# Why DataFlow?¶

There are many other data loading solutions for deep learning. Here we explain why you may want to use Tensorpack DataFlow: it’s easy, and fast (enough).

## How Fast Do You Actually Need?¶

Your data pipeline only needs to be fast enough.

In practice, you should always first make sure your data pipeline runs asynchronously with your training. The method to do so is different in each training framework, and in tensorpack this is automatically done by the InputSource interface.

Once the data pipeline runs async with your training, the data pipeline only needs to be as fast as the training. Getting faster brings no gains to overall throughput. It only needs to be fast enough.

If you have used other data loading libraries, you may doubt how easy it is to make data pipeline fast enough with pure Python. Python is hard to parallelize, but the DataFlow library has included best practices to squeeze the most performance out of Python.

For example: if you train a ResNet-50 on ImageNet, DataFlow is fast enough for you unless you use 8 V100s with both FP16 and XLA enabled, which most people don’t. For tasks that are less data-hungry (e.g., object detection, or most NLP tasks), DataFlow is already an overkill. See the Efficient DataFlow tutorial on how to build a fast Python loader with DataFlow.

There is no reason to try a more complicated solution, when you don’t know whether a simple Python solution like DataFlow is fast enough.

## Which Data Format?¶

Certain libraries advocate for a new binary data format (e.g., TFRecords, RecordIO). We think you usually do not need them, because these formats are:

1. Not Easy: To use the new binary format, you need to process data from its original format to this new format. Then you read data from this format to training workers. It’s a waste of effort: the intermediate format does not have to exist.

2. Not Easy: Even when you do need to use an intermediate format that’s different from your original data format (for performance reasons, for example), there are many formats you can choose from.

Why use a special binary format when you could use something else? A different format may bring you:

• Easier visualization.

• Interoperability with other libraries.

• More functionalities.

Different formats have their strength and weakness in the above aspects. Forcing a single binary format on users is not ideal. We should let users make the choice.

3. Not Necessarily Faster: Having an intermediate format can be useful for performance reasons, for example to apply some one-shot expensive preprocessing to your dataset. But other formats are probably equally fast.

Formats like TFRecords and RecordIO are just as fast as your disk, and of course, as fast as other similar libraries. Decades of engineering in dataset systems have provided many other competitive formats like LMDB, HDF5 that are:

• Equally fast

• More generic (not tied to your training framework)

• With more features (e.g. random access)

The only unique benefit a format like TFRecords or RecordIO may give you, is the native integration with the training framework, which may bring a small performance gain.

On the other hand, DataFlow is:

1. Easy: Any Python function that produces data can be made a DataFlow and used for training. No need for a fixed format.

2. Flexible: Since it is in pure Python, any data format can be used. When needed, you can still easily serialize your dataflow to a single-file format with a few lines of code.

Some frameworks have also provided good framework-specific solutions for data loading. On the contrary, DataFlow is framework-agnostic: you can use it in any Python environment. In addition to this advantage, there are other reasons you might prefer DataFlow over the alternatives:

### tf.data and other graph operations¶

tf.data loads data using computation graph. Generally speaking, a computation graph system is useful for the following reasons:

• Automatic differentiation

• Run the computation on different devices

• Serialize the description of computation

• Automatic performance optimization

These are important for training neural networks, but less for data loading: Autodiff is never needed. The others are useful, but often not needed or not so effective for data loading.

Unlike running a neural network model, data processing is a complicated and poorly-structured task. You need to handle different formats, handle corner cases, noisy data, combination of data. These require conditional operations, loops, data structures, even exception handling. These operations are naturally difficult for a symbolic graph. As a result, users are asking for many “trivial” TF data loading features that could’ve been achieved with 3 lines of code in Python:

The flexibility issue of tf.data disappears only when the data is originally very clean and well-formatted. If not, you may feel like writing a Python script to reformat your data, but then you’re almost writing a DataFlow (a DataFlow can be made from a Python iterator)!

In the design, torch.utils.data.Dataset is simply a Python container/iterator, similar to DataFlow. However it has made some bad assumptions: it assumes your dataset supports __getitem__, which does not work when you have a dynamic/unreliable data source, or when you need to filter your data on the fly.

torch.utils.data.DataLoader is quite good, despite that it also makes some bad assumptions on batching and is not always efficient:

1. torch.utils.data.DataLoader assumes that:

1. You do batch training

2. You use a constant batch size

3. Indices are sufficient to determine which samples to batch together

4. Multiprocessing is a better parallelization strategy than multithreading.

They are often true, but not necessarily so.

2. Its multiprocessing implementation is efficient on torch.Tensor, but inefficient for generic data type or numpy arrays.

PyTorch starts to improve on bad assumptions (e.g., with IterableDataset). But the interface still bears the history of these assumptions. On the other hand, DataFlow:

1. Is an iterator, not necessarily has a length or can be indexed. This is more generic.

2. Does not assume batches, and allow you to implement different batching logic easily.

3. Is optimized for generic data type and numpy arrays.

Note

An iterator interface is more generic than __getitem__?

DataFlow’s iterator interface can perfectly simulate the behavior of indexing interface like this:

# A dataflow which produces indices, like [0], [1], [2], ...
# The indices can be either sequential, or more fancy, akin to torch.utils.data.Sampler.
df = SomeIndexGenerator()
# Map the indices to datapoints by __getitem__.
df = MapData(df, lambda idx: dataset[idx[0]])