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[SM90] MegaMoE: fix workspace cleanup; enable push dispatch on H200; support other models - #6517

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flashinfer-ai:mainfrom
AichenF:megamoe-sm90-cleanup-offset-fix
Oct 21, 2026
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yyihuang merged 9 commits into
flashinfer-ai:mainfrom
AichenF:megamoe-sm90-cleanup-offset-fix

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@ouershua2299 ouershua2299 commented Oct 15, 2026 •

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📌 Description

Follow-up to #6343 (SM90 FP4 MegaMoE):

  1. Cleanup fix: with single_active_dispatch_warp, the inactive dispatch warp cleaned the workspace with empty expert counts and left pool blocks ≥ 32 of hot experts stale, so the next launch failed (H20, 97–128 tokens per rank, e.g. CUDA-graph capture); it now fetches the counts first (outputs bitwise unchanged, timing within noise).
  2. H200 push dispatch: add measured H200 EP8 buckets that enable push for M ≤ 32 on DeepSeek-V4-Flash and MiMo-V2.5-Pro, and M ≤ 64 on DeepSeek-V4-Pro (rotated pool at M 33–64), saving about 10 µs per call (1.03–1.16x kernel time, bitwise-identical outputs; H20 and EP4 plans unchanged).
  3. Other models: accept any expert count that divides across ranks, keep the selected plans compilable for untuned geometries (H20 buckets match the full geometry; the planner checks the full fused shared-memory layout, which grows with the expert count, and falls back to the next legal smaller tile when the chosen one does not fit; unsupported shapes raise ValueError), and allow push only on validated plans, so Kimi-K2, DeepSeek-V3, GLM-4.x and Qwen3 shapes now run correctly while plans of the three tuned geometries are unchanged.
  4. Usage docs: the weight-preparation docs now state what callers must convert before transform_{nvfp4,mxfp4}_weights_for_mega_moe: Marlin byte order (checkpoints are usually low-nibble-first), scales inside the decoder range (NVFP4 6 * scale <= 448; MXFP4 2^-5–2^6 for groups with nonzero codes, otherwise outputs silently saturate or flush), and the excess moved into per-expert global scales passed at run time; the module example shows the full flow.
  5. Framework hooks: add MegaMoESymmBuffer.prewarm(), which builds every NVFP4 module nvfp4_mega_moe can select up to capacity (sharded across ranks, before CUDA graph capture), and commit_staged(), which accepts rows a producer wrote directly into the now-documented staging views (x, x_sf, topk_idx, topk_weights), so integrations no longer reach into JIT internals or buffer fields.

How to use: FlashInfer only provides the kernels and the layout transforms; frameworks convert checkpoint tensors to the format above at load time, call prewarm() once, and stage inputs with stage() or in place plus commit_staged() (the SGLang integration does this for nvidia/DeepSeek-V4-Flash-NVFP4, where about 51% of blocks would saturate without rebalancing).

🔍 Related Issues

Follow-up to #6343.

🚀 Pull Request Checklist

✅ Pre-commit Checks

  • I have installed pre-commit by running pip install pre-commit (or used your preferred method).
  • I have installed the hooks with pre-commit install.
  • I have run the hooks manually with pre-commit run --all-files and fixed any reported issues.

🧪 Tests

  • Tests have been added or updated as needed.

  • All tests are passing (unittest, etc.).

  • test_hot_expert_workspace_cleanup reproduces fix 1: it fails before the fix on 8×H20 and passes after.

  • New CPU tests cover the H200 buckets, the counter-dispatch limits and plan legality for other geometries, including 2048 and 8192 experts (every plan fits shared memory); every fused and split plan selected for 12 geometries (the three tuned models, six common shapes, 1024 / 2048 / 8192 experts) compiles on SM90, and plans of the tuned and previously validated geometries are unchanged.

  • test_nvfp4_prewarm_and_direct_staging checks that after prewarm() no fused or split call compiles a new module, that in-place staging plus commit_staged() gives correct results under CUDA graphs, and that out-of-range counts are rejected.

  • tests/moe/test_megamoe_fp4_{sm90,ep,weights}.py: 73 passed on 8×H200.

Reviewer Notes

Summary by CodeRabbit

  • New Features

    • Added optional JIT prewarming for NVFP4 workloads across supported token counts.
    • Added direct staging support for efficiently preparing dispatch inputs.
    • Added support for transformed NVFP4 weights with per-expert global scales.
    • Improved dispatch planning across supported hardware configurations and workload shapes.
  • Bug Fixes

    • Fixed workspace cleanup for inactive dispatch paths, improving repeated CUDA Graph execution reliability.
    • Improved expert-count handling during dispatch cleanup.
  • Documentation

    • Clarified supported weight formats, scale requirements, dispatch behavior, and configuration limits.

…anup

Single-dispatch-warp plans fetch the per-expert counts only in warp 0, but
cleanup_workspace() runs on every dispatch warp and derives pool block offsets
from each warp's cached counts. Warp 1 used offset 0, so blocks 32+ of every
local expert except expert 0 kept stale L1 arrival counts / L2 masks and the
next launch failed (H20, M 97..128, one expert > 32 BM24 blocks, e.g. CUDA graph
capture). Let inactive dispatch warps fetch the final counts before cleanup.

Adds test_hot_expert_workspace_cleanup (hot routing, M=97/128, repeated calls and
CUDA graph replay): fails before the fix on 8xH20, passes after (41 passed).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
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📝 Walkthrough

MegaMoE FP4 changes update fused-plan selection, add buffer staging and NVFP4 JIT prewarming, and fetch expert receive counts before workspace cleanup. The changes also add validation tests and document FP4 weight packing and scale constraints.

Changes: MegaMoE FP4 dispatch and staging

Layer / File(s) Summary
Select and validate fused plans
flashinfer/megamoe/heuristics.py, tests/moe/test_megamoe_fp4_ep.py
Counter policies and fused-plan selection now include geometry and shape validation, bounded push dispatch, and shared-memory fallback selection. EP tests cover policy boundaries, unsupported shapes, and publishing-SM limits.
Add staging and NVFP4 prewarming
flashinfer/megamoe/__init__.py, flashinfer/megamoe/weights.py, tests/moe/test_megamoe_fp4_sm90.py
MegaMoESymmBuffer adds commit_staged and prewarm. Tests cover direct staging, prewarm validation, and JIT cache behavior. Documentation describes token-count requirements, packing, and scale constraints.
Fetch counts before workspace cleanup
include/flashinfer/megamoe/sm90_mxfp4_mega_moe_fused_body.inl, include/flashinfer/megamoe/sm90_nvfp4_mega_moe_fused_body.inl, tests/moe/test_megamoe_fp4_sm90.py
Dispatch warps that did not fetch expert receive counts in the pull loop fetch them before cleanup. NVFP4 push dispatch skips this fetch. A hot-expert test covers both FP4 formats.

Priority: ➖ Normal

Estimated code review effort: 4 (Complex) | ~45 minutes

Change: Feature

Sequence Diagram(s)

sequenceDiagram
  participant Caller
  participant MegaMoESymmBuffer
  participant PlanSelection
  participant Rank
  participant JITModuleCache
  Caller->>MegaMoESymmBuffer: call prewarm
  MegaMoESymmBuffer->>PlanSelection: create plans for token counts
  PlanSelection->>MegaMoESymmBuffer: return fused and split plans
  MegaMoESymmBuffer->>Rank: distribute module builds
  Rank->>JITModuleCache: build assigned modules
  MegaMoESymmBuffer->>Rank: synchronize and load every module
Loading
Merge Risk: ⚪ Minimal · up to dc0ab

The previously reported weight-conversion error path is no longer present, and no remaining issue was established that should delay merging.

Pre-merge checks: Passed 4 · Failed 1

Failed Failed checks: 1 warning

Check name Status Explanation Resolution
Docstring Coverage Warning Docstring coverage is 40.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 30 functions across 6 files. Write docstrings for the functions missing them to satisfy the coverage threshold.
Passed Passed checks: 4 passed
Check name Status Explanation
Title check Passed The title clearly summarizes the main changes: fixing workspace cleanup, enabling H200 push dispatch, and supporting additional models.
Description check Passed The description follows the repository template. It explains the changes, links the related issue, completes the pre-commit and test checklists, and provides test coverage details. The optional review…
Linked Issues check Passed Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check Passed Check skipped because no linked issues were found for this pull request.
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@ouershua2299
ouershua2299 marked this pull request as draft October 16, 2026 07:33
The counter policy only had H20 buckets, so H200 always took the pull path with
NVLink barrier #1. Add measured H200 EP8 buckets: push for M 1..32 on the
V4-Flash (H4096/E256/K6) and MiMo (H6144/E384/K8) geometries, and push for
M 1..64 with the rotated pool for M 33..64 on Pro (H7168/E384/K6).

Push removes ~10 us per call: 1.03-1.16x kernel time at the enabled M on
8xH200 across uniform, Zipf-skewed and hot-experts-on-one-rank routers; each
bound is the largest M that does not lose on any of them (push loses at BM64,
0.89-0.95x at M=256). Outputs stay bitwise identical to pull. H20 and EP4
policies are unchanged.

Adds test_h200_ep8_counter_policy (CPU).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
ouershua2299 and others added 2 commits October 16, 2026 05:46
… set

Counter dispatch:
- Allow push / rotated pool only on the validated small-M plans (M <= 64,
  BLOCK_M <= 24) and when the push row copy supports the hidden size
  (hidden % 1024 == 0); force_* overrides outside that range fall back to the
  barrier path (forced push at BM64 could produce run-to-run differences).
- Geometries without measured buckets push (without the rotated pool) up to
  M = 32 on H20 and H200.

Untuned geometries (e.g. DeepSeek-V3 H7168/E256/K8, Kimi-K2 H7168/E384/K8):
- The H20 tile buckets assume the RS arm of the three tuned geometries; match
  them on (hidden, experts, top-k) instead of hidden alone. Before, any other
  geometry with the same hidden got 6 stages without RS and failed to compile
  ("Unexpected SM90 pipeline depth", shared-memory capacity).
- Replace the Pro-only BLOCK_M=8 stage clamp with the shared-memory budget the
  4-stage layout leaves for the expert-count table and dispatch send rows;
  H7168/E384 (Kimi-K2) and H >= 7680 no longer overflow at M <= 16.
- Reject shapes the kernel cannot compile (hidden alignment, intermediate not a
  multiple of 128 or larger than hidden / 2) with a ValueError.

Plans of the three tuned geometries are unchanged on H20/H200, EP4/EP8,
M 1..256. 198 distinct fused/split plans of six untuned geometries compile;
Kimi-K2 and DeepSeek-V3 shapes pass the reference check on 8xH200 (fused
M 1..128 random/hot routing, split M 384/1024).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
The NVFP4 fused plan only accepted 32/48 local experts at EP8 (64/96 at EP4),
the counts of the tuned models, although the kernels handle the expert count
generically (per-lane expert caches, SM-strided cleanup, C++-sized split plan).
Remove that restriction; GLM-4.5/4.6/4.7 (E128/E160), Qwen3 (E128) and
Qwen3-Next / Qwen3.5-397B (E512) now select a plan.

Push dispatch publishes local expert e from SM e, so it additionally requires
experts per rank <= SMs.

192 distinct fused/split plans of the GLM / Qwen geometries compile (H20/H200,
EP4/EP8). On 8xH200 the GLM-4.7 (EP8, EP4), GLM-4.5-Air, Qwen3-30B and
Qwen3.5-397B shapes pass the reference check (fused M 1..128, random/hot
routing; split M=384).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
@ouershua2299 ouershua2299 changed the title [SM90] Fix MegaMoE workspace cleanup when one expert exceeds 32 pool blocks [SM90] MegaMoE: fix workspace cleanup; enable push dispatch on H200; support other models Oct 16, 2026
@ouershua2299
ouershua2299 marked this pull request as ready for review October 16, 2026 16:08
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

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Actionable comments posted: 1


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Inline comments:
Review comments at @flashinfer/megamoe/heuristics.py:
- Line 344: Update `select_fused_plan` to validate the complete shared-memory
layout for the selected tile and stage configuration against `SMEM_CAPACITY`
before returning or compiling a plan. Reject configurations that exceed the
limit or select a fitting schedule; apply this check to generic plans, not only
the existing `BLOCK_M=8` adjustment.

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Comment thread flashinfer/megamoe/heuristics.py Resolved

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Review comments at @flashinfer/megamoe/weights.py:
- Around line 182-195: In the gated `[E, 2]` branch of the weight-scale folding
logic, validate that the ratio-adjusted local scales remain exactly
representable in E4M3 before applying the power-of-two shift; raise a
gate/up-specific error if folding is lossy, rather than allowing the later
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Comment thread flashinfer/megamoe/weights.py Resolved
@ouershua2299
ouershua2299 marked this pull request as draft October 20, 2026 05:51
State in the weight-preparation docs what callers convert before the
transforms: Marlin byte order (checkpoints are usually low-nibble-first),
the decoder scale ranges (NVFP4 6 * scale <= 448; MXFP4 2^-5..2^6 for groups
with nonzero codes, else silent saturation or flush), and per-expert global
scales passed at run time. The module usage example shows the full flow.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
@ouershua2299
ouershua2299 force-pushed the megamoe-sm90-cleanup-offset-fix branch from f5696c2 to 3f75dc1 Compare October 20, 2026 08:04
ouershua2299 and others added 2 commits October 20, 2026 02:43
prewarm() builds every NVFP4 module nvfp4_mega_moe can select up to the
buffer capacity, sharded across ranks, so frameworks need not reach into the
JIT internals before CUDA graph capture. commit_staged() marks rows a producer
wrote directly into the now-documented staging views (x, x_sf, topk_idx,
topk_weights), replacing writes to num_staged_tokens. The kernel and stage()
paths are unchanged.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
The fused kernel keeps a per-expert count table in shared memory, so its
footprint grows with the expert count; at 8192 experts the generic BM64
3-stage plan needs 236576 B and failed the kernel static assert. The planner
now computes the whole fused layout (mirroring sm90_nvfp4_mega_moe_fused_body.inl)
and falls back to BM8 3-stage or the next smaller legal tile when the chosen
one does not fit, raising ValueError if none does. This replaces the
BM8-only headroom check; plans of the tuned and previously validated
geometries are unchanged.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
@ouershua2299
ouershua2299 marked this pull request as ready for review October 20, 2026 14:03

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Inline comments:
Review comments at @flashinfer/megamoe/heuristics.py:
- Around line 418-427: The push-plan selection around the capacity check must be
collective: validate equal num_tokens across ranks or derive a cross-rank bound,
then use the same push, no_clean, and BLOCK_M decisions on every rank. Update
nvfp4_mega_moe, MegaMoESymmBuffer.stage, and commit_staged documentation to
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Comment thread flashinfer/megamoe/heuristics.py Resolved
The fused plan (family, tile, push dispatch) is selected from num_tokens and
every rank must launch the same kernel, so state on nvfp4_mega_moe,
mxfp4_mega_moe, MegaMoESymmBuffer.stage and commit_staged that the token count
must match across ranks, with short ranks padded by rows routed to -1.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
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@flashinfer-bot run tests/moe/test_megamoe_fp4_sm90.py tests/moe/test_megamoe_fp4_ep.py

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