MonoMoE: An Efficient Fused Mega-kernel for Quantized MoE Decoding
Mixture-of-Experts (MoE) layers increase model capacity without proportionally increasing arithmetic, but their sparse expert computation is difficult to execute efficiently during autoregressive decode. Existing grouped and batched GEMMs are token-major: they construct expert-local token tiles and obtain parallelism from the token dimension. When few tokens reach each expert, this organization incurs tile padding and preprocessing, launches short-lived grids that underutilize memory bandwidth, and exposes quantization, activation, and reduction as separate stages. We present \textbf{MonoMoE}, a weight-major persistent megakernel for block-wise quantized MoE decode. MonoMoE places the complete decode-step token tile on the fine-grained tensor-core $N$ dimension and partitions CTAs over expert-weight tiles, eliminating expert-local token materialization and reducing padded arithmetic. A persistent grid fuses routing, top-$k$ selection, quantization, both expert projections, activation, and reduction in one launch; warp specialization and readiness flags overlap auxiliary work with the dominant expert-weight stream. MonoMoE is integrated with vLLM and supports multiple model shapes through generated kernel specializations and offline schedule tuning. On NVIDIA H200 GPUs, MonoMoE accelerates the complete routed-MoE operator by up to $\mathbf{1.54\times}$ over vLLM Triton Grouped GEMM, is $\mathbf{2.20}$--$\mathbf{3.84\times}$ faster than FlashMoE-FP8 adaptation across various models and batch sizes, and reduces end-to-end time per output token by up to $\mathbf{18.7\%}$ across the evaluated FP8 models, while preserving task accuracy. The MonoMoE implementation and supporting artifacts are open source and available in the \href{https://github.com/flashinfer-ai/flashinfer/tree/main/csrc/fused_moe/monomoe}{FlashInfer repository}.