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On TP-only deployments with the modular TRTLLM-Gen MXFP4 experts, a MoE call that the next layer's fused all-reduce + mHC does not take runs the experts' top-k finalize and then a separate shared_output + routed_output: two passes over the hidden states. These experts can already stop after GEMM2 (vllm-project#58635), so the MoE layer now finalizes such calls itself, in one Triton kernel that also adds the shared-expert output. - _finalize_shared_add_kernel reduces each token's top-k rows of the permuted GEMM2 output with their routing weights, adds the shared-expert output and writes the bf16 sum. - The model calls DeepseekV4MoE.defer_finalize(next_layer_max_tokens) for every local layer. An eligible layer defers every call that fits one experts launch and records how many tokens the next layer's fused all-reduce + mHC takes: MHC_OVERLAP_MAX_TOKENS when the model fuses the mHC all-reduce and the successor is a local layer without engram, 0 otherwise. - The decoder layer still hands the calls the next layer takes to forward_unfinalized. DeepseekV4MoE.forward finalizes every other deferred call with the kernel, then all-reduces when reduce_results is set, as the runner does. Dispatch is automatic, with no new flag. A call takes the kernel when the experts are TrtLlmMxfp4ExpertsModular (not native MegaMoE), the runner applies no routed scale to their output and the layer has shared experts; the deployment can defer (use_deferred_moe_finalize: TP > 1 without DP, EP, PCP or sequence parallelism, no hidden-dim padding); the call is non-empty and fits one experts launch; and the next layer's fused all-reduce + mHC does not take it: the model does not fuse that all-reduce, the layer is the last local one or precedes an engram layer, or the call has more than MHC_OVERLAP_MAX_TOKENS tokens. The choice depends only on the token count, so each CUDA graph captures one path and the host never waits on the device. Other experts, deployments and the DSpark drafter's layers keep the runner's path, and the calls handed to the next layer unfinalized are unchanged. The MoE before the decoder replay layers may now defer, so their input comment now says that input is never an unfinalized MoE output. Exactness: per element, the kernel does the finalize's arithmetic in the same order, an FP32 FMA over the top-k slots in slot order from +0.0 that skips routes to other ranks, rounded to bf16, then the runner's FP32 shared + routed add, rounded to bf16. Given the same GEMM2 output the result is bit-identical (NaN payloads aside). With deferral no longer capped at MHC_OVERLAP_MAX_TOKENS, the deferred FlashInfer autotune pass in kernel_warmup coincides with the max_num_batched_tokens pass whenever that fits one experts launch, and tunes the GEMM2-only variant. The kernel's constexprs depend only on the model, so the startup profile run compiles it whenever max_num_batched_tokens fits one experts launch. Tests: the kernel is checked bit for bit against moe_unpermute plus the shared add, including a route to another rank; dispatch tests on a real FusedMoEConfig cover eligibility, the next layer's token limit and forward's finalize + all-reduce; and test_trtllm_mxfp4_deferred_finalize gains a 1000-token case, holding moe_unpermute to the experts' own finalize at zero tolerance for prefill-sized calls. Signed-off-by: bofengl <bofengl@nvidia.com> Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
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Since MHC_OVERLAP_MAX_TOKENS = 16, decode batches <=16 already fold the finalize into the fused all-reduce on all but the last/pre-engram layers, so I’d expect ~no change there. If there is a a gain at batch <= 16, it’s either noise or something I’ve missed.
Could you share the actual decode step latency (in ms, before and after) that the −2.1% comes from, and at which batch size it was measured? I generally recommend to repeat e2e runs 3-4 times.
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Thanks for the careful read — the ≤16-token fold you describe is real, but it doesn't cover the configuration the −2.1% came from. The decode number was measured at TP2 (the Where the fold does exist (TP4 + MNNVL), this PR leaves it alone: a deferred call still routes through On the measurement: agreed that percentages from single runs aren't enough. I'll rerun the decode case ( |
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Absolute numbers, 4 repeats per arm — and you were right to push back on the result, though for a different reason than the ≤16 fold. Setup. B300 (2 GPUs, TP2), vLLM at commit 00b6c4c; arm A stock, arm B = this PR's runtime hunks applied on top (they apply cleanly).
So on this machine at today's main, the PR is a net regression, and the −2.1%/−2.5% from the description (measured on a different SM10x system) does not reproduce. Where it goes. A profiled pair ( Next. I'm converting this to a draft until it's reworked so the fused finalize consumes the experts' output on their already-tuned path (or the deferred-call shapes are covered by the tuning buckets), with the A/B above re-run as the acceptance gate. Thanks for insisting on the absolute numbers — the percentages hid exactly this. |
Overview
Makes DeepSeek-V4.1's MoE top-k finalize + shared-expert add 1.3-2.3x faster by fusing them into one Triton kernel, which makes the TP2 model step 2.1% faster in decode and 2.5% faster in a 4096-token prefill, with unchanged accuracy. On TP-only
deployments with the modular TRTLLM-Gen MXFP4 experts, every MoE call that the next layer's fused all-reduce + mHC does not take now runs its deferred top-k finalize and the shared-expert add as one kernel, instead of the experts' finalize followed by
a separate add.
DeepSeek V4.1 (#57448), building on #58635 and #58586. Related but not overlapping: #61868, #60355, #62174, #62366 (see Details).
Claims
[num_tokens, hidden]bf16 intermediate is no longer writtenand read back. This covers every deferrable call on deployments without the fused all-reduce + mHC, and elsewhere the last local layer, layers before an engram layer and calls above
MHC_OVERLAP_MAX_TOKENS(16) tokens, such as prefill.vllm bench forward, decode (8 requests at 8192 context) is 2.1% faster and a 4096-token prefill 2.5% faster.shared_output + routed_output(tested on the raw bits). GSM8K: 0.894 on the basecommit, 0.911 with this PR (one run each).
Validation
Tests
pytest -v -s tests/kernels/test_dsv41_moe_finalize.py(dispatch tests run on CPU)pytest -v -s "tests/kernels/moe/test_ocp_mx_moe.py::test_trtllm_mxfp4_deferred_finalize"pytest -v -s tests/models/test_deepseek_v4_mega_moe.py(buildsDeepseekV4MoE)test_finalize_shared_add_matches_finalize_then_add: the kernel againstmoe_unpermuteplus the shared add atatol=rtol=0on the int16 view, for 1 and 1000 tokens, DeepSeek V4.1's shape (hidden 5120, top-6) and a hidden size ending in a partialblock (2880, top-4), with one route to another rank's expert.
test_ineligible_moe_keeps_runner_finalize,test_deferred_calls_fold_into_next_layer_up_to_its_limit,test_forward_finalizes_deferred_calls_with_shared_add: dispatch on a realFusedMoEConfig(CPU).test_trtllm_mxfp4_deferred_finalize[False-1000-None](new case): the existing zero-tolerance check ofmoe_unpermuteagainst the experts' own finalize, now at a prefill-sized call, since calls above 16 tokens now defer too.Kernel microbenchmark (hidden 5120, top-6;
python bench_dsv41_finalize_shared_add.pybelow: FlashInfer's timing, which falls back to CUDA events withoutcupti-python, median; two runs agree within 6%).moe_unpermuteruns the TRTLLM-Genfinalize's algorithm and matches it bit for bit, so it stands in for the finalize that runs inside FlashInfer; the script asserts both arms are bit-identical before timing.
moe_unpermute+ add / fused)bench_dsv41_finalize_shared_add.py
End to end, DeepSeek-V4.1-Flash (MXFP4 experts) at TP2, base commit
aa3564cevs. this PR with identical flags. The in-tree DeepSeek V4.1 GSM8K config serves the NVFP4 checkpoint, whose experts do not take this path, so it is run here against theMXFP4 checkpoint. The accuracy server runs with
VLLM_GPU_SYNC_CHECK=error, to check that the new path never syncs the host with the device, and with--jit-monitor-mode warn, whose log must not list_finalize_shared_add_kernelamong the kernelscompiled after warmup. (
errormode is not usable here: on main, the DSA indexer's_block_scores_kernelalready compiles at the first request, which stops the server.)bench forward, median): decode8*1@8192-2.1%, prefill4096-2.5%._finalize_shared_add_kernel(1.6%); in the decode step, the finalize(1.7%) and the adds (0.9%) by the fused kernel (1.2%). The GEMM2 kernel switches to its GEMM2-only variant at the same cost.
VLLM_GPU_SYNC_CHECK=errordetected no host sync in either run, and the JIT monitor listed only the DSA indexer's_block_scores_kernel(already on main) as compiled after warmup in both runs, never_finalize_shared_add_kernel.Details
Motivation. Since #58635 the TRTLLM-Gen MXFP4 modular experts can stop after GEMM2 and return an
UnfinalizedMoEOutput. On DeepSeek V4.1 the only consumer is the fused all-reduce + mHC boundary (#58586), for calls of up toMHC_OVERLAP_MAX_TOKENStokens on deployments where
fuse_mhc_all_reduceholds. Every other call runs the experts' finalize kernel and then the runner's elementwiseshared_output + routed_output.Design.
_finalize_shared_add_kernel: a(num_tokens, cdiv(hidden, 1024))grid; each program reduces one token's top-k GEMM2 rows over 1024 columns, adds the shared output and stores bf16.DeepseekV4MoE.defer_finalize(next_layer_max_tokens)on every local layer, passingMHC_OVERLAP_MAX_TOKENSwhen the model fuses the mHC all-reduce and the next local layer has no engram, else 0 (the last local layer, a layer beforean engram layer, or a model without that fusion). An eligible layer then defers every call that fits one experts launch.
forward_unfinalized, now viafinalizes_in_next_layer.DeepseekV4MoE.forwardfinalizes every other deferred call with the kernel and all-reduces whenreduce_resultsis set, whichis exactly when the runner's late all-reduce would run on these deployments.
MoERunneris unchanged, and the kernel depends only onMoEOutput/UnfinalizedMoEOutput, so it can move next to them if another model with shared experts wants the same consumer. No existing kernelfuses the finalize with the shared add outside an all-reduce (
moe_unpermuteand the experts' finalize stop at the top-k sum; the LoRA experts'_finalize_lora_kerneladds the W2 LoRA delta to the unrounded sum instead).Tradeoffs.
kernel_warmupcoincides with themax_num_batched_tokenspass whenever that fits one experts launch: deployments that already deferred run one warmup pass fewer,and the remaining pass tunes the GEMM2-only variant that eligible calls now use. Bit-exactness holds given the same GEMM2 output, and that variant is tuned on its own, which is why end-to-end accuracy is checked with GSM8K rather than token by token.
max_num_batched_tokensfits one experts launch; the JIT monitor log above checks that it never compiles later.Limitations. TP1 keeps the runner path (
use_deferred_moe_finalizeneeds TP > 1), as do calls split across experts launches, the DSpark drafter's MoE layers (they never request deferral) and every other experts class, including the NVFP4 TRTLLM-Genexperts.
Compatibility.
TrtLlmMxfp4ExpertsModular(an identity check, so the LoRA experts class is excluded) with no routed scale left in the runner and with shared experts; native MegaMoE and every other experts class are unchanged.use_deferred_moe_finalize. The split keys onfuse_mhc_all_reduce, so it holds for whatever TP sizessupports_mhc_all_reduceadmits (TP4 on main; [Perf][DSV4.1] Enable the fused all-reduce + mHC boundary at TP2 #60355proposes TP2). With pipeline parallelism, each stage's last local layer finalizes in the MoE layer.
forward_unfinalized, as the fused all-reduce + mHC path does onmain.
VLLM_BATCH_INVARIANT=1the TRTLLM-Gen MXFP4 experts are not selected, so this path is not taken; the kernel itself reduces each token in a fixed order without atomics.CI coverage.
tests/kernels/test_dsv41_moe_finalize.pyruns in ":nvidia: (B200) Miscellaneous Kernels". The newtest_ocp_mx_moe.pycase runs in ":nvidia: (B200) Kernels" shard 0, which this diff does not trigger (the L4 MoE job skips SM100-onlytests), so it is run manually above. The optional ":nvidia: (B200) LM Eval Spec Decode" job serves the NVFP4 checkpoint at TP4, which covers the unchanged runner path only; no CI job serves DeepSeek V4.1 with MXFP4 experts at TP > 1, so the end-to-end
runs above are manual.
Not a duplicate. Searches of open PRs and PRs since 2026-08-15 for "finalize shared expert", "defer finalize" and "MoE finalize shared add" found no PR that fuses the deferred TRTLLM-Gen top-k finalize with the shared-expert add on DeepSeek V4.1's
default path.
--additional-config '{"decode_backend": "latency_specialized"}', GB300 + TP4 + DSpark only) whose own fused all-reduce + mHC boundary takes the deferred finalizes of decode steps. This PR changes the defaultpath on every eligible deployment and step, prefill included, and does the finalize and the shared add in one kernel inside the MoE layer, without fusing an all-reduce. The two merge cleanly in
vllm/models/deepseek_v41/nvidia/model.py(git merge-file). [Perf][DSv4.1] DeepSeek-V4.1-Flash specialized decode on GB300 #61868's MoE step saves and restores the deferral fields ofFusedMoEConfigaround its own calls and callsffn(...)for layers it cannot defer, which keeps working with this change.AI assistance. This change, its tests and this description were prepared with AI assistance (Claude Code). The submitter reviewed every changed line and ran the commands in Validation.
Pull Request Checklist
I used vLLM's
/pr-checklistskill. (Mandatory for agents, optional for humans).AI assistance was used during the creation of this PR.
Design Fit: Minimizes impact on core components, reuses existing functionality, and justifies added complexity.
Testing and Validation: Validates the change and ensures any added tests are meaningful and reliable, with CI coverage or documented CI resource constraints and validation performed outside CI.
Code Quality and Style: Keeps code and comments clear and concise, and updates relevant documentation and examples.
Pull Request Contents: Includes a brief summary and relevant links, supports claims with evidence, explains root causes and implementation trade-offs, and follows the contributing guide.