Kernel fusion is a GPU optimization technique that merges multiple neural network operations into a single CUDA kernel, eliminating intermediate writes to global memory and reducing kernel launch overhead. Inference engines like TensorRT and JIT compilers like torch.compile use kernel fusion extensively, often delivering 15–30% latency improvements on deep learning workloads. This article explains how fusion works, when it helps, and the implementation tradeoffs it requires.
What is Kernel Fusion?
Kernel Fusion is a technique that combines multiple neural network operations into unified kernels, reducing memory bandwidth usage and improving computational efficiency. Research such as Making Deep Learning Go Brrrr has shown that operator fusion is one of the most impactful optimizations for GPU workloads. This optimization is particularly effective in deep learning models where multiple operations can be fused into a single GPU kernel call.
Key Benefits
- Reduced memory bandwidth usage across the GPU memory hierarchy
- Fewer kernel launches
- Better cache utilization
- Improved overall throughput, especially on hardware with Tensor Cores
Implementation Details
The implementation of Kernel Fusion requires careful consideration of:
- Operation dependencies
- Memory access patterns - as explored in Data Movement Is All You Need, data movement is often the dominant bottleneck in transformer workloads
- Register pressure
- Shared memory utilization
Performance Impact
When properly implemented, Kernel Fusion can lead to:
- 20-40% reduction in memory bandwidth usage
- 15-30% improvement in inference speed
- Significant reduction in power consumption
Sources
- NVIDIA CUDA Programming Guide
- Deep Learning Performance Guide
- Research papers on kernel optimization