Tokamax

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Tokamax is a library of custom accelerator kernels, supporting both NVIDIA GPUs and Google TPUs. Tokamax provides state-of-the-art custom kernel implementations built on top of JAX and Pallas.

Tokamax also provides tooling for users to build and autotune their own custom accelerator kernels.

Status

Tokamax is still heavily under development. Incomplete features and API changes are to be expected.

We currently support the following GPU kernels:

• tokamax.dot_product_attention

• tokamax.gated_linear_unit

• tokamax.layer_norm

And the following for both GPU and TPU:

• tokamax.ragged_dot

And the following TPU kernels:

• tokamax.linear_softmax_cross_entropy_loss

Installation

The latest Tokamax PyPI release:

pip install -U tokamax

The latest bleeding edge version from Github, with no stability guarantees:

pip install git+https://github.com/openxla/tokamax.git

Using Tokamax

Consider a function containing Tokamax functions running on an H100 GPU:

import jax
import jax.numpy as jnp
import tokamax
def loss(x, scale):
  x = tokamax.layer_norm(
      x, scale=scale, offset=None, implementation="triton"
  )
  x = tokamax.dot_product_attention(x, x, x, implementation="xla_chunked")
  x = tokamax.layer_norm(x, scale=scale, offset=None, implementation=None)
  x = tokamax.dot_product_attention(x, x, x, implementation="mosaic")
  return jnp.sum(x)
f_grad = jax.jit(jax.grad(loss))

With implementation=None, Tokamax is allowed to select the best implementation for each kernel shape. It is even allowed to choose different implementations for the forward pass and gradient. It will also always be supported, as it can fall back to an XLA implementation implementation='xla'.

However, you may want to choose a specific implementation of the kernel, and fail if it is unsupported. For instance, implementation="mosaic" will try to use a Pallas:Mosaic GPU kernel if possible, and throw an exception if this is unsupported for any reason. For example, using FP64 inputs are unsupported, or older GPUs.

Evaluate the Gradient

channels, seq_len, batch_size, num_heads = (64, 2048, 32, 16)
scale = jax.random.normal(jax.random.key(0), (channels,), dtype=jnp.float32)
x = jax.random.normal(
    jax.random.key(1),
    (batch_size, seq_len, num_heads, channels),
    dtype=jnp.bfloat16,
)
out = f_grad(x, scale)

Autotuning

To get the best performance, autotune all Tokamax kernels in f_grad:

autotune_result: tokamax.AutotuningResult = tokamax.autotune(f, x, scale)

autotune_result can be used as a context-manager, using the autotuned configs for all Tokamax kernels in f_grad:

with autotune_result:
  out_autotuned = f_grad(x, scale)

To serialize and reuse the result of a potentially expensive tokamax.autotuning call:

autotune_result_json: str = autotune_result.dumps()
autotune_result = tokamax.AutotuningResult.loads(autotune_result_json)

Users can autotune their own kernels with tokamax.autotune by inheriting from the tokamax.Op class and overriding the tokamax.Op._get_autotuning_configs method to define the autotuning search-space.

Note that autotuning is fundamentally non-deterministic: measuring kernel execution times is noisy. As different configs chosen during autotuning can lead to different numerics, this is a potential source of numerical non-determinism. Serializing and reusing fixed autotuning results is a way to ensure the same numerics across sessions.

Serialization

Kernels can be serialized to StableHLO. Kernel calls are JAX custom calls, which are by default banned in jax.export, requiring the use of tokamax.DISABLE_JAX_EXPORT_CHECKS to allow all Tokamax kernels to be exported:

from jax import export
f_grad_exported = export.export(f_grad, disabled_checks=tokamax.DISABLE_JAX_EXPORT_CHECKS)(
    jax.ShapeDtypeStruct(x.shape, x.dtype),
    jax.ShapeDtypeStruct(scale.shape, scale.dtype),
)

Note that functions serialized with Tokamax kernels lose the device-independence of standard StableHLO. Tokamax makes two serialization guarantees:

1. A deserialized function serialized on a specific device will be guaranteed

1. Tokamax gives the same

Cross-Compilation and CPU Export

Tokamax kernels can be exported or cross-compiled on a CPU host:

• Explicit implementation selection (e.g. implementation="mosaic_tpu_v2")

• Alternatively, enable cross-compilation globally using

with tokamax.config.cross_compile(True):
  f_exported = export.export(
      f, disabled_checks=tokamax.DISABLE_JAX_EXPORT_CHECKS
  )(
      jax.ShapeDtypeStruct(x.shape, x.dtype),
      ...,
  )

Benchmarking

JAX Python overhead is often much larger than the actual accelerator kernel execution time. This means the usual approach of timing jax.block_until_ready(f_grad(x, scale)) won't be useful. Tokamax has utilities for only measuring accelerator execution time:

f_std, args = tokamax.standardize_function(f, kwargs={'x': x, 'scale': scale})
bench: tokamax.BenchmarkData = tokamax.benchmark(f_std, args)

There are different measurement techniques: for example, on GPU, there is the CUPTI profiler that can be specified via tokamax.benchmark(f_std, args, method='cupti'). This instruments the kernel and adds some a small overhead. The default method=None allows Tokamax to choose the method, and works for both TPU and GPU. Benchmark noise can be reduced by increasing the number of iterations:

tokamax.benchmark(f_std, args, iterations=10)

Disclaimer

This is not an official Google product.