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PyTorch 2.15 made a rewritten NCCL backend the default and added FP8 with faster decode on Apple Silicon

AspireAI community · PyTorch Foundation
PyTorch 2.15 ships 3,117 commits from 583 contributors since 2.14. Every job using an nccl process group now runs on the rewritten backend by default, with TORCH_DIST_USE_NCCL2=0 or the nccl-legacy name as rollback paths, and a backend-agnostic flight recorder writes a post-mortem to disk when a collective hangs. FSDP2 can now keep precision-sensitive parameters such as MoE router gates in fp32 while the rest trains in bf16, capture a full gradient-accumulation step in one CUDA graph, and overlap gradient reduction with compute. On Apple Silicon, float8_e4m3fn and _scaled_mm bring FP8, a new grouped_mm decode fast path gives 1.32-1.34x MoE decode throughput on an M3 Ultra, and native Metal kernels make norm reductions more than 100x faster. The default wheel moves to CUDA 13.2 with 13.4 alongside it. It is a broad release rather than a single headline feature, and the NCCL default is the piece most likely to change behaviour on an existing cluster.
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One distance function matched sparse-autoencoder features across layers and compressed feature circuits into supernodes
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.
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