WRF系列教程4：WRF并行核数如何确定？

气象互助社 发表于 2024-4-10 17:15:53

使用WRF模式进行模拟，一个常涉及的问题就是应该使用多少核用于并行计算？并行计算，并不是核数越多越好，核数增加，则相应的通信成本也会增加。如下图所示，当核数提升到一定程度时，模式每个积分步长所用时间就难以继续下降。

模式并行scaling，图片来自WRF官网培训PPT,Dave Gill

WRF运行的并行核数如何确定?
mpirun -np? ./wrf.exe
并行的最小核数，取决于运行的个例需要多大内存
例如北美大陆1500×1500网格数的个例，常规的节点(每个节点36核)则最少需要4个节点，如果使用内存大的节点，则只需要2个节点。

CONUS模拟配置，图片来自WRF官网培训PPT,Dave Gill

对于一个不考虑内存的计算任务，并行和串行都能完成计算，用时长短的区别而已。但是分布式内存的并行方式(编译WRF时并行方式选择dm)，与串行最大的区别是能提供更大的内存，承担规模更大的任务。举个例子，一个小区域即使模拟一万年，个人PC和超算都能完成，快慢的区别而已。但是模拟一个全球高分辨率的任务，个人PC提供的内存无法存储这么大的内容，而超算分布式内存并行的方式就可以将其分解存储在不同的节点中。

对WRF模拟而言，具体的意思就是分布式内存并行的方式提供的内存，能够满足模拟更大的区域以及更高的分辨率。内存不够，就会出现段错误。

并行的最大核数，取决于WRF模式内部的通信模板
当并行核数增加时，相应的通信成本会增加，WRF会将模拟区域剖分成一个个小块，每个核计算一小块，每个小块差分计算需要用到相邻小块的值，因此每步积分都需要通信获取相邻块的值。

WRF分解模拟区域时，每个小块的长和宽必须不小于10。因此运行模式时，使用过多的核也会报错。

例如北美大陆1500×1500网格数的个例，在X和Y方向分别最多可以分解为150份，也就是150×150小块，最多能使用150×150=22500核。

最小最大核数经验的计算方法
如果是多层嵌套，并行使用的最小核数应基于网格数最多的domain计算，
而并行最大核数应基于网格数最少的domain。例如，假设有2个domain，并且domain01是200x200网格数，而domain02是300x300网格数，可用来下述公式大致估计：
最大核数 =（200/25）*（200/25）= 64个处理器
最小核数 =（300/100）*（ 300/100）= 9个处理器

推荐的核数
因为不同用户的模拟区域范围、分辨率以及嵌套层数都不一样，并且每个人所拥有的计算资源以及对时效性的要求都不一致，最终使用的并行核数其实难以有一个统一的推荐。简单粗暴的方法就是不同核数测试一下，得到一个合适的值。并行核数最好是节点核数的整数倍，比如一个节点是24核，那么可以采用24，48，64等整数倍。

参考
https://www2.mmm.ucar.edu/wrf/users/tutorial/presentation_pdfs/202101/gill_computation.pdf
https://www.cnblogs.com/jiangleads/p/12885651.html

文章来源于微信公众号：气海同途

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