Neural gradients are lognormally distributed: understanding sparse and quantized training

Neural gradient compression remains a main bottleneck in improving training efficiency, as most existing neural network compression methods (e.g., pruning or quantization) focus on weights, activations, and weight gradients. However, these methods are not suitable for compressing neural gradients, which have a very different distribution. Specifically, we find that the neural gradients follow a lognormal distribution. Taking this into account, we suggest two methods to reduce the computational and memory burdens of neural gradients. The first one is stochastic gradient pruning, which can accurately set the sparsity level -- up to 85% gradient sparsity without hurting validation accuracy (ResNet18 on ImageNet). The second method determines the floating-point format for low numerical precision gradients (e.g., FP8). Our results shed light on previous findings related to local scaling, the optimal bit-allocation for the mantissa and exponent, and challenging workloads for which low-precision floating-point arithmetic has reported to fail. Reference implementation accompanies the paper.