launch overhead

A chain of five elementwise operations reads and writes the tensor five times when once would do. The byte arithmetic for an unfused chain against a fused one, the launch overhead that matters at small sizes, the three fusion shapes, and the operations where fusion changes nothing because the matmul was at the roof.

At batch 1 a decode step is a few milliseconds of GPU work behind several hundred kernel launches, and the CPU cannot issue them fast enough. The arithmetic showing the GPU starve, what replay changes, and the four constraints capture imposes on memory.

Graphs remove the per-kernel launch cost, which is a large share of a decode step at low batch and a small one at high batch, so whether it matters is a question about your operating point. The arithmetic, the four reasons an engine turns them off, and the measurement that settles it.