Optimizes transformer attention with Flash Attention for 2-4x speedup and 10-20x memory reduction. Use when training/running transformers with long sequences (>512 tokens), encountering GPU memory issues with attention, or need faster inference. Supports PyTorch native SDPA,
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Optimizes transformer attention with Flash Attention for 2-4x speedup and 10-20x memory reduction. Use when training/running transformers with long sequences (>512 tokens), encountering GPU memory issues with attention, or need faster inference. Supports PyTorch native SDPA,
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optimization-flash-attention.SKILL.md
---name: optimizing-attention-flash
description: Optimizes transformer attention with Flash Attention for 2-4x speedup and 10-20x memory reduction. Use when training/running transformers with long sequences (>512 tokens), encountering GPU memory issues with attention, or need faster inference. Supports PyTorch native SDPA, flash-attn library, H100 FP8, and sliding window attention.
version: 1.0.0
author: Orchestra Research
license: MIT
tags: [Optimization, Flash Attention, Attention Optimization, Memory Efficiency, Speed Optimization, Long Context, PyTorch, SDPA, H100, FP8, Transformers]
dependencies: [flash-attn, torch, transformers]
---# 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+)**:
```python
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
k = torch.randn(2, 4096, 32, 64, device='cuda', dtype=torch.float16)
v = torch.randn(2, 4096, 32, 64, device='cuda', dtype=torch.float16)
# Convert to float8_e4m3 (FP8)
q_fp8 = q.to(torch.float8_e4m3fn)
k_fp8 = k.to(torch.float8_e4m3fn)
v_fp8 = v.to(torch.float8_e4m3fn)
```
**Step 4: Run with FP8 attention**
```python
from flash_attn import flash_attn_func
# FlashAttention-3 automatically uses FP8 kernels on H100
out = flash_attn_func(q_fp8, k_fp8, v_fp8)
# Result: ~1.2 PFLOPS, 1.5-2x faster than FP16
```
## 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:
```bash
pip install flash-attn --no-build-isolation
```
Or install CUDA toolkit first:
```bash
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:
```python
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):
```python
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](references/transformers-integration.md) for enabling Flash Attention in BERT, GPT, Llama models.
**Performance benchmarks**: See [references/benchmarks.md](references/benchmarks.md) for detailed speed and memory comparisons across GPUs and sequence lengths.
**Algorithm details**: See [references/algorithm.md](references/algorithm.md) for tiling strategy, recomputation, and IO complexity analysis.
**Advanced features**: See [references/advanced-features.md](references/advanced-features.md) for rotary embeddings, ALiBi, paged KV cache, and custom attention masks.
## 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)
