composable_kernel/client_example/08_fused_attention/README.md

Fused Attention Examples

This directory contains comprehensive examples demonstrating CK's high-performance fused attention implementations, which are critical for modern transformer architectures and large language models.

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Theory

Fused Multi-Head Attention Operation: The fused attention mechanism performs the core transformer operation in a single, optimized kernel:

\text{Attention}(Q, K, V) = \text{Softmax}(Q K^T / \sqrt{d_k}) V

Detailed Mathematical Steps:

1. Query-Key Attention Scores: S = Q K^T
2. Scale: S_{\text{scaled}} = S / \sqrt{d_k}
3. Softmax: A = \text{Softmax}(S_{\text{scaled}})
4. Weighted Value Sum: \text{Output} = A V

• Multi-head extension: Each head computes attention independently, then results are concatenated and projected.
• Tensor shapes: Q, K, V, Output are typically [Batch, Seq_len, Num_heads, Head_dim].

Algorithmic Background:

• Fused attention combines two GEMMs and a softmax in a single kernel, minimizing memory traffic.
• Supports bias, masking, and permutation for transformer and LLM workloads.

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How to Run

Prerequisites

Please follow the instructions in the main Build Guide section as a prerequisite to building and running this example.

Build and run

cd composable_kernel/client_example/08_fused_attention
mkdir build && cd build
cmake -DCMAKE_CXX_COMPILER=/opt/rocm/bin/hipcc ..
make -j

# Example run (basic fused attention)
./fused_attention

# Example run (fused attention with bias)
./fused_attention_bias

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Source Code Structure

Directory Layout

client_example/08_fused_attention/
├── fused_attention.cpp         # Main client example: fused attention (Q, K, V)
├── fused_attention_bias.cpp    # Fused attention with bias
├── CMakeLists.txt              # Build configuration for the example

Key Functions

• main() (in each .cpp):
  Sets up Q, K, V tensors, configures attention parameters, launches the fused kernel, and verifies the result.
• Fused attention kernel invocation:
  Uses the Composable Kernel device API to launch the fused attention operation, optionally with bias.

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Additional Details

• Supports FP16, BF16, FP32, and mixed precision.
• Handles causal and generic masking for autoregressive and variable-length models.
• Optimized for memory efficiency (no intermediate attention matrix in global memory).
• Example parameters can be adjusted in the source for different transformer workloads.

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Related Examples

• 01_gemm: GEMM for Q×K^T and Attn×V
• 06_softmax: Softmax client API usage
• 03_gemm_layernorm: Fused GEMM + layer normalization
• 07_grouped_convnd_fwd: Grouped convolution for vision transformers

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