神威太湖之光上分子动力学模拟的性能优化

“神威·太湖之光”国产超级计算机的特点是适用于高通量计算系统，此类系统往往存储器访问延迟，网络延迟较长.在实际应用中，有一大类问题是时间演化的模拟问题，往往需要高频状态迭代，每次迭代需要通信.此类应用问题的典型代表是分子动力学模拟，分子的性质依赖于时间演化，导致状态相关的时间尺度上难以并行化.实际应用中，全原子模型需要模拟超过ms时间尺度，每一步的物理时间为1fs~2.5fs，这意味着所需时间步个数超过10¹²个.众核处理器中，不同核心访存时需较长的“排队”等待，造成访存延迟.另外，网卡通信延迟以及较长的数据通路会带来网络延迟，由此导致在长延迟的众核处理器上进行一次有效的模拟几乎是不可能的.解决此类问题的主要挑战是提高迭代频率，即每秒执行尽可能多的迭代步.针对神威高性能芯片处理器的体系结构特点，以分子动力学模拟为例，研究了一系列优化策略以提高迭代频率：（1）单核通信与片上核间同步相结合，降低通信成本；（2）共享内存等待与从核同步相结合，优化异构体系结构中的核间同步；（3）改变计算模式，减少核间数据关联和依赖关系；（4）数据传输与计算重叠，掩盖访存延迟；（5）规则化问题，以提高访存凝聚性.

Performance Optimization of Molecular Dynamics Simulation on Sunway TaihuLight System

Sunway TaihuLight supercomputer is suitable for high-throughput computing systems, which tend to have memory access latency and network latency. There is a large class of problems namely time-to-solution, which requires high frequency iterations. The typical application of time-to-solution problems is molecular dynamics simulation. Computations in molecular dynamics simulation depend on the time. Therefore, the iterative computations are difficult to be parallelized. Time scale usually exceeds microsecond, which means that the number of steps is more than 10¹². It is impossible to finish effective simulation in a limited time on long latency system. Therefore, the main performance bottleneck on long latency Sunway system is how to increase the iterative frequency. This study proposes a series of optimization strategies to improve the iterative frequency:(1) Reducing communication overhead and network competition costs through single-core communication combined with on-chip synchronization; (2) Optimizating the speed of synchronization between cores through waiting the shared memory variable and synchronizing the computing processing elements; (3) Reducing the data dependencies by changing the computation patterns; (4) Covering up the memory access latency by overlapping computation and communication; (5) Regulating the data structure to improve accessibility.