spec_decode: propagate DCP size to V1 draft config (fixes MTP collapse under DCP)#72
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create_draft_parallel_config() does not copy decode_context_parallel_size from the target parallel config, so drafts built via the V1 proposer path run their attention with dcp_world_size=1 (no q-allgather, no LSE merge, top-k indices consumed with replicated-cache semantics) while the draft KV, metadata, and sparse-indexer state are DCP-sharded. The V2 runner path (gpu/spec_decode/eagle/utils.py) already restores the target DCP size for native MTP drafts; this mirrors that logic in the V1 path, gated by VLLM_DCP_SHARD_DRAFT with the same method==mtp default. On a 4x DGX Spark (SM121) cluster serving GLM-5.2 NVFP4 at TP4/DCP4/128K, this takes MTP3 per-position acceptance from 0.72/0.30/0.07 to 0.90/0.79/0.67 and decode from 14.6 to 22-23 tok/s.
Three companion fixes needed before the draft DCP config fix is reachable for GLM checkpoints routed through glm4_moe (which do not pass a topk_scores_buffer at construction): - sparse_attn_indexer: lazily allocate topk_scores_buffer when DCP>1 and the B12X sparse indexer is active (otherwise the first decode step fails with 'B12X sparse indexer DCP requires topk_scores_buffer'). - deepseek_mtp: allocate the MTP layer scores buffer whenever DCP>1. - mla/indexer: add build_for_drafting() with an unpadded decode view for draft_index>0 (otherwise iterative draft steps fail the num_decode_tokens+num_prefill_tokens==num_tokens assertion), and skip dcp-local KV addressing for dcp_replicated specs.
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Quick note: fable driven PR, I've been grinding for days w/ various models trying to figure out why at DCP=2/DCP=4 MTP acceptance collapsed. This was the reason. I expect (because on my 8x RTX6000 Pro Blackwell, this is the case) that MTP=4 will still be +tp/s. However, traveling tomorrow because of holiday, but wanted to throw this up after I got the results rather than let the bits rot; so draft. |
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thank you for this. |
2 participants
Problem
Drafts built via the V1 proposer path (
vllm/v1/spec_decode/llm_base_proposer.py) run their attention withdcp_world_size=1while their KV cache, metadata, and sparse-indexer state are DCP-sharded.Root cause:
SpeculativeConfig.create_draft_parallel_config()builds the draftParallelConfigby copying selected fields from the target — anddecode_context_parallel_sizeis not among them, so it silently defaults to 1._create_draft_vllm_config()then uses it verbatim. The V2 runner path (gpu/spec_decode/eagle/utils.py) already restores the target DCP size for native MTP drafts; the V1 path has no equivalent.The result for native MTP under DCP>1 on V1: no q-allgather, no LSE merge (
merged_out == partial_out), global top-k indices consumed with replicated-cache addressing against a sharded cache, and draft forwards where 3 of 4 DCP ranks see an all--1top-k row and emit all-zero attention output for their head groups.o_proj's TP all-reduce then blends the inconsistent per-rank results into one identical-everywhere (wrong) hidden state — which makes the failure invisible to cross-rank divergence checks and immune to every runtime knob (interleave size,ag_rsvsa2a, global vs local top-k, eager vs CUDA graphs were all tested; none changed the signature).Because the corruption compounds each recursive draft step, MTP1 mostly survives on the hidden-state pathway while MTP2/MTP3 collapse — and the damage scales with
dcp_world_size(step-1 conditional acceptance ≈ 0.80 / 0.68 / 0.42 at DCP 1/2/4).Fix
Commit 1 mirrors the V2 logic into the V1
_create_draft_vllm_config: whenVLLM_DCP_SHARD_DRAFTis enabled (default formethod=="mtp", same gating as V2), replace the draft parallel config'sdecode_context_parallel_sizewith the target's.Commit 2 carries three companion fixes needed before that path is reachable for GLM checkpoints routed through
glm4_moe(which don't pass atopk_scores_bufferat construction):topk_scores_bufferallocation inSparseAttnIndexerat DCP>1 with the B12X sparse indexer (otherwise the first decode step raisesB12X sparse indexer DCP requires topk_scores_buffer);deepseek_mtp;build_for_drafting()onDeepseekV32IndexerMetadataBuilderwith an unpadded decode view fordraft_index>0(otherwise iterative draft steps failnum_decode_tokens + num_prefill_tokens == num_tokens), plus skipping dcp-local KV addressing fordcp_replicatedspecs.An alternative (possibly preferable) form of the core fix is to copy
decode_context_parallel_sizeinsidecreate_draft_parallel_config()itself so every consumer inherits it; I kept the change local to the V1 MTP path to match the existing V2 gating and avoid changing behavior for external Eagle/DFlash drafts.Validation
4× DGX Spark (GB10/SM121, unified memory), GLM-5.2 NVFP4 + MTP overlay, TP4/DCP4, 131072 max len, fp8 KV,
B12X_MLA_SPARSE, MTP3, greedy 512-token codegen bench:DCP1 on the same checkpoint gives 0.85 / 0.68 / 0.59, so the fixed DCP4 path now matches/exceeds the no-DCP baseline, in line with the numbers this repo reports on SM120 rigs (which run the V2 path).
Diagnosed with a tensor-level tap on the draft layer's
forward_mqa/merge site: at DCP1 a fp64 reference attention over the dequantizedfp8_ds_mlacache reproduces the kernel partials at cos ≥ 0.9999; the DCP4 dumps showimpl.dcp_world_size==1, no merge, per-rank zero partials, and DCP-local seq lens (6/6/5/5 for a global 22) feeding a non-DCP attention. Happy to share the dumps/harness if useful.