ParaTTT makes test-time training layers parallel enough to match linear recurrences
AspireAI community · arXiv
Test-time training layers replace the linear fast-weight memory of models like Gated DeltaNet and Mamba-2 with a small neural network trained online by gradient descent, which is more expressive but painfully sequential. ParaTTT recovers parallelism by iteratively refining an estimate of the inner-loop gradient trajectory, with each refinement made of three chunkwise-parallel passes; the exact online recurrence turns out to be a fixed point of that iteration, and two refinements suffice in practice. At 4K context the authors report a 32x speedup over the block-approximated TTT-MLP at matched error, and a 340M model trained 4.9x faster, with 340M and 1.3B variants matching or beating Gated DeltaNet and Mamba-2 on perplexity and zero-shot accuracy. The convergence proof only covers a two-layer variant under sufficient contraction conditions.
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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.