Meet mKernel: A Multi-GPU, Multi-Node Fused Kernel Library for GPU-Driven Communication

GPU communication overhead is a measurable bottleneck in production AI workloads. According to data cited by the mKernel project, communication can consume 43.6% of the forward pass and 32% of end-to-end training time. Across popular Mixture-of-Experts (MoE) models, inter-device communication can account for up to 47% of total execution time. Researchers from UC Berkeley’s UCCL project have released mKernel, a library of persistent CUDA kernels that fuse intra-node NVLink communication, inter-node RDMA, and compute into a single kernel.

The Problem: Host-Driven Communication

The standard model for multi-GPU communication is host-driven: the CPU runs the control path and calls into a library like NCCL or NVSHMEM. The library issues the collective operation — an AllReduce, an AllGather, etc. — across GPUs. Compute and communication run on separate CUDA streams and overlap at kernel boundaries.

The research team identifies two problems with this approach:

(1) CPUs are not scaling with GPU compute. A GB300 NVL72 rack integrates 72 Blackwell Ultra GPUs and 36 Grace CPUs, delivering 720 PFLOP/s FP8/FP6, 1.44 EFLOP/s FP4 Tensor Core performance, and 130 TB/s of all-to-all intra-rack NVLink bandwidth. At those speeds, microsecond-scale host orchestration overhead — a cudaLaunchKernel call, a CPU-side “all writes done” check, an inter-stream event — shows up directly as pipeline bubbles.

(2) Host-driven systems overlap compute and communication at coarse kernel boundaries. Finer-grained overlap at the tile or chunk level is not possible from the host side.

The alternative is GPU-driven communication: the GPU itself triggers transfers, with communication fused into the same kernel as the compute. Most existing fused kernel libraries operate within a single node, or a single GPU. mKernel targets the multi-node case.

What mKernel Does

mKernel is a library of persistent CUDA kernels. Each kernel fuses intra-node NVLink communication, inter-node RDMA, and dense compute into a single kernel.

Multi-GPU + multi-node, in one kernel: Both intra-node NVLink and inter-node RDMA live inside the same persistent kernel.

Fine-grained intra-kernel overlap: Compute and communication overlap at tile/chunk granularity, covering both intra-node and inter-node GPU communication.

Persistent kernel with SM specialization: CTAs self-assign roles: compute, intra-comm, inter-send, inter-reduce. The number of SMs dedicated to each role is tunable per shape.

GPU-driven networking built on libibverbs: mKernel uses GPU-initiated RDMA writes without depending on NCCL or NVSHMEM. The communication backend is written from scratch to maximize performance and support heterogeneous networking devices.

The Five Fused Kernels

Evaluation Setup

The research team evaluated mKernel on two 2-node × 8-H200 clusters that differ only in their inter-node fabric:

mKernel was benchmarked against NCCL, Triton-distributed, Flux, Mercury, MagiAttention, Transformer-Engine, and ring-flash-attention. The team notes that further benchmarking at larger scale is still in progress.

Backends and Requirements

mKernel supports two networking backends:

Both backends share the same host-side API and the same on-GPU kernel. Only the proxy/session implementation differs (session.h for CX7, session_efa.h for EFA). Requirements: NVIDIA Hopper GPUs (default build targets sm_90a), CUDA 12.9, Python with PyTorch. The CX7 backend requires libibverbs development headers and libraries. The EFA backend requires AWS EFA installation with libfabric, libibverbs, efadv, and EFA headers under EFA_HOME=/opt/amazon/efa by default.

Marktechpost’s Visual Explainer

UCCL

mKernel — Multi-GPU, Multi-Node Fused Kernels Guide

01 / 07 — Overview

What is mKernel?

mKernel is an open-source library of persistent CUDA kernels from UC Berkeley’s UCCL project. It fuses intra-node NVLink communication, inter-node RDMA, and dense compute into a single kernel.

Most existing fused kernel libraries operate within a single node or a single GPU. mKernel is designed from the start to span node boundaries.

43.6%

of forward pass consumed by communication in production

47%

of total execution time in popular MoE models

32%

of end-to-end training time consumed by communication

02 / 07 — The Problem

Why Host-Driven Communication Falls Short

The standard model is host-driven: the CPU calls NCCL or NVSHMEM, which issues collective operations across GPUs. The UCCL team identifies two problems.

CPUs are not scaling with GPUs. A GB300 NVL72 rack delivers 720 PFLOP/s FP8/FP6 and 1.44 EFLOP/s FP4. At those speeds, microsecond-scale overhead from cudaLaunchKernel, CPU-side sync checks, and inter-stream events shows up directly as pipeline bubbles.

Overlap is too coarse. Host-driven systems overlap compute and communication only at kernel boundaries. Finer-grained overlap at the tile or chunk level is not possible from the host side.

The answer: GPU-driven communication. The GPU itself triggers fine-grained transfers, fused into the same kernel as the compute.

03 / 07 — Design

Four Core Design Properties

🖧

Multi-GPU + multi-node, in one kernel. Intra-node NVLink and inter-node RDMA both live inside the same persistent kernel.

Fine-grained intra-kernel overlap. Compute and communication overlap at tile/chunk granularity, covering both intra-node and inter-node communication.

Persistent kernel with SM specialization. CTAs self-assign roles: compute, intra-comm, inter-send, inter-reduce. SM split is tunable per shape.

GPU-driven networking via libibverbs. Uses GPU-initiated RDMA writes. No NCCL or NVSHMEM dependency. Communication backend is written from scratch.

04 / 07 — Kernels

The Five Fused Kernels

AllGather + GEMM

AllGather —> GEMM

Each rank holds a shard of A. The local GEMM consumes tiles over NVLink/RDMA as they arrive — matmul starts before the collective finishes.

GEMM + AllReduce

GEMM —> AllReduce

Computes C = A @ B and reduces partial outputs across all ranks in one launch. Output tiles enter the reduction tree the instant they are produced.

MoE Dispatch + GEMM

All-to-All dispatch —> grouped GEMM

Routes MoE tokens to expert ranks via NVLink + inter-node all-to-all, then runs per-expert grouped GEMM in the same kernel. No staging buffer round-trip.

Ring Attention

Ring KV exchange —> FlashAttention

Sequence-parallel attention across ranks. Each step rotates a KV chunk around the ring while the local FlashAttention consumes the previously-received chunk.

GEMM + ReduceScatter

GEMM —> ReduceScatter

Computes C = A @ B and reduce-scatters the output. Each tile is reduced and forwarded to its owning rank as soon as it is produced.

05 / 07 — Evaluation

Evaluation Setup

Tested on two 2-node × 8-H200 clusters differing only in inter-node fabric.

Testbed	Nodes × GPUs	Inter-node	NIC
AWS EFA	2 × 8 H200	AWS EFA / SRD	16 × 200 Gb/s EFA per node
ConnectX-7	2 × 8 H200	InfiniBand	8 × 400 Gb/s CX7 per node

Both testbeds use NVLink intra-node. Benchmarked against: NCCL, Triton-distributed, Flux, Mercury, MagiAttention, Transformer-Engine, and ring-flash-attention. Larger-scale benchmarking is still in progress.

06 / 07 — Backends & Requirements

Backends & Requirements

Backend	Transport	Where it runs
CX7	libibverbs RC	ConnectX-7 / InfiniBand / RoCE
EFA	libibverbs + efadv (SRD)	AWS p5/p5e (H200, EFA)

Requirements: NVIDIA Hopper GPUs (default sm_90a), CUDA 12.9, Python with PyTorch. CX7 needs libibverbs headers. EFA needs libfabric, libibverbs, efadv under EFA_HOME=/opt/amazon/efa.

License & Attribution: MIT licensed. MMA/compute code adapted from ThunderKittens (HazyResearch).

07 / 07 — Roadmap & Key Takeaways

Roadmap & Key Takeaways

✅

Fused GPU-driven multi-node kernels (AG+GEMM, GEMM+AR, MoE Dispatch+GEMM, Ring Attention, GEMM+RS)

✅

ConnectX-7 and AWS EFA backends

🚧

Full heterogeneous accelerator/NIC support with topology-aware discovery, placement, routing

🚧

Inter-node megakernels: collapsing several fused steps into a single megakernel spanning a transformer layer

🚧

Blackwell GPU support

Fuses NVLink, inter-node RDMA, and compute into a single persistent CUDA kernel

Five kernels: AllGather+GEMM, GEMM+AllReduce, MoE Dispatch+GEMM, Ring Attention, GEMM+ReduceScatter

GPU-initiated RDMA via libibverbs — no NCCL or NVSHMEM dependency

Requires Hopper GPUs (sm_90a) and ConnectX-7 or AWS EFA networking

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