add multi-proc in `to_json`
Thank you for working on this, @bhavitvyamalik 

10% is not solving the issue, we want 5-10x faster on a machine that has lots of resources, but limited processing time.

So let's benchmark it on an instance with many more cores, I can test with 12 on my dev box and 40 on JZ. 

Could you please share the test I could run with both versions?

Should we also test the sharded version I shared in https://github.com/huggingface/datasets/issues/2663#issue-946552273 so optionally 3 versions to test. Since I was facing `OSError: [Errno 12] Cannot allocate memory` in CircleCI tests, I've added `num_proc` option instead of always using full `cpu_count`. You can test both v1 and v2 through this branch (some redundancy needs to be removed). 

Update: I was able to convert into json which took 50% less time as compared to v1 on `ascent_kb` dataset. Will post the benchmarking script with results here. Here are the benchmarks with the current branch for both v1 and v2 (dataset: `ascent_kb`, 8.9M samples):
| batch_size | time (in sec) | time (in sec) |
|------------|---------------|---------------|
|            | num_proc = 1  | num_proc = 4  |
| 10k        | 185.56        | 170.11        |
| 50k        | 175.79        | 86.84         |
| **100k**       | 191.09        | **78.35**         |
| 125k       | 198.28        | 90.89         |

Increasing the batch size on my machine helped in making v2 around 50% faster as compared to v1. Timings may vary depending on the machine. I'm including the benchmarking script as well. CircleCI errors are unrelated (something related to `bertscore`)
```
import time
from datasets import load_dataset
import pathlib
import os
from pathlib import Path
import shutil
import gc

batch_sizes = [10_000, 50_000, 100_000, 125_000]
num_procs = [1, 4]  # change this according to your machine

SAVE_LOC = "./new_dataset.json"

for batch in batch_sizes:
    for num in num_procs:
        dataset = load_dataset("ascent_kb")

        local_start = time.time()
        ans = dataset['train'].to_json(SAVE_LOC, batch_size=batch, num_proc=num)
        local_end = time.time() - local_start

        print(f"Time taken on {num} num_proc and {batch} batch_size: ", local_end)

        # remove that dataset and its contents from cache and newly generated json
        new_json = pathlib.Path(SAVE_LOC)
        new_json.unlink()

        try:
            shutil.rmtree(os.path.join(str(Path.home()), ".cache", "huggingface"))
        except OSError as e:
            print("Error: %s - %s." % (e.filename, e.strerror))

        gc.collect()
```
This will download the dataset in every iteration and run `to_json`. I didn't do multiple iterations here for `to_json` (for a specific batch_size and num_proc) and took average time as I found that v1 got faster after 1st iteration (maybe it's caching somewhere). Since you'll be doing this operation only once, I thought it'll be better to report how both v1 and v2 performed in single iteration only. 

Important: Benchmarking script will delete the newly generated json and `~/.cache/huggingface/` after every iteration so that it doesn't end up using any cached data (just to be on a safe side) Thank you for sharing the benchmark, @bhavitvyamalik. Your results look promising.

But if I remember correctly the sharded version at https://github.com/huggingface/datasets/issues/2663#issue-946552273 was much faster. So we probably should compare to it as well? And if it's faster than at least document that manual sharding version?

-------

That's a dangerous benchmark as it'd wipe out many other HF things. Why not wipe out:
```
~/.cache/huggingface/datasets/ascent_kb/
```

Running the benchmark now. Weird, I tried to adapt your benchmark to using shards and the program no longer works. It instead quickly uses up all available RAM and hangs. Has something changed recently in `datasets`? You can try:

```
import time
from datasets import load_dataset
import pathlib
import os
from pathlib import Path
import shutil
import gc
from multiprocessing import cpu_count, Process, Queue

batch_sizes = [10_000, 50_000, 100_000, 125_000]
num_procs = [1, 8]  # change this according to your machine

DATASET_NAME = ("ascent_kb")
num_shards = [1, 8]
for batch in batch_sizes:
    for shards in num_shards:
        dataset = load_dataset(DATASET_NAME)["train"]
        #print(dataset)

        def process_shard(idx):
            print(f"Sharding {idx}")
            ds_shard = dataset.shard(shards, idx, contiguous=True)
            # ds_shard = ds_shard.shuffle() # remove contiguous=True above if shuffling
            print(f"Saving {DATASET_NAME}-{idx}.jsonl")
            ds_shard.to_json(f"{DATASET_NAME}-{idx}.jsonl", orient="records", lines=True, force_ascii=False)

        local_start = time.time()
        queue = Queue()
        processes = [Process(target=process_shard, args=(idx,)) for idx in range(shards)]
        for p in processes:
            p.start()

        for p in processes:
            p.join()
        local_end = time.time() - local_start

        print(f"Time taken on {shards} shards and {batch} batch_size: ", local_end)
```

Just careful, so that it won't crash your compute environment. As it almost crashed mine. So this part seems to no longer work:
```
        dataset = load_dataset("ascent_kb")["train"]
        ds_shard = dataset.shard(1, 0, contiguous=True)
        ds_shard.to_json("ascent_kb-0.jsonl", orient="records", lines=True, force_ascii=False)
``` If you are using `to_json` without any `num_proc`or `num_proc=1` then essentially it'll fall back to v1 only and I've kept it as it is (the tests were passing as well)

> That's a dangerous benchmark as it'd wipe out many other HF things. Why not wipe out:

That's because some dataset related files were still left inside `~/.cache/huggingface/datasets` folder. You can wipe off datasets folder inside your cache maybe

> dataset = load_dataset("ascent_kb")["train"]
> ds_shard = dataset.shard(1, 0, contiguous=True)
> ds_shard.to_json("ascent_kb-0.jsonl", orient="records", lines=True, force_ascii=False)

I tried this `lama` dataset (1.3M) and it worked fine. Trying it with `ascent_kb` currently, will update it here. I don't think the issue has anything to do with your work, @bhavitvyamalik. I forgot to mention I tested to see the same problem with the latest datasets release.

Interesting, I tried your suggestion. This:
```
python -c 'import datasets; ds="lama"; dataset = datasets.load_dataset(ds)["train"]; \
dataset.shard(1, 0, contiguous=True).to_json(f"{ds}-0.jsonl", orient="records", lines=True, force_ascii=False)'
```
works fine and takes just a few GBs to complete.

this on the other hand blows up memory-wise:
```
python -c 'import datasets; ds="ascent_kb"; dataset = datasets.load_dataset(ds)["train"]; \
dataset.shard(1, 0, contiguous=True).to_json(f"{ds}-0.jsonl", orient="records", lines=True, force_ascii=False)'
```
and I have to kill it before it uses up all RAM. (I have 128GB of it, so it should be more than enough) > That's because some dataset related files were still left inside ~/.cache/huggingface/datasets folder. You can wipe off datasets folder inside your cache maybe

I think recent datasets added a method that will print out the path for all the different components for a given dataset, I can't recall the name though. It was when we were discussing a janitor program to clear up space selectively. > and I have to kill it before it uses up all RAM. (I have 128GB of it, so it should be more than enough)

Same thing just happened on my machine too. Memory leak somewhere maybe? Even if you were to load this dataset in your memory it shouldn't take more than 4GB. You were earlier doing this for `oscar` dataset. Is it working fine for that? Hmm, looks like `datasets` has changed and won't accept my currently cached oscar-en (crashes), so I'd rather not download 0.5TB again. 

Were you able to reproduce the memory blow up with `ascent_kb`? It's should be a much quicker task to verify.

But yes, oscar worked just fine with `.shard()` which is what I used to process it fast. What I tried is:
```
HF_DATASETS_OFFLINE=1 HF_DATASETS_CACHE=cache python -c 'import datasets; ds="oscar"; \
dataset = datasets.load_dataset(ds, "unshuffled_deduplicated_en")["train"]; \
dataset.shard(1000000, 0, contiguous=True).to_json(f"{ds}-0.jsonl", orient="records", lines=True, force_ascii=False)'
```
and got:
```
Using the latest cached version of the module from /gpfswork/rech/six/commun/modules/datasets_modules/datasets/oscar/e4f06cecc7ae02f7adf85640b4019bf476d44453f251a1d84aebae28b0f8d51d (last modified on Fri Aug  6 01:52:35 2021) since it couldn't be found locally at oscar/oscar.py or remotely (OfflineModeIsEnabled).
Reusing dataset oscar (cache/oscar/unshuffled_deduplicated_en/1.0.0/e4f06cecc7ae02f7adf85640b4019bf476d44453f251a1d84aebae28b0f8d51d)
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/gpfswork/rech/six/commun/conda/stas/lib/python3.8/site-packages/datasets/load.py", line 755, in load_dataset
    ds = builder_instance.as_dataset(split=split, ignore_verifications=ignore_verifications, in_memory=keep_in_memory)
  File "/gpfswork/rech/six/commun/conda/stas/lib/python3.8/site-packages/datasets/builder.py", line 737, in as_dataset
    datasets = utils.map_nested(
  File "/gpfswork/rech/six/commun/conda/stas/lib/python3.8/site-packages/datasets/utils/py_utils.py", line 203, in map_nested
    mapped = [
  File "/gpfswork/rech/six/commun/conda/stas/lib/python3.8/site-packages/datasets/utils/py_utils.py", line 204, in <listcomp>
    _single_map_nested((function, obj, types, None, True)) for obj in tqdm(iterable, disable=disable_tqdm)
  File "/gpfswork/rech/six/commun/conda/stas/lib/python3.8/site-packages/datasets/utils/py_utils.py", line 142, in _single_map_nested
    return function(data_struct)
  File "/gpfswork/rech/six/commun/conda/stas/lib/python3.8/site-packages/datasets/builder.py", line 764, in _build_single_dataset
    ds = self._as_dataset(
  File "/gpfswork/rech/six/commun/conda/stas/lib/python3.8/site-packages/datasets/builder.py", line 834, in _as_dataset
    dataset_kwargs = ArrowReader(self._cache_dir, self.info).read(
  File "/gpfswork/rech/six/commun/conda/stas/lib/python3.8/site-packages/datasets/arrow_reader.py", line 217, in read
    return self.read_files(files=files, original_instructions=instructions, in_memory=in_memory)
  File "/gpfswork/rech/six/commun/conda/stas/lib/python3.8/site-packages/datasets/arrow_reader.py", line 238, in read_files
    pa_table = self._read_files(files, in_memory=in_memory)
  File "/gpfswork/rech/six/commun/conda/stas/lib/python3.8/site-packages/datasets/arrow_reader.py", line 173, in _read_files
    pa_table: Table = self._get_table_from_filename(f_dict, in_memory=in_memory)
  File "/gpfswork/rech/six/commun/conda/stas/lib/python3.8/site-packages/datasets/arrow_reader.py", line 308, in _get_table_from_filename
    table = ArrowReader.read_table(filename, in_memory=in_memory)
  File "/gpfswork/rech/six/commun/conda/stas/lib/python3.8/site-packages/datasets/arrow_reader.py", line 327, in read_table
    return table_cls.from_file(filename)
  File "/gpfswork/rech/six/commun/conda/stas/lib/python3.8/site-packages/datasets/table.py", line 450, in from_file
    table = _memory_mapped_arrow_table_from_file(filename)
  File "/gpfswork/rech/six/commun/conda/stas/lib/python3.8/site-packages/datasets/table.py", line 43, in _memory_mapped_arrow_table_from_file
    memory_mapped_stream = pa.memory_map(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
``` > Were you able to reproduce the memory blow up with ascent_kb? It's should be a much quicker task to verify.

Yes, this blows up memory-wise on my machine too. 

I found that a [similar error](https://discuss.huggingface.co/t/saving-memory-with-run-mlm-py-with-wikipedia-datasets/4160) was posted on the forum on 5th March. Since you already knew how much time [#2663 comment](https://github.com/huggingface/datasets/issues/2663#issue-946552273) took, can you try benchmarking v1 and v2 for now maybe until we have a fix for this memory blow up? OK, so I benchmarked using "lama" though it's too small for this kind of test, since the sharding is much slower than one thread here.

Results: https://gist.github.com/stas00/dc1597a1e245c5915cfeefa0eee6902c

So sharding does really bad there, and your json over procs is doing great!

Any suggestions to a somewhat bigger dataset, but not too big? say 10 times of lama? Looks great! I had a few questions/suggestions related to `benchmark-datasets-to_json.py`:
 
1. You have used only 10_000 and 100_000 batch size. Including more batch sizes may help you find the perfect batch size for your machine and even give you some extra speed-up. 
For eg, I found `load_dataset("cc100", lang="eu")` with batch size 125_000 took less time as compared to batch size 100_000 (71.16 sec v/s 67.26 sec) since this dataset has 2 fields only `['id', 'text']`, so that's why we can go for higher batch size here. 
 
2. Why have you used `num_procs` 1 and 4 only? 

You can use:
1. `dataset = load_dataset("cc100", lang="af")`. Even though it has only 2 fields but there are around 9.9 mil samples. (lama had around 1.3 mil samples)
2. `dataset = load_dataset("cc100", lang="eu")` -> 16 mil samples. (if you want something more than 9.9 mil)
3. `dataset = load_dataset("neural_code_search", 'search_corpus')` -> 4.7 mil samples Thank you, @bhavitvyamalik 

My apologies, at the moment I have not found time to do more benchmark with the proposed other datasets. I will try to do it later, but I don't want it to hold your PR, it's definitely a great improvement based on the benchmarks I did run! And the comparison to sharded is really just of interest to me to see if it's on par or slower.

So if other reviewers are happy, this definitely looks like a great improvement to me and addresses the request I made in the first place.

> Why have you used num_procs 1 and 4 only?

Oh, no particular reason, I was just comparing to 4 shards on my desktop. Typically it's sufficient to go from 1 to 2-4 to see whether the distributed approach is faster or not. Once hit larger numbers you often run into bottlenecks like IO, and then numbers can be less representative. I hope it makes sense. Tested it with a larger dataset (`srwac`) and memory utilisation remained constant with no swap memory used. @lhoestq should I also add test for the same? Last time I tried this, I got `OSError: [Errno 12] Cannot allocate memory` in CircleCI tests
Closes #2663. I've tried adding multiprocessing in `to_json`. Here's some benchmarking I did to compare the timings of current version (say v1) and multi-proc version (say v2). I did this with `cpu_count` 4 (2015 Macbook Air)

1. Dataset name: `ascent_kb` - 8.9M samples (all samples were used, reporting this for a single run)
v1- ~225 seconds for converting whole dataset to json
v2- ~200 seconds for converting whole dataset to json

2. Dataset name: `lama` - 1.3M samples (all samples were used, reporting this for 2 runs)
v1- ~26 seconds for converting whole dataset to json
v2- ~23.6 seconds for converting whole dataset to json

I think it's safe to say that v2 is 10% faster as compared to v1. Timings may improve further with better configuration.

The only bottleneck I feel is writing to file from the output list. If we can improve that aspect then timings may improve further. 

Let me know if any changes/improvements can be done in this @stas00, @lhoestq, @albertvillanova. @lhoestq even suggested to extend this work with other export methods as well like `csv` or `parquet`.