Network Endpoint Congestion Control for Fine-Grained Communication

Endpoint congestion in HPC networks creates tree saturation that is detrimental to performance. Endpoint congestion can be alleviated by reducing the injection rate of traffic sources, but requires fast reaction time to avoid congestion buildup. Congestion control becomes more challenging as application communication shift from traditional two-sided model to potentially fine-grained, one-sided communication embodied by various global address space programming models. Existing hardware solutions, such as Explicit Congestion Notification (ECN) and Speculative Reservation Protocol (SRP), either react too slowly or incur too much overhead for small messages. In this study we present two new endpoint congestion-control protocols, Small-Message SRP (SMSRP) and Last-Hop Reservation Protocol (LHRP), both targeted specifically for small messages. Experiments show they can quickly respond to endpoint congestion and prevent tree saturation in the network. Under congestion-free traffic conditions, the new protocols generate minimal overhead with performance comparable to networks with no endpoint congestion control.

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