@no_stp_on_snek: DeepSeek-V4.1-Flash on 2 Sparks. thinking high, TP=2. 55M tokens. Actual kernel work, not a demo. it read the notes and…

DeepSeek-V4.1-Flash on 2 Sparks. thinking high, TP=2. 55M tokens. Actual kernel work, not a demo. it read the notes and prds, ran A against B, checked a win, then opened a PR. i watched it publish. i did not write it. The fix at a glance: causal attention is a "triangle". the kernel was still launching the square. every strictly-future tile still took a threadgroup slot, woke up, asked "am i even allowed?", and retired. on a giant GPU that's rounding error. on Metal, at the 128–192 token prefills you actually type into a local model, launch cost is a real slice of time-to-first-token. so only launch the on/below-diagonal tiles. T(T+1)/2 instead of T*T. same math. fewer wakes. M5 Max, full attn chain, interleaved A/B, 15 rounds: bf16 seq=192: 0.924x (−7.6%), 15/15 paired wins. at 128 you keep 10 of 16 tiles. a third of the launches were dead on arrival. not a new matmul. we just stopped paying for work the mask already threw away. the win is small. that's how you know it's the tax, not a bench trick.