Hybrid PDES Simulation of HPC Networks Using Zombie Packets

High-fidelity network simulations provide insights into new realms for high-performance computing (HPC) architectures, although at a high cost. Surrogate models offer a significant reduction in runtime, yet they cannot serve as complete replacements and should be only used when appropriate. Thus the need for hybrid modeling, where high-fidelity simulation and surrogates run side-by-side. We present a surrogate model for HPC networks in which packets bypass the network, and the network state itself is suspended when switching to the surrogate. To bypass the network, every packet is scheduled to arrive at a predicted time in the future estimated from historical data; to suspend the network, all in-flight packets are delivered to their destinations, but they are kept in the system to awaken as zombies when switching back to high-fidelity. Speedup for a hybrid model is relative to the proportion of surrogate to high-fidelity. We obtained a 3 × speedup for a simulation where 70% of virtual time was spent in surrogate mode. When considering the surrogate portion only, the speedup jumps to nearly 20 × on a uniform random network traffic example. The accuracy of the overall simulation increased when the network state was suspended instead of ignored, which demonstrates the need for modeling the network state when transitioning from surrogate back to high-fidelity mode.