flashoptim-jax

JAX/Optax reimplementation of FlashOptim: Optimizers for Memory Efficient Training (PyTorch original). See project page for writeup.

Fused kernels that match the speed of the standard optimizers by reducing memory > 2x for mixed precision training using Adam, SGD, or LION.

Installation

uv pip install flashoptim-jax

Usage

All optimizers share the same small interface: construct the optimizer, call init(params), then call step(params, opt_state, grads).

import jax
import jax.numpy as jnp

from flashoptim_jax import flash_adamw, flash_lion, flash_sgdw

params = {
    "w": jnp.ones((8, 8), dtype=jnp.bfloat16),
    "b": jnp.zeros((8,), dtype=jnp.float32),
}
grads = jax.tree.map(jnp.ones_like, params)

tx = flash_adamw(learning_rate=1e-3, weight_decay=1e-2)
# tx = flash_lion(learning_rate=1e-4, weight_decay=1e-2)
# tx = flash_sgdw(learning_rate=1e-2, momentum=0.9, weight_decay=1e-2)

opt_state = tx.init(params)
params, opt_state = tx.step(params, opt_state, grads)

For checkpointing, all optimizers expose state roundtrip helpers:

from flashoptim_jax import (
    flash_adamw_state_dict,
    flash_lion_state_dict,
    flash_sgd_state_dict,
    load_flash_adamw_state_dict,
    load_flash_lion_state_dict,
    load_flash_sgd_state_dict,
)

adamw_state_dict = flash_adamw_state_dict(adamw_state)
lion_state_dict = flash_lion_state_dict(lion_state)
sgd_state_dict = flash_sgd_state_dict(sgd_state)

adamw_state = load_flash_adamw_state_dict(adamw_state_dict)
lion_state = load_flash_lion_state_dict(lion_state_dict)
sgd_state = load_flash_sgd_state_dict(sgd_state_dict)

There are also helpers to save and restore reconstructed FP32 master weights. Use the same master_weight_bits value for restore that you used during training:

from flashoptim_jax import (
    flash_adamw,
    flash_adamw_state_dict,
    load_flash_adamw_state_dict,
)
from flashoptim_jax.compression import reconstruct_weights, set_fp32_params

tx = flash_adamw(learning_rate=1e-3, weight_decay=1e-2, master_weight_bits=24)
opt_state = tx.init(params)
params, opt_state = tx.step(params, opt_state, grads)

checkpoint = {
    "params_fp32": reconstruct_weights(params, opt_state.ecc),
    "opt_state": flash_adamw_state_dict(opt_state),
}

# ... save `checkpoint` with your preferred checkpoint library ...

restored_opt_state = load_flash_adamw_state_dict(checkpoint["opt_state"])
restored_params, restored_ecc = set_fp32_params(
    checkpoint["params_fp32"],
    param_template=params,
    master_weight_bits=24,
)
restored_opt_state = restored_opt_state._replace(ecc=restored_ecc)
params = restored_params

Benchmarks

End-to-end correctness

Below are training loss curves comparing the Flash optimizer versus an optax reference, for AdamW, LION, and SGD across 3 benchmarks: GPT-2 pretraining, MNIST, and ImageNet (each with its own architecture).

Note that LION on ImageNet diverges between the two: this is also the case with the original Pytorch implementation. PyTorch loss curves match these curves, ommitted for clarity.

# To re-produce runs
bash examples/run_benchmark.sh [gpt2|mnist|imagenette]

# To re-make the plot
python examples/plot_all_benchmarks_3x3.py [--jax-only]

Throughput and memory savings

The JAX kernels are slightly slower at large matrix multiplications (with 10%), but significantly faster for smaller matrices due to less kernel launch overhead (> 2x for matrices 2048x2048 and below).

# Run Torch/JAX Flash/reference on many different matrix sizes
bash examples/benchmark_square_step_all.sh

# Plot the results
python examples/plot_benchmark_step_logs.py \
    --input-dir examples/out/benchmark_step_square \
    --output benchmark_timings.png \
    --memory-output benchmark_memory.png \
    --log --flash-only

Reproducing Figure 3

To reproduce the FP32 reconstruction-error plot from the paper, run:

python examples/plot_reconstruction_error.py --output compression_comparison_jax.png

Citation

This is an independent JAX/Optax reimplementation of FlashOptim. If you use this software, please cite the original paper:

@article{gonzalezblalock2026flashoptim,
  title={FlashOptim: Optimizers for Memory Efficient Training},
  author={Gonzalez Ortiz, Jose Javier and Gupta, Abhay and Rinard, Christopher and Blalock, Davis},
  journal={arXiv preprint arXiv:2602.23349},
  year={2026}
}

Next steps

Implement Pallas TPU kernels so this is fast on TPUs :)