformers/blob/v4.3.2/examples/language-modeling/run_mlm.py 
```
python run_mlm.py --model_name_or_path bert-base-multilingual-cased --dataset_name wikipedia --dataset_config_name 20200501.en --do_train --do_eval --output_dir /dara/test  --max_seq_length 128
```

I am using transformer version: 4.3.2 

But I got memory erorr using this dataset, is there a way I could save on memory with dataset library with wikipedia dataset?
Specially I need to train a model with multiple of wikipedia datasets concatenated. thank you very much @lhoestq  for your help and suggestions:

```
  File "run_mlm.py", line 441, in <module>
    main()
  File "run_mlm.py", line 233, in main
    split=f"train[{data_args.validation_split_percentage}%:]",
  File "/dara/libs/anaconda3/envs/code/lib/python3.7/site-packages/datasets-1.3.0-py3.7.egg/datasets/load.py", line 750, in load_dataset
    ds = builder_instance.as_dataset(split=split, ignore_verifications=ignore_verifications, in_memory=keep_in_memory)
  File "/dara/libs/anaconda3/envs/code/lib/python3.7/site-packages/datasets-1.3.0-py3.7.egg/datasets/builder.py", line 740, in as_dataset
    map_tuple=True,
  File "/dara/libs/anaconda3/envs/code/lib/python3.7/site-packages/datasets-1.3.0-py3.7.egg/datasets/utils/py_utils.py", line 225, in map_nested
    return function(data_struct)
  File "/dara/libs/anaconda3/envs/code/lib/python3.7/site-packages/datasets-1.3.0-py3.7.egg/datasets/builder.py", line 757, in _build_single_dataset
    in_memory=in_memory,
  File "/dara/libs/anaconda3/envs/code/lib/python3.7/site-packages/datasets-1.3.0-py3.7.egg/datasets/builder.py", line 829, in _as_dataset
    in_memory=in_memory,
  File "/dara/libs/anaconda3/envs/code/lib/python3.7/site-packages/datasets-1.3.0-py3.7.egg/datasets/arrow_reader.py", line 215, in read
    return self.read_files(files=files, original_instructions=instructions, in_memory=in_memory)
  File "/dara/libs/anaconda3/envs/code/lib/python3.7/site-packages/datasets-1.3.0-py3.7.egg/datasets/arrow_reader.py", line 236, in read_files
    pa_table = self._read_files(files, in_memory=in_memory)
  File "/dara/libs/anaconda3/envs/code/lib/python3.7/site-packages/datasets-1.3.0-py3.7.egg/datasets/arrow_reader.py", line 171, in _read_files
    pa_table: pa.Table = self._get_dataset_from_filename(f_dict, in_memory=in_memory)
  File "/dara/libs/anaconda3/envs/code/lib/python3.7/site-packages/datasets-1.3.0-py3.7.egg/datasets/arrow_reader.py", line 302, in _get_dataset_from_filename
    pa_table = ArrowReader.read_table(filename, in_memory=in_memory)
  File "/dara/libs/anaconda3/envs/code/lib/python3.7/site-packages/datasets-1.3.0-py3.7.egg/datasets/arrow_reader.py", line 322, in read_table
    stream = stream_from(filename)
  File "pyarrow/io.pxi", line 782, in pyarrow.lib.memory_map
  File "pyarrow/io.pxi", line 743, in pyarrow.lib.MemoryMappedFile._open
  File "pyarrow/error.pxi", line 122, in pyarrow.lib.pyarrow_internal_check_status
  File "pyarrow/error.pxi", line 99, in pyarrow.lib.check_status
OSError: Memory mapping file failed: Cannot allocate memory
```




Comments:
Do you think this is trying to bring the dataset into memory and if I can avoid it to save on memory so it only brings a batch into memory? @lhoestq  thank you

It's not trying to bring the dataset into memory.

Actually, it's trying to memory map the dataset file, which is different. It allows to load large dataset files without filling up memory.

What dataset did you use to get this error ?
On what OS are you running ? What's your python and pyarrow version ?

Dear @lhoestq 
thank you so much for coming back to me. Please find info below:
1) Dataset name: I used  wikipedia with config 20200501.en
2) I got these pyarrow in my environment:
pyarrow                   2.0.0                     <pip>
pyarrow                   3.0.0                     <pip>

3) python version 3.7.10
4) OS version 

lsb_release -a
No LSB modules are available.
Distributor ID:	Debian
Description:	Debian GNU/Linux 10 (buster)
Release:	10
Codename:	buster


Is there a way I could solve the memory issue and if I could run this model, I am using  GeForce GTX 108, 
thanks 


I noticed that the error happens when loading the validation dataset.
What value of `data_args.validation_split_percentage` did you use ?

Dear @lhoestq 

thank you very much for the very sharp observation, indeed, this happens there, I use the default value of 5, I basically plan to subsample a part of the large  dataset and choose it as validation set. Do you think this is bringing the data into memory during subsampling? Is there a way I could avoid this?

Thank you very much for the great help.


On Mon, Mar 8, 2021 at 11:28 AM Quentin Lhoest ***@***.***>
wrote:

> I noticed that the error happens when loading the validation dataset.
> What value of data_args.validation_split_percentage did you use ?
>
> —
> You are receiving this because you authored the thread.
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Methods like `dataset.shard`, `dataset.train_test_split`, `dataset.select` etc. don't bring the dataset in memory. 
The only time when samples are brought to memory is when you access elements via `dataset[0]`, `dataset[:10]`, `dataset["my_column_names"]`.

But it's possible that trying to use those methods to build your validation set doesn't fix the issue since, if I understand correctly, the error happens when when the dataset arrow file is opened (just before the 5% percentage is applied).

Did you try to reproduce this issue in a google colab ? This would be super helpful to investigate why this happened.

Also maybe you can try clearing your cache at `~/.cache/huggingface/datasets` and try again. If the arrow file was corrupted somehow, removing it and rebuilding may fix the issue.