The MMA flash-attention dispatcher only instantiated ncols2 = 8 and 4 for
head_dim 512, so any other GQA ratio hit GGML_ABORT. Gemma 4 12B's global
attention layers use head_dim 512 with a 16:1 GQA ratio (16 query heads /
1 KV head), which aborts at load. Because MTP speculative decoding requires
flash attention, this also blocks the Gemma 4 12B MTP drafter entirely.
Instantiating ncols2 = 16 there is not viable: it exceeds the maximum dynamic
shared memory on Ada (cudaFuncSetAttribute returns invalid argument). Instead,
route gqa_ratio % 8 == 0 (covering 8 and 16) through the existing ncols2 = 8
kernel, which already iterates over Q-head groups (iter_z = ceil(gqa_ratio /
ncols2)). gqa_ratio 8 and 4 behavior is unchanged; this mirrors the divisor
dispatch already used for the 576x512 case below.
Verified on RTX 4070 Ti SUPER (Ada, cc 8.9): Gemma 4 12B + MTP drafter now
runs with flash attention; draft acceptance 43-95% by workload, 1.5-2.2x
end-to-end speedup. The 26B-A4B drafter (gqa_ratio 8) is unaffected.
Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
19dcc1f7d1