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jax-flash-attn2

A flexible and efficient implementation of Flash Attention 2.0 for JAX, supporting multiple backends (GPU/TPU/CPU) and platforms (Triton/Pallas/JAX).

Features

  • 🚀 Multiple backend support: GPU, TPU, and CPU
  • 🔧 Multiple platform implementations: Triton, Pallas, and JAX
  • ⚡ Efficient caching of attention instances
  • 🔄 Support for Grouped Query Attention (GQA) and headdims up to 256.
  • 📊 JAX sharding-friendly implementation
  • 🎯 Automatic platform selection based on backend
  • 🧩 Compatible with existing JAX mesh patterns

Installation

pip install jax-flash-attn2

Quick Start

from jax_flash_attn2 import get_cached_flash_attention

# Get a cached attention instance
attention = get_cached_flash_attention(
	backend="gpu", # 'gpu', 'tpu', or 'cpu'
	platform="triton", # 'triton', 'pallas', or 'jax'
	blocksize_q=64, # BLOCK SIZE Q
	blocksize_k=128, # BLOCK SIZE K
	softmax_scale=headdim ** -0.5 # Optional scaling factor
)

# Use with your tensors
outputs = attention(
	query=query_states,
	key=key_states,
	value=value_states,
	bias=attention_bias, # Optional
)

Usage with JAX Sharding

with mesh:
	attention_outputs = get_cached_flash_attention(
		backend="gpu",
		platform="triton",
		blocksize_q=128,
		blocksize_k=128,
		softmax_scale=None,
	)(
		query=with_sharding_constraint(query_states, qps).astype(dtype),
		key=with_sharding_constraint(key_states, kps).astype(dtype),
		value=with_sharding_constraint(value_states, vps).astype(dtype),
		bias=with_sharding_constraint(bias, bps).astype(dtype),
	)

📊 Benchmarks

Supported Configurations

Backends

  • gpu: CUDA/AMD-capable GPUs
  • tpu: Google Cloud TPUs
  • cpu: CPU fallback

Platforms

  • triton: Optimized for NVIDIA/AMD GPUs
  • pallas: Optimized for TPUs and supported on GPUs
  • jax: Universal fallback, supports all backends

Valid Backend-Platform Combinations

Backend Supported Platforms
GPU - AMD/NVIDIA Triton, JAX
GPU - NVIDIA Triton, Pallas, JAX
TPU Pallas, JAX
CPU JAX

Advanced Configuration

Custom Block Sizes

attention = get_cached_flash_attention(
    backend="gpu",
    platform="triton",
    blocksize_q=128,    # Customize query block size # Ignored for Triton
    blocksize_k=128,    # Customize key block size Ignored for Triton
    softmax_scale=1.0,  # Custom softmax scaling
)

Environment Variables

  • "GPU_IDX_FLASH_ATTN" to define GPU INDEX force for computing triton attention
  • "CACHE_TRITON_KERNELS" whenever to cache triton kernels (defualt true)
  • "_JAX_TRITON_DUMP_DIR" path to save triton kernels
  • "BLOCKSIZE_M_FLASH_ATTN" block size q seq length for backward
  • "BLOCKSIZE_N_FLASH_ATTN" block size kv seq length for backward

Performance Tips

  1. Platform Selection:

    • For NVIDIA GPUs: prefer triton
    • For TPUs: prefer pallas
    • For CPU or fallback: use jax

  2. Caching: The get_cached_flash_attention function automatically caches instances based on parameters. No need to manage caching manually.

Requirements

  • JAX
  • einops
  • chex
  • jax.experimental.pallas (for TPU support)
  • triton (for GPU optimized implementation)

Limitations

  • Triton platform is only available on NVIDIA GPUs.
  • Some platform-backend combinations are not supported (see table above).
  • Custom attention masks are not yet supported (use bias instead).

Contributing

Contributions are welcome! Please feel free to submit a Pull Request.

Citation

If you use this implementation in your research, please cite:

@software{jax_flash_attn2,
    title = {JAX Flash Attention 2.0},
    year = {2024},
    url = {https://github.com/erfanzar/jax-flash-attn2}
}

refrence citations

@inproceedings{dao2022flashattention,
  title={Flash{A}ttention: Fast and Memory-Efficient Exact Attention with {IO}-Awareness},
  author={Dao, Tri and Fu, Daniel Y. and Ermon, Stefano and Rudra, Atri and R{\'e}, Christopher},
  booktitle={Advances in Neural Information Processing Systems (NeurIPS)},
  year={2022}
}
@inproceedings{dao2023flashattention2,
  title={Flash{A}ttention-2: Faster Attention with Better Parallelism and Work Partitioning},
  author={Dao, Tri},
  booktitle={International Conference on Learning Representations (ICLR)},
  year={2024}
}

Acknowledgments And Refrences

  1. This implementation (MHA) is based on:

  1. Custom Triton Uses JAX-Triton

  2. All of kernels are copied from EasyDeL