Communication efficiency and robustness are two important concerns for distributed learning systems. Gradient sparsification that only transmits partial dimensions of the original gradient is one of the major methods to reduce the communication cost and gradient filter that filters out gradients with abnormal value is the mainstream to tolerate Byzantine attackers. However, in this paper, we identify that existing gradient sparsification techniques are not compatible with the gradient filter because the sparsified gradient may be viewed as abnormal and thus be filtered out. To tackle this challenge, we propose DGSB that sparsifies the differential gradient which is the difference between the gradients in two adjacent iterations instead of the original gradient. The principle is that the server can recover the approximately original gradient of each worker by using their sent sparsified differential gradient, and thus the server can apply the gradient filter over these recovered gradients which are close to the original gradients. Experiments conducted on various training tasks demonstrate that, in the presence of Byzantine workers, DGSB achieves over 30-fold communication compression while maintaining stable test accuracy in the evaluated settings, while the traditional gradient sparsification empowered with Byzantine-resilient methods sometimes cannot converge.



