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c13737d | 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 | # Datasets 🤝 Arrow
## What is Arrow?
[Arrow](https://arrow.apache.org/) enables large amounts of data to be processed and moved quickly. It is a specific data format that stores data in a columnar memory layout. This provides several significant advantages:
* Arrow's standard format allows [zero-copy reads](https://en.wikipedia.org/wiki/Zero-copy) which removes virtually all serialization overhead.
* Arrow is language-agnostic so it supports different programming languages.
* Arrow is column-oriented so it is faster at querying and processing slices or columns of data.
* Arrow allows for copy-free hand-offs to standard machine learning tools such as NumPy, Pandas, PyTorch, and TensorFlow.
* Arrow supports many, possibly nested, column types.
## Memory-mapping
🤗 Datasets uses Arrow for its local caching system. It allows datasets to be backed by an on-disk cache, which is memory-mapped for fast lookup.
This architecture allows for large datasets to be used on machines with relatively small device memory.
For example, loading the full English Wikipedia dataset only takes a few MB of RAM:
```python
>>> import os; import psutil; import timeit
>>> from datasets import load_dataset
# Process.memory_info is expressed in bytes, so convert to megabytes
>>> mem_before = psutil.Process(os.getpid()).memory_info().rss / (1024 * 1024)
>>> wiki = load_dataset("wikipedia", "20220301.en", split="train")
>>> mem_after = psutil.Process(os.getpid()).memory_info().rss / (1024 * 1024)
>>> print(f"RAM memory used: {(mem_after - mem_before)} MB")
RAM memory used: 50 MB
```
This is possible because the Arrow data is actually memory-mapped from disk, and not loaded in memory.
Memory-mapping allows access to data on disk, and leverages virtual memory capabilities for fast lookups.
## Performance
Iterating over a memory-mapped dataset using Arrow is fast. Iterating over Wikipedia on a laptop gives you speeds of 1-3 Gbit/s:
```python
>>> s = """batch_size = 1000
... for batch in wiki.iter(batch_size):
... ...
... """
>>> elapsed_time = timeit.timeit(stmt=s, number=1, globals=globals())
>>> print(f"Time to iterate over the {wiki.dataset_size >> 30} GB dataset: {elapsed_time:.1f} sec, "
... f"ie. {float(wiki.dataset_size >> 27)/elapsed_time:.1f} Gb/s")
Time to iterate over the 18 GB dataset: 31.8 sec, ie. 4.8 Gb/s
```
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