[IEEE 2019 IEEE 21st International Conference on High Performance Computing and Communications; IEEE 17th International Conference on Smart City; IEEE 5th International Conference on Data Science and Systems (HPCC/SmartCity/DSS) - Zhangjiajie, China (2019.8.10-2019.8.12)] 2019 IEEE 21st International Conference on High Performance Computing and Communications; IEEE 17th International Conference on Smart City; IEEE 5th International Conference on Data Science and Systems (HPCC/SmartCity/DSS) - Accelerating Distributed Training in Heterogeneous Clusters via a Straggler-Aware Parameter Server