mtp: combined-forward speculative decode beats plain on GB10 (+2.4 t/s) (stacked on #11)

[TrevorS]

mtp: combined-forward speculative decode beats plain on GB10

Stacked on #11 (clean-spark-backend).

Summary

Makes --mtp faster than plain decode on DGX Spark / GB10 by replacing the canonical MTP draft+verify sequence (eval first_token, then run the verifier separately) with a single batched-N=2 forward over [first_token, drafts[0]]. The verifier reads both tokens' logits in one graph eval, so the cost amortizes.

Strict mode (--quality or DS4_MTP_STRICT=1) falls back to canonical decode2_exact for byte-equality with plain decode.

Speed — ds4-bench standard sweep (promessi_sposi.txt, gen=128, GB10)

ds4-bench now takes --mtp (see below), so this is the same harness CONTRIBUTING.md specifies, just with speculative decode enabled. Full CSVs: speed-bench/gb10.csv (plain), speed-bench/gb10_mtp.csv (--mtp).

ctx	plain	--mtp	Δ
2048	14.24	16.13	+1.89
10240	14.04	15.56	+1.52
18432	13.88	14.82	+0.94
26624	13.59	14.77	+1.18
34816	12.95	14.91	+1.96
43008	12.79	13.79	+1.00
51200	12.57	12.95	+0.38
59392	12.32	12.91	+0.59

MTP is faster at every context (+0.4 to +2.0 t/s on this prompt). The margin tracks the speculative accept rate, which depends on how predictable the continuation is — prose like promessi_sposi sits on the lower end.

Prompt-dependence (chat-style prompts accept more drafts)

Measured with ds4 -p ... -n 256 --temp 0 (plain vs default --mtp), not in the CSV:

Prompt	plain	--mtp	Δ
"knight" (short)	16.12	18.54	+2.42
long story prompt (~30k ctx)	13.46	17.54-19.91	+3.9 to +6.4

These are higher-accept-rate cases; the CSV table above is the conservative standard-prompt number.

What's in the PR

1. Small-N matmul kernel polish

• Pair-fuse Q_A + KV_A in qkv_rms_fused decode (one weight load, two outputs).
• Fuse head_rms_norm + rope_tail on Q.

2. Q8 share-weight batched matmul

• matmul_q8_0_preq_batch_share_warp_kernel<N_TOK>: one Q8 weight row per warp, N_TOK F32 dot-products against N_TOK token activations.
• Bit-equal to N=1 matmul_q8_0_preq_warp8_kernel at any block count: same per-lane stride, same warp_sum_f32, explicit __fmaf_rn locks the FMA contraction to the N=1 SASS. Dispatched at n_tok=2..4 under !DS4_MTP_STRICT.

3. Combined-forward verifier

• metal_graph_eval_mtp_draft_n_from_hc: batched MTP-draft primitive.
• ds4_session_eval_speculative_argmax_combined: single batched verifier forward over [first_token, drafts[0]], accept drafts matching target argmax, strict-mode fallback to decode2_exact.
• combined_prev_hc bootstrap from canonical eval for the first iter.

4. Dispatch + default

• K4 share-warp dispatch gate: replace the cuBLAS-cache-availability check (always-false on Spark, where every Q8 weight has a cached F16 copy) with a !DS4_MTP_STRICT gate, so the share-warp kernel fires at decode time.
• mtp_draft_tokens default 1 → 2 (combined-forward needs ==2 to fire; the old default of 1 routed --mtp through canonical decode2 with no win).

5. Bench tooling

• ds4-bench gains --mtp FILE / --mtp-draft N, mirroring the CLI decode loop so the standard harness can measure real --mtp throughput. Logs the chosen decode path. speed-bench/gb10_mtp.csv generated with it.

Correctness

DS4_MTP_STRICT=1 / --quality: combined-forward declines, path is canonical decode2_exact. Plain decode byte-identical to upstream/main (chat-formatted test prompts). The ds4-bench plain sweep matches the no-MTP baseline at every context.

Tested

• make clean && make cuda-spark — clean
• make cpu — clean
• ./ds4_test --long-context, --tool-call-quality, --server, --metal-kernels — OK
• ds4-bench plain + --mtp full 2048→65536 sweeps (both CSVs included)
• Two ds4_test checks fail identically on raw upstream/main (bfe070a) — not introduced here:
  • --logprob-vectors short_code_completion — fixture's official continuation is one greedy token off
  • --metal-tensor-equivalence — intrinsically flaky on GB10 (batched-prefill run-to-run non-determinism; reproduces on raw upstream). Verified the combined-forward strip/changes don't worsen it: PR1 (no MTP changes) and raw upstream show the same behavior.

Hardware: NVIDIA DGX Spark (GB10 / sm_121), driver 580.142, CUDA 13.0
Model: DeepSeek-V4-Flash-IQ2XXS-w2Q2K-AProjQ8-SExpQ8-OutQ8-chat-v2-imatrix.gguf
MTP: DeepSeek-V4-Flash-MTP-Q4K-Q8_0-F32.gguf

AGENT.md compliance

• "Preserve correctness before speed" — plain decode byte-identical; strict mode preserves byte-equal canonical path.
• "Do not add permanent semantic variants behind flags" — no new flag; --mtp-draft is pre-existing (only its default changes), DS4_MTP_STRICT is pre-existing.
• "Diagnostic switches are fine when they validate the one release path" — DS4_CUDA_NO_Q8_SHARE_BATCH=1 opt-out preserved as the kill switch.

[TrevorS]

Recreated as #14 after renaming the branch to gb10-mtp-combined-forward. Continue at #14.