Flash Attention | Hermes Skill

Speed up long-sequence transformer training and inference.

Skill metadata


Source	Optional — install with `hermes skills install official/mlops/flash-attention`
Path	`optional-skills/mlops/flash-attention`
Version	`1.0.1`
Author	Orchestra Research
License	MIT
Dependencies	`flash-attn`, `torch`, `transformers`
Platforms	linux, macos
Tags	`Optimization`, `Flash Attention`, `Attention Optimization`, `Memory Efficiency`, `Speed Optimization`, `Long Context`, `PyTorch`, `SDPA`, `H100`, `FP8`, `Transformers`

Reference: full SKILL.md

info

The following is the complete skill definition that Hermes loads when this skill is triggered. This is what the agent sees as instructions when the skill is active.

Flash Attention - Fast Memory-Efficient Attention

Quick start

Flash Attention provides 2-4x speedup and 10-20x memory reduction for transformer attention through IO-aware tiling and recomputation.

PyTorch native (easiest, PyTorch 2.2+):

import torch
import torch.nn.functional as F

q = torch.randn(2, 8, 512, 64, device='cuda', dtype=torch.float16)  # [batch, heads, seq, dim]
k = torch.randn(2, 8, 512, 64, device='cuda', dtype=torch.float16)
v = torch.randn(2, 8, 512, 64, device='cuda', dtype=torch.float16)

# Automatically uses Flash Attention if available
out = F.scaled_dot_product_attention(q, k, v)

flash-attn library (more features):

pip install flash-attn --no-build-isolation

from flash_attn import flash_attn_func

# q, k, v: [batch, seqlen, nheads, headdim]
out = flash_attn_func(q, k, v, dropout_p=0.0, causal=True)

Common workflows

Workflow 1: Enable in existing PyTorch model

Copy this checklist:

Flash Attention Integration:
- [ ] Step 1: Check PyTorch version (≥2.2)
- [ ] Step 2: Enable Flash Attention backend
- [ ] Step 3: Verify speedup with profiling
- [ ] Step 4: Test accuracy matches baseline

Step 1: Check PyTorch version

python -c "import torch; print(torch.__version__)"
# Should be ≥2.2.0

If <2.2, upgrade:

pip install --upgrade torch

Step 2: Enable Flash Attention backend

Replace standard attention:

# Before (standard attention)
attn_weights = torch.softmax(q @ k.transpose(-2, -1) / math.sqrt(d_k), dim=-1)
out = attn_weights @ v

# After (Flash Attention)
import torch.nn.functional as F
out = F.scaled_dot_product_attention(q, k, v, attn_mask=mask)

Force Flash Attention backend (torch.backends.cuda.sdp_kernel is deprecated; use torch.nn.attention.sdpa_kernel with SDPBackend):

from torch.nn.attention import SDPBackend, sdpa_kernel

with sdpa_kernel(SDPBackend.FLASH_ATTENTION):
    out = F.scaled_dot_product_attention(q, k, v)

Step 3: Verify speedup with profiling

import torch.utils.benchmark as benchmark

def test_attention(use_flash):
    q, k, v = [torch.randn(2, 8, 2048, 64, device='cuda', dtype=torch.float16) for _ in range(3)]

    if use_flash:
        from torch.nn.attention import SDPBackend, sdpa_kernel
        with sdpa_kernel(SDPBackend.FLASH_ATTENTION):
            return F.scaled_dot_product_attention(q, k, v)
    else:
        attn = (q @ k.transpose(-2, -1) / 8.0).softmax(dim=-1)
        return attn @ v

# Benchmark
t_flash = benchmark.Timer(stmt='test_attention(True)', globals=globals())
t_standard = benchmark.Timer(stmt='test_attention(False)', globals=globals())

print(f"Flash: {t_flash.timeit(100).mean:.3f}s")
print(f"Standard: {t_standard.timeit(100).mean:.3f}s")

Expected: 2-4x speedup for sequences >512 tokens.

Step 4: Test accuracy matches baseline

# Compare outputs
q, k, v = [torch.randn(1, 8, 512, 64, device='cuda', dtype=torch.float16) for _ in range(3)]

# Flash Attention
out_flash = F.scaled_dot_product_attention(q, k, v)

# Standard attention
attn_weights = torch.softmax(q @ k.transpose(-2, -1) / 8.0, dim=-1)
out_standard = attn_weights @ v

# Check difference
diff = (out_flash - out_standard).abs().max()
print(f"Max difference: {diff:.6f}")
# Should be <1e-3 for float16

Workflow 2: Use flash-attn library for advanced features

For multi-query attention, sliding window, or H100 FP8.

Copy this checklist:

flash-attn Library Setup:
- [ ] Step 1: Install flash-attn library
- [ ] Step 2: Modify attention code
- [ ] Step 3: Enable advanced features
- [ ] Step 4: Benchmark performance

Step 1: Install flash-attn library

# NVIDIA GPUs (CUDA 12.0+)
pip install flash-attn --no-build-isolation

# Verify installation
python -c "from flash_attn import flash_attn_func; print('Success')"

Step 2: Modify attention code

from flash_attn import flash_attn_func

# Input: [batch_size, seq_len, num_heads, head_dim]
# Transpose from [batch, heads, seq, dim] if needed
q = q.transpose(1, 2)  # [batch, seq, heads, dim]
k = k.transpose(1, 2)
v = v.transpose(1, 2)

out = flash_attn_func(
    q, k, v,
    dropout_p=0.1,
    causal=True,  # For autoregressive models
    window_size=(-1, -1),  # No sliding window
    softmax_scale=None  # Auto-scale
)

out = out.transpose(1, 2)  # Back to [batch, heads, seq, dim]

Step 3: Enable advanced features

Multi-query attention (shared K/V across heads):

from flash_attn import flash_attn_func

# q: [batch, seq, num_q_heads, dim]
# k, v: [batch, seq, num_kv_heads, dim]  # Fewer KV heads
out = flash_attn_func(q, k, v)  # Automatically handles MQA

Sliding window attention (local attention):

# Only attend to window of 256 tokens before/after
out = flash_attn_func(
    q, k, v,
    window_size=(256, 256),  # (left, right) window
    causal=True
)

Step 4: Benchmark performance

import torch
from flash_attn import flash_attn_func
import time

q, k, v = [torch.randn(4, 4096, 32, 64, device='cuda', dtype=torch.float16) for _ in range(3)]

# Warmup
for _ in range(10):
    _ = flash_attn_func(q, k, v)

# Benchmark
torch.cuda.synchronize()
start = time.time()
for _ in range(100):
    out = flash_attn_func(q, k, v)
    torch.cuda.synchronize()
end = time.time()

print(f"Time per iteration: {(end-start)/100*1000:.2f}ms")
print(f"Memory allocated: {torch.cuda.max_memory_allocated()/1e9:.2f}GB")

Workflow 3: H100 FP8 optimization (FlashAttention-3)

For maximum performance on Hopper GPUs (H100).

Important: The pip package flash-attn (2.8.x) ships FlashAttention-2 only — it does not contain FA3 or FP8 H100 kernels, and flash_attn_func does not auto-use FP8. FlashAttention-3 is a separate beta build compiled from source from the repo's hopper/ directory, exposed via the flash_attn_interface module. FA3 supports FP16/BF16 forward+backward and FP8 forward only.

FP8 Setup:
- [ ] Step 1: Verify Hopper (H100) GPU available
- [ ] Step 2: Build & install FlashAttention-3 from source (hopper/)
- [ ] Step 3: Use the FA3 interface (FP8 forward)

Step 1: Verify H100 GPU

nvidia-smi --query-gpu=name --format=csv
# Should show "H100" or "H800"

Step 2: Build & install FlashAttention-3 from source

FA3 is NOT included in pip install flash-attn. Build it from the hopper/ subdirectory:

git clone https://github.com/Dao-AILab/flash-attention.git
cd flash-attention/hopper
python setup.py install
# (compilation is heavy and requires a CUDA toolchain + Hopper GPU)

Step 3: Use the FA3 interface (FP8 forward)

FA3 exposes its own module flash_attn_interface (distinct from the FA2 flash_attn). FP8 is a forward-only path and expects float8_e4m3fn inputs:

import torch
from flash_attn_interface import flash_attn_func  # FA3 (hopper build), not `flash_attn`

# q, k, v: [batch, seqlen, nheads, headdim]
q = torch.randn(2, 4096, 32, 64, device='cuda', dtype=torch.float16)
k = torch.randn(2, 4096, 32, 64, device='cuda', dtype=torch.float16)
v = torch.randn(2, 4096, 32, 64, device='cuda', dtype=torch.float16)

# FP8 forward (inference / forward-only): cast to float8_e4m3fn
q_fp8 = q.to(torch.float8_e4m3fn)
k_fp8 = k.to(torch.float8_e4m3fn)
v_fp8 = v.to(torch.float8_e4m3fn)

out = flash_attn_func(q_fp8, k_fp8, v_fp8, causal=True)
# FP16/BF16 forward+backward is also supported by the FA3 interface.

When to use vs alternatives

Use Flash Attention when:

Training transformers with sequences >512 tokens
Running inference with long context (>2K tokens)
GPU memory constrained (OOM with standard attention)
Need 2-4x speedup without accuracy loss
Using PyTorch 2.2+ or can install flash-attn

Use alternatives instead:

Standard attention: Sequences <256 tokens (overhead not worth it)
xFormers: Need more attention variants (not just speed)
Memory-efficient attention: CPU inference (Flash Attention needs GPU)

Common issues

Issue: ImportError: cannot import flash_attn

Install with no-build-isolation flag:

pip install flash-attn --no-build-isolation

Or install CUDA toolkit first:

conda install cuda -c nvidia
pip install flash-attn --no-build-isolation

Issue: Slower than expected (no speedup)

Flash Attention benefits increase with sequence length:

<512 tokens: Minimal speedup (10-20%)
512-2K tokens: 2-3x speedup
2K tokens: 3-4x speedup

Check sequence length is sufficient.

Issue: RuntimeError: CUDA error

Verify GPU supports Flash Attention:

import torch
print(torch.cuda.get_device_capability())
# Should be ≥(7, 5) for Turing+

Flash Attention requires:

Ampere (A100, A10): ✅ Full support
Turing (T4): ✅ Supported
Volta (V100): ❌ Not supported

Issue: Accuracy degradation

Check dtype is float16 or bfloat16 (not float32):

q = q.to(torch.float16)  # Or torch.bfloat16

Flash Attention uses float16/bfloat16 for speed. Float32 not supported.

Advanced topics

Integration with HuggingFace Transformers: See references/transformers-integration.md for enabling Flash Attention in BERT, GPT, Llama models.

Performance benchmarks: See references/benchmarks.md for detailed speed and memory comparisons across GPUs and sequence lengths.

Hardware requirements

GPU: NVIDIA Ampere+ (A100, A10, A30) or AMD MI200+
VRAM: Same as standard attention (Flash Attention doesn't increase memory)
CUDA: 12.0+ (11.8 minimum)
PyTorch: 2.2+ for native support

Not supported: V100 (Volta), CPU inference

Resources

Paper: "FlashAttention: Fast and Memory-Efficient Exact Attention with IO-Awareness" (NeurIPS 2022)
Paper: "FlashAttention-2: Faster Attention with Better Parallelism and Work Partitioning" (ICLR 2024)
Blog: https://tridao.me/blog/2024/flash3/
GitHub: https://github.com/Dao-AILab/flash-attention
PyTorch docs: https://pytorch.org/docs/stable/generated/torch.nn.functional.scaled_dot_product_attention.html