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Local and Distributed Large LLM Inference with MLX and RDMA on Mac Studio

machinelearning.apple.com6 points1 comments
Screenshot of Local and Distributed Large LLM Inference with MLX and RDMA on Mac Studio

MLX on Mac Studio demonstrates that current frontier LLMs can be run locally and scaled across a small Mac cluster using RDMA over Thunderbolt. Benchmarks compare M5 Ultra and M3 Ultra Mac Studios running Qwen 3.8 (27B), Mistral Medium 3.5 (128B) and Muse Spark Open (816B with 42B active experts in MoE form). The M5 Ultra’s GPU Neural Accelerator and 1.2 TB/s unified memory bandwidth produce large gains: up to 4.6× faster time-to-first-token (compute-bound) and about 1.6× higher decode throughput (memory-bandwidth-bound) versus M3 Ultra. Energy draw remains modest (≈200W for M3 Ultra, ≈300W for M5 Ultra). Muse Spark Open, quantized to NVFP4, reduces model storage to ≈493GB but still exceeds single-machine capacity; on a four-Mac cluster it achieves over 36 tokens/sec at an 8,192-token context.

Large models are handled across machines via two MLX-supported schemes. Pipeline parallelization (split by depth) is simple, low-overhead, and can improve prefill throughput for very long contexts by chunked overlap (up to ~1.8× speedup at 65,536 tokens). Tensor parallelization (split by width) yields better per-layer parallelism and higher decode throughput but requires many fine-grained collectives and extremely low-latency links. MLX leverages macOS 26.2 RDMA-over-Thunderbolt (jaccl backend) and helper tools (mlx.launch, mlx_lm.benchmark) to orchestrate these distributed runs with practical trade-offs between implementation complexity and runtime speed.

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