MonoMoE: An Efficient Fused Mega-kernel for Quantized MoE Decoding
arXiv:2609.04244
Abstract
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 dimension and partitions CTAs over expert-weight tiles, eliminating expert-local token materialization and reducing padded arithmetic. A persistent grid fuses routing, top- 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 over vLLM Triton Grouped GEMM, is -- faster than FlashMoE-FP8 adaptation across various models and batch sizes, and reduces end-to-end time per output token by up to 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}.
14 pages, 10 figures