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Composable transformations of Python+NumPy programs: differentiate, vectorize, JIT to GPU/TPU, and more
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Composable transformations of Python+NumPy programs: differentiate, vectorize, JIT to GPU/TPU, and more

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# JAX: Autograd and XLA



![Continuous integration](https://github.com/google/jax/actions/workflows/ci-build.yaml/badge.svg)

![PyPI version](https://img.shields.io/pypi/v/jax)



[**Quickstart**](#quickstart-colab-in-the-cloud)

| [**Transformations**](#transformations)

| [**Install guide**](#installation)

| [**Neural net libraries**](#neural-network-libraries)

| [**Change logs**](https://jax.readthedocs.io/en/latest/changelog.html)

| [**Reference docs**](https://jax.readthedocs.io/en/latest/)



## What is JAX?



JAX is [Autograd](https://github.com/hips/autograd) and [XLA](https://www.tensorflow.org/xla),

brought together for high-performance numerical computing, including

large-scale machine learning research.



With its updated version of [Autograd](https://github.com/hips/autograd),

JAX can automatically differentiate native

Python and NumPy functions. It can differentiate through loops, branches,

recursion, and closures, and it can take derivatives of derivatives of

derivatives. It supports reverse-mode differentiation (a.k.a. backpropagation)

via [`grad`](#automatic-differentiation-with-grad) as well as forward-mode differentiation,

and the two can be composed arbitrarily to any order.



What’s new is that JAX uses [XLA](https://www.tensorflow.org/xla)

to compile and run your NumPy programs on GPUs and TPUs. Compilation happens

under the hood by default, with library calls getting just-in-time compiled and

executed. But JAX also lets you just-in-time compile your own Python functions

into XLA-optimized kernels using a one-function API,

[`jit`](#compilation-with-jit). Compilation and automatic differentiation can be

composed arbitrarily, so you can express sophisticated algorithms and get

maximal performance without leaving Python. You can even program multiple GPUs

or TPU cores at once using [`pmap`](#spmd-programming-with-pmap), and

differentiate through the whole thing.



Dig a little deeper, and you'll see that JAX is really an extensible system for

[composable function transformations](#transformations). Both

[`grad`](#automatic-differentiation-with-grad) and [`jit`](#compilation-with-jit)

are instances of such transformations. Others are

[`vmap`](#auto-vectorization-with-vmap) for automatic vectorization and

[`pmap`](#spmd-programming-with-pmap) for single-program multiple-data (SPMD)

parallel programming of multiple accelerators, with more to come.



This is a research project, not an official Google product. Expect bugs and

[sharp edges](https://jax.readthedocs.io/en/latest/notebooks/Common_Gotchas_in_JAX.html).

Please help by trying it out, [reporting

bugs](https://github.com/google/jax/issues), and letting us know what you

think!



```python

import jax.numpy as jnp

from jax import grad, jit, vmap



def predict(params, inputs):

  for W, b in params:

    outputs = jnp.dot(inputs, W) + b

    inputs = jnp.tanh(outputs)  # inputs to the next layer

  return outputs                # no activation on last layer



def loss(params, inputs, targets):

  preds = predict(params, inputs)

  return jnp.sum((preds - targets)**2)



grad_loss = jit(grad(loss))  # compiled gradient evaluation function

perex_grads = jit(vmap(grad_loss, in_axes=(None, 0, 0)))  # fast per-example grads

```



### Contents

* [Quickstart: Colab in the Cloud](#quickstart-colab-in-the-cloud)

* [Transformations](#transformations)

* [Current gotchas](#current-gotchas)

* [Installation](#installation)

* [Neural net libraries](#neural-network-libraries)

* [Citing JAX](#citing-jax)

* [Reference documentation](#reference-documentation)



## Quickstart: Colab in the Cloud

Jump right in using a notebook in your browser, connected to a Google Cloud GPU.

Here are some starter notebooks:

- [The basics: NumPy on accelerators, `grad` for differentiation, `jit` for compilation, and `vmap` for vectorization](https://jax.readthedocs.io/en/latest/notebooks/quickstart.html)

- [Training a Simple Neural Network, with TensorFlow Dataset Data Loading](https://colab.research.google.com/github/google/jax/blob/main/docs/notebooks/neural_network_with_tfds_data.ipynb)



**JAX now runs on Cloud TPUs.** To try out the preview, see the [Cloud TPU

Colabs](https://github.com/google/jax/tree/main/cloud_tpu_colabs).



For a deeper dive into JAX:

- [The Autodiff Cookbook, Part 1: easy and powerful automatic differentiation in JAX](https://jax.readthedocs.io/en/latest/notebooks/autodiff_cookbook.html)

- [Common gotchas and sharp edges](https://jax.readthedocs.io/en/latest/notebooks/Common_Gotchas_in_JAX.html)

- See the [full list of

notebooks](https://github.com/google/jax/tree/main/docs/notebooks).



## Transformations



At its core, JAX is an extensible system for transforming numerical functions.

Here are four transformations of primary interest: `grad`, `jit`, `vmap`, and

`pmap`.



### Automatic differentiation with `grad`



JAX has roughly the same API as [Autograd](https://github.com/hips/autograd).

The most popular function is

[`grad`](https://jax.readthedocs.io/en/latest/jax.html#jax.grad)

for reverse-mode gradients:



```python

from jax import grad

import jax.numpy as jnp



def tanh(x):  # Define a function

  y = jnp.exp(-2.0 * x)

  return (1.0 - y) / (1.0 + y)



grad_tanh = grad(tanh)  # Obtain its gradient function

print(grad_tanh(1.0))   # Evaluate it at x = 1.0

# prints 0.4199743

```



You can differentiate to any order with `grad`.



```python

print(grad(grad(grad(tanh)))(1.0))

# prints 0.62162673

```



For more advanced autodiff, you can use

[`jax.vjp`](https://jax.readthedocs.io/en/latest/jax.html#jax.vjp) for

reverse-mode vector-Jacobian products and

[`jax.jvp`](https://jax.readthedocs.io/en/latest/jax.html#jax.jvp) for

forward-mode Jacobian-vector products. The two can be composed arbitrarily with

one another, and with other JAX transformations. Here's one way to compose those

to make a function that efficiently computes [full Hessian

matrices](https://jax.readthedocs.io/en/latest/_autosummary/jax.hessian.html#jax.hessian):



```python

from jax import jit, jacfwd, jacrev



def hessian(fun):

  return jit(jacfwd(jacrev(fun)))

```



As with [Autograd](https://github.com/hips/autograd), you're free to use

differentiation with Python control structures:



```python

def abs_val(x):

  if x > 0:

    return x

  else:

    return -x



abs_val_grad = grad(abs_val)

print(abs_val_grad(1.0))   # prints 1.0

print(abs_val_grad(-1.0))  # prints -1.0 (abs_val is re-evaluated)

```



See the [reference docs on automatic

differentiation](https://jax.readthedocs.io/en/latest/jax.html#automatic-differentiation)

and the [JAX Autodiff

Cookbook](https://jax.readthedocs.io/en/latest/notebooks/autodiff_cookbook.html)

for more.



### Compilation with `jit`



You can use XLA to compile your functions end-to-end with

[`jit`](https://jax.readthedocs.io/en/latest/jax.html#just-in-time-compilation-jit),

used either as an `@jit` decorator or as a higher-order function.



```python

import jax.numpy as jnp

from jax import jit



def slow_f(x):

  # Element-wise ops see a large benefit from fusion

  return x * x + x * 2.0



x = jnp.ones((5000, 5000))

fast_f = jit(slow_f)

%timeit -n10 -r3 fast_f(x)  # ~ 4.5 ms / loop on Titan X

%timeit -n10 -r3 slow_f(x)  # ~ 14.5 ms / loop (also on GPU via JAX)

```



You can mix `jit` and `grad` and any other JAX transformation however you like.



Using `jit` puts constraints on the kind of Python control flow

the function can use; see

the [Gotchas

Notebook](https://jax.readthedocs.io/en/latest/notebooks/Common_Gotchas_in_JAX.html#python-control-flow-+-JIT)

for more.



### Auto-vectorization with `vmap`



[`vmap`](https://jax.readthedocs.io/en/latest/jax.html#vectorization-vmap) is

the vectorizing map.

It has the familiar semantics of mapping a function along array axes, but

instead of keeping the loop on the outside, it pushes the loop down into a

function’s primitive operations for better performance.



Using `vmap` can save you from having to carry around batch dimensions in your

code. For example, consider this simple *unbatched* neural network prediction

function:



```python

def predict(params, input_vec):

  assert input_vec.ndim == 1

  activations = input_vec

  for W, b in params:

    outputs = jnp.dot(W, activations) + b  # `activations` on the right-hand side!

    activations = jnp.tanh(outputs)        # inputs to the next layer

  return outputs                           # no activation on last layer

```



We often instead write `jnp.dot(activations, W)` to allow for a batch dimension on the

left side of `activations`, but we’ve written this particular prediction function to

apply only to single input vectors. If we wanted to apply this function to a

batch of inputs at once, semantically we could just write



```python

from functools import partial

predictions = jnp.stack(list(map(partial(predict, params), input_batch)))

```



But pushing one example through the network at a time would be slow! It’s better

to vectorize the computation, so that at every layer we’re doing matrix-matrix

multiplication rather than matrix-vector multiplication.



The `vmap` function does that transformation for us. That is, if we write



```python

from jax import vmap

predictions = vmap(partial(predict, params))(input_batch)

# or, alternatively

predictions = vmap(predict, in_axes=(None, 0))(params, input_batch)

```



then the `vmap` function will push the outer loop inside the function, and our

machine will end up executing matrix-matrix multiplications exactly as if we’d

done the batching by hand.



It’s easy enough to manually batch a simple neural network without `vmap`, but

in other cases manual vectorization can be impractical or impossible. Take the

problem of efficiently computing per-example gradients: that is, for a fixed set

of parameters, we want to compute the gradient of our loss function evaluated

separately at each example in a batch. With `vmap`, it’s easy:



```python

per_example_gradients = vmap(partial(grad(loss), params))(inputs, targets)

```



Of course, `vmap` can be arbitrarily composed with `jit`, `grad`, and any other

JAX transformation! We use `vmap` with both forward- and reverse-mode automatic

differentiation for fast Jacobian and Hessian matrix calculations in

`jax.jacfwd`, `jax.jacrev`, and `jax.hessian`.



### SPMD programming with `pmap`



For parallel programming of multiple accelerators, like multiple GPUs, use

[`pmap`](https://jax.readthedocs.io/en/latest/jax.html#parallelization-pmap).

With `pmap` you write single-program multiple-data (SPMD) programs, including

fast parallel collective communication operations. Applying `pmap` will mean

that the function you write is compiled by XLA (similarly to `jit`), then

replicated and executed in parallel across devices.



Here's an example on an 8-GPU machine:



```python

from jax import random, pmap

import jax.numpy as jnp



# Create 8 random 5000 x 6000 matrices, one per GPU

keys = random.split(random.PRNGKey(0), 8)

mats = pmap(lambda key: random.normal(key, (5000, 6000)))(keys)



# Run a local matmul on each device in parallel (no data transfer)

result = pmap(lambda x: jnp.dot(x, x.T))(mats)  # result.shape is (8, 5000, 5000)



# Compute the mean on each device in parallel and print the result

print(pmap(jnp.mean)(result))

# prints [1.1566595 1.1805978 ... 1.2321935 1.2015157]

```



In addition to expressing pure maps, you can use fast [collective communication

operations](https://jax.readthedocs.io/en/latest/jax.lax.html#parallel-operators)

between devices:



```python

from functools import partial

from jax import lax



@partial(pmap, axis_name='i')

def normalize(x):

  return x / lax.psum(x, 'i')



print(normalize(jnp.arange(4.)))

# prints [0.         0.16666667 0.33333334 0.5       ]

```



You can even [nest `pmap` functions](https://colab.research.google.com/github/google/jax/blob/main/cloud_tpu_colabs/Pmap_Cookbook.ipynb#scrollTo=MdRscR5MONuN) for more

sophisticated communication patterns.



It all composes, so you're free to differentiate through parallel computations:



```python

from jax import grad



@pmap

def f(x):

  y = jnp.sin(x)

  @pmap

  def g(z):

    return jnp.cos(z) * jnp.tan(y.sum()) * jnp.tanh(x).sum()

  return grad(lambda w: jnp.sum(g(w)))(x)



print(f(x))

# [[ 0.        , -0.7170853 ],

#  [-3.1085174 , -0.4824318 ],

#  [10.366636  , 13.135289  ],

#  [ 0.22163185, -0.52112055]]



print(grad(lambda x: jnp.sum(f(x)))(x))

# [[ -3.2369726,  -1.6356447],

#  [  4.7572474,  11.606951 ],

#  [-98.524414 ,  42.76499  ],

#  [ -1.6007166,  -1.2568436]]

```



When reverse-mode differentiating a `pmap` function (e.g. with `grad`), the

backward pass of the computation is parallelized just like the forward pass.



See the [SPMD

Cookbook](https://colab.research.google.com/github/google/jax/blob/main/cloud_tpu_colabs/Pmap_Cookbook.ipynb)

and the [SPMD MNIST classifier from scratch

example](https://github.com/google/jax/blob/main/examples/spmd_mnist_classifier_fromscratch.py)

for more.



## Current gotchas



For a more thorough survey of current gotchas, with examples and explanations,

we highly recommend reading the [Gotchas

Notebook](https://jax.readthedocs.io/en/latest/notebooks/Common_Gotchas_in_JAX.html).

Some standouts:



1. JAX transformations only work on [pure functions](https://en.wikipedia.org/wiki/Pure_function), which don't have side-effects and respect [referential transparency](https://en.wikipedia.org/wiki/Referential_transparency) (i.e. object identity testing with `is` isn't preserved). If you use a JAX transformation on an impure Python function, you might see an error like `Exception: Can't lift Traced...`  or `Exception: Different traces at same level`.

1. [In-place mutating updates of

   arrays](https://jax.readthedocs.io/en/latest/notebooks/Common_Gotchas_in_JAX.html#in-place-updates), like `x[i] += y`, aren't supported, but [there are functional alternatives](https://jax.readthedocs.io/en/latest/jax.ops.html). Under a `jit`, those functional alternatives will reuse buffers in-place automatically.

1. [Random numbers are

   different](https://jax.readthedocs.io/en/latest/notebooks/Common_Gotchas_in_JAX.html#random-numbers), but for [good reasons](https://github.com/google/jax/blob/main/docs/jep/263-prng.md).

1. If you're looking for [convolution

   operators](https://jax.readthedocs.io/en/latest/notebooks/convolutions.html),

   they're in the `jax.lax` package.

1. JAX enforces single-precision (32-bit, e.g. `float32`) values by default, and

   [to enable

   double-precision](https://jax.readthedocs.io/en/latest/notebooks/Common_Gotchas_in_JAX.html#double-64bit-precision)

   (64-bit, e.g. `float64`) one needs to set the `jax_enable_x64` variable at

   startup (or set the environment variable `JAX_ENABLE_X64=True`).

   On TPU, JAX uses 32-bit values by default for everything _except_ internal

   temporary variables in 'matmul-like' operations, such as `jax.numpy.dot` and `lax.conv`.

   Those ops have a `precision` parameter which can be used to simulate

   true 32-bit, with a cost of possibly slower runtime.

1. Some of NumPy's dtype promotion semantics involving a mix of Python scalars

   and NumPy types aren't preserved, namely `np.add(1, np.array([2],

   np.float32)).dtype` is `float64` rather than `float32`.

1. Some transformations, like `jit`, [constrain how you can use Python control

   flow](https://jax.readthedocs.io/en/latest/notebooks/Common_Gotchas_in_JAX.html#control-flow).

   You'll always get loud errors if something goes wrong. You might have to use

   [`jit`'s `static_argnums`

   parameter](https://jax.readthedocs.io/en/latest/jax.html#just-in-time-compilation-jit),

   [structured control flow

   primitives](https://jax.readthedocs.io/en/latest/jax.lax.html#control-flow-operators)

   like

   [`lax.scan`](https://jax.readthedocs.io/en/latest/_autosummary/jax.lax.scan.html#jax.lax.scan),

   or just use `jit` on smaller subfunctions.



## Installation



### Supported platforms



|            | Linux x86_64 | Linux aarch64 | Mac x86_64   | Mac ARM      | Windows x86_64 | Windows WSL2 x86_64 |

|------------|--------------|---------------|--------------|--------------|----------------|---------------------|

| CPU        | yes          | yes           | yes          | yes          | yes            | yes                 |

| NVIDIA GPU | yes          | yes           | no           | n/a          | no             | experimental        |

| Google TPU | yes          | n/a           | n/a          | n/a          | n/a            | n/a                 |

| AMD GPU    | experimental | no            | no           | n/a          | no             | no                  |

| Apple GPU  | n/a          | no            | experimental | experimental | n/a            | n/a                 |



### Instructions



| Hardware   | Instructions                                                                                                    |

|------------|-----------------------------------------------------------------------------------------------------------------|

| CPU        | `pip install -U "jax[cpu]"`                                                                                       |

| NVIDIA GPU on x86_64 | `pip install -U "jax[cuda12_pip]" -f https://storage.googleapis.com/jax-releases/jax_cuda_releases.html`        |

| Google TPU | `pip install -U "jax[tpu]" -f https://storage.googleapis.com/jax-releases/libtpu_releases.html`                 |

| AMD GPU    | Use [Docker](https://hub.docker.com/r/rocm/jax) or [build from source](https://jax.readthedocs.io/en/latest/developer.html#additional-notes-for-building-a-rocm-jaxlib-for-amd-gpus). |

| Apple GPU  | Follow [Apple's instructions](https://developer.apple.com/metal/jax/).                                          |



See [the documentation](https://jax.readthedocs.io/en/latest/installation.html)

for information on alternative installation strategies. These include compiling

from source, installing with Docker, using other versions of CUDA, a

community-supported conda build, and answers to some frequently-asked questions.



## Neural network libraries



Multiple Google research groups develop and share libraries for training neural

networks in JAX. If you want a fully featured library for neural network

training with examples and how-to guides, try

[Flax](https://github.com/google/flax).



Google X maintains the neural network library

[Equinox](https://github.com/patrick-kidger/equinox). This is used as the

foundation for several other libraries in the JAX ecosystem.



In addition, DeepMind has open-sourced an [ecosystem of libraries around

JAX](https://deepmind.com/blog/article/using-jax-to-accelerate-our-research)

including [Optax](https://github.com/deepmind/optax) for gradient processing and

optimization, [RLax](https://github.com/deepmind/rlax) for RL algorithms, and

[chex](https://github.com/deepmind/chex) for reliable code and testing. (Watch

the NeurIPS 2020 JAX Ecosystem at DeepMind talk

[here](https://www.youtube.com/watch?v=iDxJxIyzSiM))



## Citing JAX



To cite this repository:



```

@software{jax2018github,

  author = {James Bradbury and Roy Frostig and Peter Hawkins and Matthew James Johnson and Chris Leary and Dougal Maclaurin and George Necula and Adam Paszke and Jake Vander{P}las and Skye Wanderman-{M}ilne and Qiao Zhang},

  title = {{JAX}: composable transformations of {P}ython+{N}um{P}y programs},

  url = {http://github.com/google/jax},

  version = {0.3.13},

  year = {2018},

}

```



In the above bibtex entry, names are in alphabetical order, the version number

is intended to be that from [jax/version.py](../main/jax/version.py), and

the year corresponds to the project's open-source release.



A nascent version of JAX, supporting only automatic differentiation and

compilation to XLA, was described in a [paper that appeared at SysML

2018](https://mlsys.org/Conferences/2019/doc/2018/146.pdf). We're currently working on

covering JAX's ideas and capabilities in a more comprehensive and up-to-date

paper.



## Reference documentation



For details about the JAX API, see the

[reference documentation](https://jax.readthedocs.io/).



For getting started as a JAX developer, see the

[developer documentation](https://jax.readthedocs.io/en/latest/developer.html).