flash-attn-triton 0.1.1

Triton-based backend for Flash Attention 2

Flash Attention Triton

This repository provides a wrapper for the Triton implementation of the Flash Attention algorithm with a Flash Attention 2 compatible API. It allows for a drop-in replacement of the original Flash Attention 2 package for supported functionality. This package provides support for Turing (eg. 2080 Ti, T4) GPUs not supported by the original FA2 CUDA package.

Installation

You can install the package directly from GitHub:

pip install git+https://github.com/rationalism/flash-attn-triton.git

Or from PyPI:

pip install flash-attn-triton

Requirements

• PyTorch 2.6 or later
• Triton 3.2 or later
• CUDA-compatible GPU (compute capability 7.5+)

Usage

The API is designed to be compatible with Flash Attention 2. You can use it in the same way:

from flash_attn_triton import flash_attn_func, flash_attn_qkvpacked_func, FlashAttention

# Basic usage
out = flash_attn_func(q, k, v, causal=True)

# Packed QKV
out = flash_attn_qkvpacked_func(qkv, causal=True)

# Module interface
flash_attn = FlashAttention()
out = flash_attn(q, k, v, causal=True)

Currently Supported Features

• Basic attention mechanism (forward and backward)
• FP16 and BF16 (BF16 only on Ampere and above)
• Causal masking
• Softmax scaling
• Basic MQA/GQA support (via tensor repetition)
• Head dims 16, 32, 64, 128
• Ampere, Turing cards

Limitations

This implementation does not currently support:

• Non-causal attention for sequence lengths not divisible by 128
• Dropout (in progress)
• Volta, Pascal, and earlier cards (in progress)
• varlen/unpadded support
• Attention bias
• Sliding window attention
• ALiBi
• KV caching with in-place updates
• Softcapping
• Deterministic backward pass

Benchmarks

RTX 3090 (Ampere)

fused-attention-batch4-head32-d64-fwd-causal=True-dropout=0.0:
     N_CTX  Triton [FP16]
0   1024.0      48.049147
1   2048.0      61.062769
2   4096.0      68.363188
3   8192.0      70.768167
4  16384.0      72.332634
fused-attention-batch4-head32-d64-fwd-causal=False-dropout=0.0:
     N_CTX  Triton [FP16]
0   1024.0      60.190653
1   2048.0      71.126662
2   4096.0      69.049310
3   8192.0      74.579215
4  16384.0      73.911621
fused-attention-batch4-head32-d64-bwd-causal=True-dropout=0.0:
     N_CTX  Triton [FP16]
0   1024.0      33.531732
1   2048.0      40.884683
2   4096.0      45.627974
3   8192.0      47.449394
4  16384.0      48.993511
fused-attention-batch4-head32-d64-bwd-causal=False-dropout=0.0:
     N_CTX  Triton [FP16]
0   1024.0      42.834959
1   2048.0      46.382862
2   4096.0      49.984253
3   8192.0      51.358497
4  16384.0      49.913040

RTX 2080 Ti (Turing)

fused-attention-batch4-head32-d64-fwd-causal=True-dropout=0.0:
     N_CTX  Triton [FP16]
0   1024.0      29.258471
1   2048.0      41.382117
2   4096.0      46.972266
3   8192.0      49.315714
4  16384.0      50.443531
fused-attention-batch4-head32-d64-fwd-causal=False-dropout=0.0:
     N_CTX  Triton [FP16]
0   1024.0      38.110175
1   2048.0      47.640577
2   4096.0      50.301599
3   8192.0      51.136501
4  16384.0      51.826783
fused-attention-batch4-head32-d64-bwd-causal=True-dropout=0.0:
     N_CTX  Triton [FP16]
0   1024.0      22.085938
1   2048.0      26.173398
2   4096.0      28.565586
3   8192.0      30.030201
4  16384.0      31.082861
fused-attention-batch4-head32-d64-bwd-causal=False-dropout=0.0:
     N_CTX  Triton [FP16]
0   1024.0      27.756566
1   2048.0      30.274265
2   4096.0      31.471025
3   8192.0      32.253811
4  16384.0      32.614130

Acknowledgements

This implementation is based on the Triton attention implementation from the original Flash Attention 2 repository by TriDao and the Triton tutorial on fused attention.

License

This project is licensed under the BSD 3-Clause License - see the LICENSE file for details.