View Source EXLA (EXLA v0.7.0)

Google's XLA (Accelerated Linear Algebra) compiler/backend for Nx.

It supports just-in-time (JIT) compilation to GPU (both CUDA and ROCm) and TPUs.

XLA binaries

EXLA relies on the XLA package to provide the necessary XLA binaries. Whenever possible it tries to download precompiled builds, but you may need to build from source if there is no version matching your target environment. For more details, including GPU/TPU support see the usage section.

Configuration

EXLA ships with a backend to store tensors and run computations on. Generally speaking, the backend is enabled globally in your config/config.exs (or config/ENV.exs) with the following:

import Config
config :nx, :default_backend, EXLA.Backend

In a script/notebook, you would do:

Mix.install([
  {:exla, "~> 0.2"}
])

Nx.global_default_backend(EXLA.Backend)

From now on, all created tensors will be allocated directly on the given EXLA.Backend. You can use functions such as Nx.backend_transfer/2 to explicitly transfer tensors.

EXLA will pick an available client to allocate and compute tensors, in this order: :cuda, :rocm, :tpu, and :host (CPU). See the "Clients" section below for more information.

To use GPUs/TPUs, you must also set the appropriate value for the XLA_TARGET environment variable. If you have GPU/TPU enabled, we recommend setting the environment variable for your machine altogether. For CUDA, setting ELIXIR_ERL_OPTIONS="+sssdio 128" is also required on more complex operations to increase CUDA's compiler stack size.

Note that setting the EXLA.Backend does not enable the EXLA compiler. You must still pass the compiler: EXLA option to Nx.Defn functions or call the functions in this module.

Options

The options accepted by EXLA backend/compiler are:

:client - an atom representing the client to use. The default client is chosen on this order: :cuda, :rocm, :tpu, and :host.
:device_id - the default device id to run the computation on. Defaults to the :default_device_id on the client
:precision - control the tradeoff between speed and accuracy for array computations on accelerator backends (i.e. TPU and GPU). It must be one of:
- :default - Fastest mode, but least accurate. Performs computations in bfloat16
- :high - Slower but more accurate. Performs float32 computations in 3 bfloat16 passes, or using tensorfloat32 where available
- :highest - Slowest but most accurate. Performs computations in float32 or float64 as applicable
:compiler_mode - the mode to use for the compiler. It must be one of:
- :mlir - The default mode. Uses MLIR to compile the computation
- :xla - Legacy implementation. Uses XLA to compile the computation

The :compiler_mode can also be set globally through a specific fallback config:

  config :exla, :compiler_mode, :mlir

Clients

The EXLA library uses a client for compiling and executing code. Those clients are typically bound to a platform, such as CPU or GPU.

Those clients are singleton resources on Google's XLA library, therefore they are treated as a singleton resource on this library too. EXLA ships with runtime client configuration for each supported platform:

config :exla, :clients,
  cuda: [platform: :cuda],
  rocm: [platform: :rocm],
  tpu: [platform: :tpu],
  host: [platform: :host]

In a script/notebook, you can set those after Mix.install/2, but before any tensor operation is performed:

Application.put_env(:exla, :clients,
  cuda: [platform: :cuda],
  rocm: [platform: :rocm],
  tpu: [platform: :tpu],
  host: [platform: :host]
)

You can provide your own list of clients, replacing the list above or configuring each client as listed below. You can also specify :default_client to set a particular client by default or :preferred_clients to change the order of clients preference, but those configurations are rarely set in practice.

Important! you should avoid using multiple clients for the same platform. If you have multiple clients per platform, they can race each other and fight for resources, such as memory. Therefore, we recommend developers to stick with the default clients above.

Client options

Each client configuration accepts the following options:

:platform - the platform the client runs on. It can be :host (CPU), :cuda, :rocm, or :tpu. Defaults to :host.
:default_device_id - the default device ID to run on. For example, if you have two GPUs, you can choose a different one as the default. Defaults to device 0 (the first device).
:preallocate- if the memory should be preallocated on GPU devices. Defaults to true.
:memory_fraction - how much memory of a GPU device to allocate. Defaults to 0.9.

Memory preallocation

XLA preallocates memory in GPU devices. This means that, if you are to run multiple notebooks or multiple instances of your application, the second, third, and so on instances won't be able to allocate memory.

You can disable this behaviour by setting preallocate: false on the client configuration, as specified above. You may also use :memory_fraction to control how much is preallocated.

GPU Runtime Issues

GPU Executions run in dirty IO threads, which have a considerable smaller stack size than regular scheduler threads. This may lead to problems with certain CUDA or cuDNN versions, leading to segmentation fails. In a development environment, it is suggested to set:

ELIXIR_ERL_OPTIONS="+sssdio 128"

To increase the stack size of dirty IO threads from 40 kilowords to 128 kilowords. In a release, you can set this flag in your vm.args.

Docker considerations

EXLA should run fine on Docker with one important consideration: you must not start the Erlang VM as the root process in Docker. That's because when the Erlang VM runs as root, it has to manage all child programs.

At the same time, Google XLA's shells out to child programs and must retain control over how child programs terminate.

To address this, simply make sure you wrap the Erlang VM in another process, such as the shell one. In other words, if you are using releases, instead of this:

CMD path/to/release start

do this:

CMD sh -c "path/to/release start"

If you are using Mix inside your Docker containers, instead of this:

CMD mix run

do this:

CMD sh -c "mix run"

Alternatively, you can pass the --init flag to docker run, so it runs an init inside the container that forwards signals and reaps processes.

The --init flag uses the tini project, so for cases where the flag may not available (e.g. kubernetes) you may want to install it.

Telemetry events

EXLA executes a telemetry event every time a function is JIT-compiled. The events are named [:exla, :compilation] and include the following measurements, given in microseconds:

:eval_time - the time spent on turning the function into XLA computation
:compile_time - the time spent on compiling the XLA computation into an executable
:total_time - the sum of :eval_time and :compile_time

The metadata is:

:key - the compilation key for debugging

Summary

Functions

cached?(function, args, options \\ [])

Checks if the compilation of function with args is cached.

compile(function, args, options \\ [])

A shortcut for Nx.Defn.compile/3 with the EXLA compiler.

jit(function, options \\ [])

A shortcut for Nx.Defn.jit/2 with the EXLA compiler.

jit_apply(function, args, options \\ [])

A shortcut for Nx.Defn.jit_apply/3 with the EXLA compiler.

stream(function, args, options \\ [])

Starts streaming the given anonymous function with just-in-time compilation.

stream_cached?(function, args, options \\ [])

Checks if the JIT compilation of stream with args is cached.

Functions

cached?(function, args, options \\ [])

Checks if the compilation of function with args is cached.

Note that hooks are part of the cache, and therefore they must be included in the options.

Examples

iex> fun = fn a, b -> Nx.add(a, b) end
iex> left = Nx.tensor(1, type: {:u, 8})
iex> right = Nx.tensor([1, 2, 3], type: {:u, 16})
iex> EXLA.jit(fun).(left, right)
iex> EXLA.cached?(fun, [left, right])
true
iex> EXLA.cached?(fun, [left, Nx.tensor([1, 2, 3, 4], type: {:u, 16})])
false

Compiled functions are also cached, unless cache is set to false:

iex> fun = fn a, b -> Nx.subtract(a, b) end
iex> left = Nx.tensor(1, type: {:u, 8})
iex> right = Nx.tensor([1, 2, 3], type: {:u, 16})
iex> EXLA.compile(fun, [left, right], cache: false)
iex> EXLA.cached?(fun, [left, right])
false
iex> EXLA.compile(fun, [left, right])
iex> EXLA.cached?(fun, [left, right])
true

compile(function, args, options \\ [])

A shortcut for Nx.Defn.compile/3 with the EXLA compiler.

iex> fun = EXLA.compile(&Nx.add(&1, &1), [Nx.template({3}, {:s, 64})])
iex> fun.(Nx.tensor([1, 2, 3]))
#Nx.Tensor<
  s64[3]
  [2, 4, 6]
>

Results are allocated on the EXLA.Backend. Note that the EXLA.Backend is asynchronous: operations on its tensors may return immediately, before the tensor data is available. The backend will then block only when trying to read the data or when passing it to another operation.

Options

It accepts the same option as Nx.Defn.compile/3 plus:

:debug - print compile and debugging information, defaults to false.
:cache - cache the results of compilation, defaults to true. You can set it to false if you plan to compile the function only once and store the compile contents somewhere.
:client - an atom representing the client to use. The default client is chosen on this order: :cuda, :rocm, :tpu, and :host.
:device_id - the default device id to run the computation on. Defaults to the :default_device_id on the client

jit(function, options \\ [])

A shortcut for Nx.Defn.jit/2 with the EXLA compiler.

iex> EXLA.jit(&Nx.add(&1, &1)).(Nx.tensor([1, 2, 3]))
#Nx.Tensor<
  s64[3]
  [2, 4, 6]
>

Options

It accepts the same option as Nx.Defn.jit/2 plus:

:cache - cache the results of compilation, defaults to true.
:client - an atom representing the client to use. The default client is chosen on this order: :cuda, :rocm, :tpu, and :host.
:debug - print compile and debugging information, defaults to false.
:device_id - the default device id to run the computation on. Defaults to the :default_device_id on the client
:lazy_transfers - when :always, it lazily transfers data to the device instead of upfront. This is useful to reduce memory allocation on GPU/TPU devices at the cost of increased latency. It is recommended to only enable this if the input tensors are allocated on host and the computation is running on GPU/TPU with a limited amount of memory

jit_apply(function, args, options \\ [])

A shortcut for Nx.Defn.jit_apply/3 with the EXLA compiler.

iex> EXLA.jit_apply(&Nx.add(&1, &1), [Nx.tensor([1, 2, 3])])
#Nx.Tensor<
  s64[3]
  [2, 4, 6]
>

See jit/2 for supported options.

stream(function, args, options \\ [])

Starts streaming the given anonymous function with just-in-time compilation.

At least two arguments are expected:

The first argument is a tensor template of the data to be streamed in
The second argument is a tensor with the stream initial state

The streaming function must return a two element tuple, the first element is the data to be sent and the second is the accumulator.

For each streamed chunk, you must call Nx.Stream.send/2 and Nx.Stream.recv/1. You don't need to call recv immediately after send, but doing so can be a useful mechanism to provide backpressure. Once all chunks are sent, you must use Nx.Stream.done/1 to receive the accumulated result. Let's see an example:

defmodule Streamed do
  import Nx.Defn

  defn sum(tensor, acc) do
    {acc, tensor + acc}
  end
end

Now let's invoke it:

stream = EXLA.stream(&Streamed.sum/2, [Nx.template({}, {:s, 64}), 0])

for i <- 1..5 do
  Nx.Stream.send(stream, i)
  IO.inspect {:chunk, Nx.Stream.recv(stream)}
end

IO.inspect {:result, Nx.Stream.done(stream)}

It will print:

{:chunk, 0}
{:chunk, 1}
{:chunk, 2}
{:chunk, 3}
{:chunk, 4}
{:result, 5}

Note: While any process can call Nx.Stream.send/2, EXLA expects the process that starts the streaming to be the one calling Nx.Stream.recv/1 and Nx.Stream.done/1.

See jit/2 for supported options.

stream_cached?(function, args, options \\ [])

Checks if the JIT compilation of stream with args is cached.

Note that hooks are part of the cache, and therefore they must be included in the options.

Examples

iex> left = Nx.tensor(1, type: {:u, 8})
iex> right = Nx.tensor([1, 2, 3], type: {:u, 16})
iex> fun = fn x, acc -> {acc, Nx.add(x, acc)} end
iex> stream = EXLA.stream(fun, [left, right])
iex> Nx.Stream.done(stream)
iex> EXLA.stream_cached?(fun, [left, right])
true
iex> EXLA.stream_cached?(fun, [left, Nx.tensor([1, 2, 3, 4], type: {:u, 16})])
false