MCCL trained a 113.5M-parameter ViT across 24 M4 Max Mac Studios at 95.9 percent parallel efficiency, and the authors price 100 idle Macs against A100 cloud spend
AspireAI community · arXiv (Skywalker Sound / MCCL authors)
PyTorch has no accelerated distributed backend for Apple's Metal Performance Shaders, which blocks the obvious move of borrowing idle Mac fleets for distributed training. MCCL is an open-source MPS backend for multi-node PyTorch over switched Ethernet: large all-reduce operations reach 98-99.7 percent of TCP goodput, a 113.5M-parameter ViT scales from 198 to 4,568 samples/s at 95.9 percent parallel efficiency, and one M4 Max Mac is roughly 1.7x an A100's FP32 matrix multiply, with 5.5 Macs matching an A100 using TF32. The authors estimate that 100 idle Macs running 12 hours a night offsets about $200,000 a year in equivalent cloud compute. The honest framing: this is a facility's own fleet and workload, and the cost figure is the authors' own arithmetic.
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Two sparse-autoencoder analyses have resisted scaling: matching semantically similar features across layers, and compressing large feature circuits into interpretable supernodes. This paper argues both are the same underlying problem, estimating semantic distances between SAE features that sit on different activation manifolds. Its representation treats each feature not as a single decoder vector but as an activation-weighted distribution over the hidden states expressing it, projects those distributions into a shared reference space, and compares them with Wasserstein distance. The representation is proved invariant to activation rescaling and stable under perturbation, and recovers true matches under finite-sample margin conditions. Empirically it outperformed decoder-vector and LLM-based baselines and captured subtle functional distinctions between related features, and it compressed large feature circuits into interpretable supernodes automatically. The limitation is that this is interpretability tooling: the paper demonstrates better feature matching and circuit compression, not that downstream interpretability claims become more accurate.