arXiv:2501.08040cs.LGmath.PR2025-01

证明了RTRL算法在特定RNN上的收敛性,可精准优化长序列数据。

Convergence Analysis of Real-time Recurrent Learning (RTRL) for a class of Recurrent Neural Networks

  • 通过前向传播梯度实现在线参数更新,避免截断误差
  • 理论证明算法收敛至损失函数的驻点,适用于无限长序列
  • 适合小中规模参数的长时序建模,如金融数据

循环神经网络(RNN)通常使用截断时间反向传播(TBPTT)训练,该方法因截断链式法则导致梯度近似误差。相比之下,实时递归学习(RTRL)是一种在线优化算法,当序列时间步数趋于无穷时,其渐近逼近真实损失梯度。RTRL通过前向传播隐藏层对参数的导数,并利用这些导数在每个时间步进行参数更新,从而可在极长序列上实现精确优化,尽管参数量大时计算成本高。本文证明了针对一类RNN的RTRL算法收敛性,建立了数据序列、RNN隐层及隐层导数联合分布的不动点,证明算法收敛至损失函数的驻点。数值实验验证了理论结果。一个潜在应用是金融数据建模,此类数据通常具有长时序特征且模型参数规模小至中等,使RTRL计算可行,具有吸引力。文中以订单簿数据为例展示了RTRL的应用。

原文摘要 · Abstract (English)

Recurrent neural networks (RNNs) are commonly trained with the truncated backpropagation-through-time (TBPTT) algorithm. For the purposes of computational tractability, the TBPTT algorithm truncates the chain rule and calculates the gradient on a finite block of the overall data sequence. Such approximation could lead to significant inaccuracies, as the block length for the truncated backpropagation is typically limited to be much smaller than the overall sequence length. In contrast, Real-time recurrent learning (RTRL) is an online optimization algorithm which asymptotically follows the true gradient of the loss on the data sequence as the number of sequence time steps $t \rightarrow \infty$. RTRL forward propagates the derivatives of the RNN hidden/memory units with respect to the parameters and, using the forward derivatives, performs online updates of the parameters at each time step in the data sequence. RTRL's online forward propagation allows for exact optimization over extremely long data sequences, although it can be computationally costly for models with large numbers of parameters. We prove convergence of the RTRL algorithm for a class of RNNs. The convergence analysis establishes a fixed point for the joint distribution of the data sequence, RNN hidden layer, and the RNN hidden layer forward derivatives as the number of data samples from the sequence and the number of training steps tend to infinity. We prove convergence of the RTRL algorithm to a stationary point of the loss. Numerical studies illustrate our theoretical results. One potential application area for RTRL is the analysis of financial data, which typically involve long time series and models with small to medium numbers of parameters. This makes RTRL computationally tractable and a potentially appealing optimization method for training models. Thus, we include an example of RTRL applied to limit order book data.

RNN优化算法收敛分析金融建模

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