[Bugfix][Spec Decode] Inherit target KV cache scales in Gemma4 MTP draft layers#49262
[Bugfix][Spec Decode] Inherit target KV cache scales in Gemma4 MTP draft layers#49262philipshurpik wants to merge 1 commit into
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…aft layers Gemma4 MTP draft layers share the target's physical KV cache (kv_sharing_target_layer_name), but their Attention objects keep their own _k_scale/_v_scale, which stay at 1.0 because assistant checkpoints carry no KV scales. On targets with calibrated FP8 KV scales (modelopt kv_cache_quant_algo / compressed-tensors kv_cache_scheme, e.g. unsloth/gemma-4-31b-it-nvfp4), the drafter dequantizes the shared cache with the wrong scale (k_scale ~0.003 read as 1.0) and MTP acceptance collapses from ~65-77% to ~34-46%, costing ~30% decode throughput. Output quality is unaffected (rejection sampling), so the bug is silent. Fix - to copy the target layer's q/k/v/prob scales onto each KV-sharing draft layer after the mapping is wired; a no-op for scale-free targets. Signed-off-by: philipshurpik <philip.shurpik@gmail.com>
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Gemma4 MTP draft layers share the target's physical KV cache (kv_sharing_target_layer_name), but their Attention objects keep their own _k_scale/_v_scale, which stay at 1.0 because assistant checkpoints carry no KV scales.
On targets with calibrated FP8 KV scales (e.g. unsloth/gemma-4-31b-it-nvfp4), the drafter dequantizes the shared cache with the wrong scale (k_scale ~0.003 read as 1.0) and MTP acceptance collapses from ~65-77% to ~34-46%, costing ~30% decode throughput.
Output quality is unaffected (rejection sampling), so the bug is silent.
Fix - to copy the target layer's q/k/v/prob scales onto each KV-sharing draft layer after the mapping is wired; a no-op for scale-free targets.
Purpose
Fix a silent MTP acceptance-rate collapse (~30% decode throughput loss) when a Gemma4 target with calibrated FP8 KV-cache scales is served with speculative decoding.
Draft layers share the target's physical KV cache but keep _k_scale/_v_scale at 1.0 (assistant checkpoints ship no scales); with calibrated-KV targets served --kv-cache-dtype fp8 the drafter reads a lot different K/V and per-position acceptance decays 0.86/0.71/0.55/0.42 -> 0.71/0.42/0.21/0.10.
The problem was both reproduced:
Fix: copy _q/_k/_v/_prob_scale + float mirrors from the target layer after process_weights_after_loading has populated them.
Test Plan
A/B of two byte-identical modelopt NVFP4 exports differing only in baked KV scales, served with MTP on RTX PRO 6000 (SM120), production workload (in our case, but could be any workload actually);
Test Result
mode | acceptance length | decode tok/s
no KV calibration baked) | 3.4 - 4.1 | 163.1
calibrated-KV before fix | 2.35 - 2.69 | 114.3
calibrated-KV after fix | 3.66 - 3.75 | 162.5 (~parity)
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