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Identifying and Harnessing Concurrency for Parallel and Distributed Network Simulation
Autor: P. Andelfinger Links:
Quelle: Dissertation, ISBN 978-3-7315-0511-2, KIT Scientific Publishing, 2016
Brief Summary: Although computer networks are inherently parallel systems, the parallel execution of network simulations on interconnected processors frequently yields only limited benefits. In this thesis, methods are proposed to estimate and understand the parallelization potential of network simulations. Further, mechanisms and architectures for exploiting the massively parallel processing resources of modern graphics cards to accelerate network simulations are proposed and evaluated.

Abstract: Since computer networks are inherently parallel systems, simulations of computer networks can in many cases be accelerated substantially through parallel and distributed execution on a set of interconnected processors. Still, simulation models of computer networks vary significantly in their parallelization potentials. Although an enormous number of works consider the efficient parallel execution of specific network models, there is still a lack of guidelines that help in decisions on parallelization and in the selection of hardware platforms, simulator architectures and synchronization approaches that enable an efficient execution. Further, the advent of commodity many-core devices has broadened the range of possible simulator realizations beyond the possibilities of traditional CPU-based approaches.

This dissertation addresses the efficient execution of simulation models of computer networks from two perspectives: we propose evaluation methods to determine a model's parallelization potential and provide simulator realizations that efficiently exploit the identified potentials using modern many-core hardware.

Identifying Concurrency: we propose an analytical approach to estimate the concurrency of network models, i.e., the number of processors that can be occupied in an idealized parallel and distributed simulation run based only on model knowledge and simple network statistics from sequential simulation runs. By not relying on an automated ``black-box'' method such as critical path analysis, our estimation approach exposes the relationships between the communication patterns in a simulated network and the resulting concurrency of the simulation. Insights into these relationships may guide model optimizations and the selection of suitable simulator architectures. Our estimations approximate the progress of simulations performed using the well-known synchronization algorithm YAWNS. After clarifying the relationship between critical path analysis and YAWNS, we provide a proof that shows the validity of the general approach and that substantiates the results of previous works that share our assumptions. Empirical results on the example of three network models implemented in popular simulators demonstrate that the estimations are sufficiently accurate to evaluate the models' parallelization potential, while avoiding an automated ``black-box'' analysis of sequential simulation runs.

In order to take into account both the properties of the considered network model as well as the execution hardware and synchronization approach, we present a tool that predicts the runtime of parallel and distributed simulations on the basis of sequential simulation runs and hardware measurements. The tool performs a simulation of an envisioned parallel and distributed simulation (``second-order simulation''). We show that reasonably accurate runtime predictions are achieved for distributed simulation runs in the case of network models where simulated messages require non-trivial amounts of computation.

Harnessing Concurrency: traditionally, parallel and distributed simulations rely on interconnected CPUs for execution of the simulation model. In some previous works, models of peer-to-peer networks have been reported to benefit only to a small degree from parallelization in CPU-based execution environments.

We analyze and compare two partitioning strategies for models of Kademlia-based networks such as the BitTorrent DHT, one of the largest public peer-to-peer networks. When applying a partitioning strategy based on the logical topology of the network, we achieve a simulation speedup of 6.0 compared to a sequential execution and a near-linear reduction of the memory requirements per execution node.

Since high-performance CPU-based parallel and distributed simulations can consume enormous amounts of hardware resources that may not be justified by the achieved runtime reductions, we consider the acceleration of network simulations using graphics processing units (GPUs). GPUs have evolved to support general-purpose computations on hundreds or thousands of parallel processing elements. An inexpensive GPU in a researcher's workstation can be used to shorten the feedback loop between changes to the network model and the retrieval of the corresponding simulation results.

We compare and evaluate architectures for a GPU-based coprocessing of detailed wireless network simulations. Although each simulated transmission already provides opportunities for parallel processing, we show that significant runtime reductions require an aggregated consideration of multiple transmissions in parallel. Since, in order to maintain correctness, the potential for interactions between multiple transmissions must be considered, the results show that even a simple GPU-based coprocessing requires synchronization mechanisms from the field of parallel and distributed simulation.

Subsequently, we propose a fully GPU-based simulation approach that executes all simulation logic as well as the network model on a GPU. Due to avoiding most interaction between a host CPU and the GPU, the fully GPU-based approach is applicable to network models where individual events require only small amounts of computation. Contrary to existing works, our approach aggregates sets of simulated nodes that are considered jointly.

The aggregation enables the exploitation of a tradeoff between the utilization of the GPU's processing elements and simulation overheads by dynamically adapting the degree of aggregation according to performance measurements at simulation runtime. We conduct a performance evaluation of our implementation on the example of a model of Kademlia-based networks and the well-known PHOLD benchmark model. Using a single commodity GPU, we achieve a speedup of up to 19.5 and event rates up to 6.8 x 10^6 for the model of Kademlia-based networks. A speedup of up to 27.5 and event rates of up to 39.3 x 10^6 events per second of wall-clock time are achieved in the case of the PHOLD model.