My name is Jiri.

I'm a software engineer on the TensorFlow team.

And today, I'm going to be talking to you about tf.data

and tf.distribute, which are TensorFlow's APIs for input

pipeline and distribution strategy, respectively.

To set the stage for what I'm going to be talking about,

let's think about what are the basic building

blocks for a machine learning workflow?

Machine learning operates over data.

It runs some computation.

And it uses some sort of hardware to do this task.

This hardware can either be a single CPU on your laptop.

Or possibly it can be on your workstation that

has either one or multiple accelerators,

either GPUs or TPUs, attached to it.

But you can also run the computation

across a large number of machines

that each have one or multiple accelerators attached to it.

Now, let's talk about the how the machine learning building

blocks are being served or reflected in the APIs

that TensorFlow provides.

So for the data handling part of the machine learning task,

TensorFlow provides a tf.data API.

It's the input pipeline API for TensorFlow.

For the computation itself, such as supervised learning,

TensorFlow offers a number of different both high level

and low level APIs.

You might be familiar with Keras or Estimators--

they've been mentioned in earlier talks today--

as well as lower level APIs for building custom training loops.

And finally, to hide the hardware details

of your computation, TensorFlow provides a tf.distribute API,

which allows you to create your input pipeline

and model in a way that's agnostic to the environment

in which it's going to execute.

So kind of thinking that your program is

going to run, perhaps, on a single device,

and with minimal changes being able to deploy it

on a large set of different devices, a possibility

of different machine learning architectures.

In this talk, I'm going to talk about the tf.data input

pipeline API.

And then, in the second part, I'm

also going to talk about the tf.distribute, the distribution

strategy API.

I'm not going to talk about Keras, and Estimator,

and other APIs for the modeling itself,

as that has been covered in previous talks.

So without further ado, let's get

started with tf.data, which is TensorFlow input pipeline API.

So let's ask ourselves a question.

Why do we need an input pipeline API in the first place?

Why don't we just load the data in memory,

maybe in our Python program as a non py array,

and pass it into a Keras model?

Well, there is actually a number of good reasons

why we need an API or why using one will benefit us.

First of all, the data might not fit into memory.

For example, the ImageNet data set

is 140 gigabytes of data, which do not necessarily

fit into memory on every laptop or workstation.

The data itself might also require randomized

preprocessing, which means that we cannot preprocess everything

ahead of time offline and then have the data to be ready

for training.

We actually need to have an input pipeline that

performs the preprocessing, such as, in the case of ImageNet,

perhaps image cropping or randomized image distortions

or transformations on the fly as we're

running dimensional learning computation.

Having an input pipeline API as an abstraction might also

allow us to, in the runtime of this API,

implement things in a way that allows the computation

to efficiently utilize the underlying hardware.

And I'm actually going to spend a fair amount of the first part

of my talk talking about how to efficiently utilize

the hardware through the tf.data input pipeline abstraction.

Last, but not least, which is something that ties the tf.data

API to the tf.distribution API, using an input pipeline API

abstraction allows us to decouple

the task of loading and preprocessing of the data

from the task of distributing the computation.

We are using the abstraction, which

allows you to create your input pipeline assuming

it's going to run on one place.

And then the distribution strategy

will somehow distribute the data without you

having to worry about the fact that the input pipeline might

actually be evaluated in multiple places in parallel.

So for those reasons, we created tf.data, TensorFlow's input

pipeline API.

And the way I like to think about tf.data is an input

pipeline API's created through tf.data--

it's an ETL process.

What I mean by that is the E, T, and L

stand for different parts of the input pipeline stages.

E stands for Extract.

This is the stage in which we read the data,

either from a memory or local or remote storage.

And we possibly parse the file format

that the data is stored in.

Perhaps it's compressed.

Then the T, the Transform stage, in this stage,

we perform either domain specific or domain

agnostic transformations.

So the domain specific transformations

are specific to the type of data we're dealing with.

So, for instance, text vectorization,

image transformation, or temporal video sampling

are examples of domain specific transformations.

While domain agnostic transformations

include things like shuffling of your data

during training or batching.

That is combining multiple elements

into a single higher dimensional element.

And, finally, the last stage of the input pipeline, Loading,

pertains to efficiently transferring

the data onto the accelerator, which is either a GPU or TPU.

What I should point out here is that, traditionally, the input

pipeline portion of your machine learning computation

happens on a CPU.

Because some of the operations are naturally

only possible on the CPU, which leaves the GPU and TPU

resources available for your machine

learning specific computations, such as your map models.

This makes-- this puts an extra pressure

on the efficiency with which the input pipeline performs.

And the reason for that is--

which is what I'm trying to illustrate here

with the graph--

is that over time the rate at which CPU performs

has plateaued, while the computational power of GPUs

and TPUs, thanks to recent hardware advances,

continues to accelerate at an exponential rate, which

opens up this performance gap between a raw CPU

and GPU/TPU processing power available in a single machine.

And that can-- the consequence of this

could be that the CPU part of your machine learning

computation, namely the input pipeline,

can be a bottleneck of your computation.

So it's really important that the CPU input pipeline performs

as efficiently as it can.

So let's take a look at an example of what a tf.data-based

input pipeline actually looks like.

Here, I'm using an example for a common image,

or how a common image processing input pipeline would look like.

We're first creating a data set using the TFRecordDataset

operation.

It's a data set constructor that takes a set of file names

or a set of file patterns and produces

elements that are stored in those files

in a sequence-like manner.

And once you create a data set, you

can chain transformations onto the data set,

thus creating new types of data sets.

A very common one and very powerful

one is the map transformation, which

allows you to apply an arbitrary processing on the elements

of the data set.

And this preprocessing can be expressed

as a function that ends up being traced

using the mechanisms available in TensorFlow,

meaning this function that is being used to transform

elements of the data set is executed as a data flow

graph, which has important implications

for the performance and how the runtime can actually

execute this function.

And the last thing that I illustrate here

is the batch transformation, which

combines multiple elements of the input data set

and produces a single element as an output that

has a higher dimension, which is a common practice for training

efficiency.

Now one thing that's not illustrated here,

but it actually does happen under the hoods inside

of tf.data runtime is that for certain combinations

of transformations, a. tf.data provides more efficient

fused implementations.

For instance, if a map transformation is followed

by a batch transformation, we actually have a highly

efficient C++ based implementation

for the combination of the two that can give you up to 2x

speed up in the performance of your input pipeline.

And that happens kind of magically behind the scenes.

And the important bit that I want to highlight here

is that the user doesn't need to worry about it.

The user here doesn't really need

to do anything with respect to optimizing the performance.

They focus on creating an input pipeline with the functional

preprocessing in mind.

And once you create the data set that you would like,

you can pass it into TensorFlow high level API such as Keras

or Estimator, which all support data set abstraction

as an input for the data.

So let's talk a bit more about the input pipeline performance.

If you were to implement the input pipeline

in naive fashion using CPU for the input pipeline processing

or data preparation and the GPU and TPU for the training

computation, you might end up in a situation

like is illustrated on the slide where

at any given point in time you're

only utilizing one of two resources available to you.

And you could probably tell that this seems rather inefficient.

Well, a common technique that can

be used to make this style of computation more efficient

is called software pipeline.

And the idea is that while you're

working on the current element for training step

on a GPU and a TPU, you're already

started preprocessing data for the next training

step on a CPU.

And thus, you overlap the computation

that happens on the two devices or two

resources available to you.

To achieve that, the effect of software pipelining in tf.data

is pretty straight forward.

All you do is you chain a .prefetch transformation

to a particular point in your input pipeline.

And the effect of doing that will

be that the producer of the data up to that point

will be decoupled from the consumer of the data,

in this case, the Keras model.

And the two will be operating independently,

coordinating through an internal buffer.

And this will have the desired effect of software

pipelining that I illustrated in the previous slide.

Another opportunity for improving

the performance of your input pipeline

is to parallelize the transformation.

So the top part of this diagram illustrates

that we're using sequential processing for applying the map

transformation of the individual elements of the batch

that we are then going to create.

But there is no reason that you need

to do that unless there would, in effect, be some sort of data

or control dependency.

But commonly, there is not.

An in that case, you can parallelize

and overlap the preprocessing of all the individual elements

for which we're going to create the batch out of.

So let's take a look at how we would

do that using the tf.data API.

And similar to the software pipelining idea,

this is pretty straightforward.