Pre-training a language model on a large dataset
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## Details
<!-- Description of your issue -->

Hi,

I'm getting a memory error when I run the example code for language modeling. I'm interested in pre-training a RoBERTa model using a 25GB text data on a virtual machine with a `v3-8` TPU on Google Cloud Platform.

I'm using the following command with `transformers/examples/xla_spawn.py` and `transformers/examples/run_language_modeling.py`.

```
python xla_spawn.py --num_cores 8 \
run_language_modeling.py \
    --output_dir=[*****] \
    --config_name=[*****] \
    --tokenizer_name=[*****] \
    --do_train \
    --per_device_train_batch_size 8 \
    --gradient_accumulation_steps 128 \
    --learning_rate 6e-4 \
    --weight_decay 0.01 \
    --adam_epsilon 1e-6 \
    --adam_beta1 0.9 \
    --adam_beta2 0.98 \
    --max_steps 500_000 \
    --warmup_steps 24_000 \
    --save_total_limit 5 \
    --save_steps=100_000 \
    --block_size=512 \
    --train_data_file=[*****] \
    --mlm \
    --line_by_line
```

When I run this, I get the following error.
```
08/20/2020 15:21:07 - INFO - transformers.data.datasets.language_modeling -   Creating features from dataset file at [*****]
Traceback (most recent call last):
  File "xla_spawn.py", line 72, in <module>
    main()
  File "xla_spawn.py", line 68, in main
    xmp.spawn(mod._mp_fn, args=(), nprocs=args.num_cores)
  File "/anaconda3/envs/torch-xla-nightly/lib/python3.6/site-packages/torch_xla/distributed/xla_multiprocessing.py", line 395, in spawn
    start_method=start_method)
  File "/anaconda3/envs/torch-xla-nightly/lib/python3.6/site-packages/torch/multiprocessing/spawn.py", line 158, in start_processes
    while not context.join():
  File "/anaconda3/envs/torch-xla-nightly/lib/python3.6/site-packages/torch/multiprocessing/spawn.py", line 108, in join
    (error_index, name)
Exception: process 0 terminated with signal SIGKILL
```

It looks like the script gets killed while it's loading the training data [here](https://github.com/huggingface/transformers/blob/573bdb0a5d2897ff6c7520ebb38693c7acfbf17e/src/transformers/data/datasets/language_modeling.py#L89-L92).


```python
with open(file_path, encoding="utf-8") as f:
    lines = [line for line in f.read().splitlines() if (len(line) > 0 and not line.isspace())]
```

When I run the above block of code separately with `transformers/examples/xla_spawn.py`, I get an error.

```
Traceback (most recent call last):
  File "xla_spawn.py", line 72, in <module>
    main()
  File "xla_spawn.py", line 68, in main
    xmp.spawn(mod._mp_fn, args=(), nprocs=args.num_cores)
  File "/anaconda3/envs/torch-xla-nightly/lib/python3.6/site-packages/torch_xla/distributed/xla_multiprocessing.py", line 395, in spawn
    start_method=start_method)
  File "/anaconda3/envs/torch-xla-nightly/lib/python3.6/site-packages/torch/multiprocessing/spawn.py", line 158, in start_processes
    while not context.join():
  File "/anaconda3/envs/torch-xla-nightly/lib/python3.6/site-packages/torch/multiprocessing/spawn.py", line 108, in join
    (error_index, name)
Exception: process 0 terminated with signal SIGKILL
```

When I run the above block of code separately using  `n1-highmem-16 (16 vCPUs, 104 GB memory)` without TPU, I still get an error.

```
Traceback (most recent call last):
  File "debug_load.py", line 7, in <module>
    lines = [line for line in f.read().splitlines() if (len(line) > 0 and not line.isspace())]
  File "/anaconda3/envs/torch-xla-nightly/lib/python3.6/codecs.py", line 321, in decode
    (result, consumed) = self._buffer_decode(data, self.errors, final)
MemoryError
```

Has anyone successfully reproduced the original RoBERTa model or pretrained a language model with a large dataset using Huggingface's transformers (with TPU)? If so, what are the specifications of your machine? Has this code (`transformers/examples/run_language_modeling.py`) tested on a large dataset?

<!-- You should first ask your question on the forum or SO, and only if
     you didn't get an answer ask it here on GitHub. -->
**A link to original question on the forum/Stack Overflow**: https://discuss.huggingface.co/t/pre-training-a-language-model-on-a-large-dataset/790
Maybe @LysandreJik can help here? <|||||>Same question...<|||||>Linked to https://github.com/huggingface/transformers/issues/6873<|||||>@go-inoue For large datasets, recommended to use mmap. I like Apache Arrow, which is also used in huggingface datasets library. Megatron LM also uses mmap, but with different implementations.<|||||>This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.
