HetSyn: Speeding Up Local SGD with Heterogeneous Synchronization

Synchronous local stochastic gradient descent (local SGD) suffers from some workers being idle and random delays due to slow and straggling workers, as it waits for the workers to complete the same amount of local updates. In this paper, to mitigate stragglers and improve communication efficiency, a novel local SGD strategy, named HetSyn, is developed. The key point is to keep all the workers computing continually at each synchronization round, and make full use of any effective (completed) local update of each worker regardless of stragglers. An analysis of the average wall-clock time, average number of local updates and average number of uploading workers per round is provided to gauge the performance of HetSyn. The convergence of HetSyn is also rigorously established even when the objective function is nonconvex. Experimental results show the superiority of the proposed HetSyn against state-of-the-art schemes through utilization of additional effective local updates at each worker, and the influence of system parameters is studied. By allowing heterogeneous synchronization with different numbers of local updates across workers, HetSyn provides substantial improvements both in time and communication efficiency.