[Bugfix] Pad Marlin FP8 MoE weight dims to tile alignment under TP > 1

[ssubhanjali]

The Marlin kernel requires size_n % 64 == 0 (tile_n_size) and size_k % 16 == 0 (tile_k_size). When tensor-parallel sharding splits MoE weights across GPUs, per-rank dimensions can violate these constraints and cause a crash at model load time on any GPU that falls back to the Marlin FP8 MoE path (CC < 9.0: L40S, A100, A10G).

Example — Nemotron Nano 3 at TP=4 (intermediate_size=1856):
w13 gate+up: size_n = 1856 / 4 * 2 = 928... actually 464 per-shard → 464 % 64 = 16 ✗
w2 down: size_k = 1856 / 4 = 464... actually 232 per-shard → 232 % 16 = 8 ✗

This error is never triggered on Hopper+ (CC >= 9.0) because vLLM selects native FP8 MoE kernels (CUTLASS/Triton) on those GPUs and never enters the Marlin path.

Fix:

• marlin_utils_fp8.py repack_weight(): pad size_n to the next multiple of 64 and size_k to the next multiple of 16 before calling gptq_marlin_repack.
• marlin_utils_fp8.py permute_scales(): pad scales to match padded size_n.
• fused_marlin_moe.py _fused_marlin_moe(): compute padded sizes, use them for GEMM calls, trim w13 padding before activation, pad intermediate activation output before w2 GEMM.

Padding with zeros is mathematically a no-op: zero weights and zero inputs contribute nothing to GEMM outputs. For already-aligned dimensions all padding amounts are zero and no tensor operations are performed.

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[mergify]

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[gemini-code-assist]

Code Review

This pull request addresses a crash in Marlin FP8 MoE with tensor parallelism by padding weight dimensions to meet kernel alignment requirements. The changes in both the weight loading and the forward pass appear to correctly implement this fix. My review includes suggestions to improve maintainability by centralizing the new padding logic and constants to avoid code duplication across different files and functions.

[gemini-code-assist]

The Marlin tile sizes (_TILE_N, _TILE_K) and the padding calculation logic are duplicated from vllm/model_executor/layers/quantization/utils/marlin_utils_fp8.py. These constants and logic are specific to the Marlin kernel and should be centralized to ensure consistency and improve maintainability. Consider defining these constants in marlin_utils_fp8.py (or a shared constants file) and exposing a utility function to compute the padded dimensions, which can then be imported and used here.

[gemini-code-assist]

This block for padding calculation is duplicated in the permute_scales function. To improve maintainability and avoid potential inconsistencies, this logic should be extracted into a single helper function. Additionally, the import torch.nn.functional as _F is a local import; it's better practice to have it at the top of the file.

[mergify]

Hi @ssubhanjali, the pre-commit checks have failed. Please run:

uv pip install pre-commit
pre-commit install
pre-commit run --all-files

Then, commit the changes and push to your branch.

For future commits, pre-commit will run automatically on changed files before each commit.

Tip

Is mypy failing?

mypy is run differently in CI. If the failure is related to this check, please use the following command to run it locally:

# For mypy (substitute "3.10" with the failing version if needed)
pre-commit run --hook-stage manual mypy-3.10

[alvinttang]

The padding approach is correct in principle: aligning size_n to MARLIN_TILE_N=64 and size_k to MARLIN_TILE_K=16 at weight-load time, then mirroring the same padding at inference time to keep the GEMM shape contract intact. A few things worth verifying: (1) The trim after the w13 GEMM (intermediate_cache1[:, :_w13_n].contiguous()) introduces an extra allocation — if this is on the hot path under high load, it may be worth only trimming when _w13_n_padded \!= _w13_n (which the code already guards, good). (2) The zero-padding of intermediate_cache2 before the w2 GEMM assumes that the repacked w2 weight's zero-padded columns produce zero output — this is only safe if the padded columns of the repacked weight are also zero; please confirm pack_fp8_to_int32 + gptq_marlin_repack preserve zero-padding in the repacked layout. (3) There are no tests added for the TP>1 path — a unit test parameterized over non-tile-aligned intermediate sizes (e.g., N=232) would make this much safer to maintain.