MPI广播集合操作的带宽节省优化
A Bandwidth-saving Optimization for MPI Broadcast Collective Operation
摘要：
MPI集体操作的效率和可扩展性，特别是广播操作，在高性能计算应用中起着不可或缺的作用。MPICH是当前广泛使用的MPI软件栈之一，它实现了基于点对点操作的广播操作。根据参数，如消息大小和进程计数，库选择使用不同的算法，例如二项式传播，递归加倍交换或环全对全广播（allgather）。然而，由于内部次优环全聚集算法，MPICH的最新版本中的现有广播设计对于大消息（\textit{lmsg}）或具有非二次幂处理计数的中等消息（\textit{mmsg-npof2}）不提供良好性能。在本文中，本机广播设计MPICH的基础上，我们提出了一种调谐广播方法与带宽节省考虑迎合\textit{lmsg}和\textit{mmsg-npof2}的情况下。在Cray XC 40集群上，针对不同的数据量和节目量，对原生广播设计和调谐广播设计进行了比较。结果表明，在用户级测试中，\textit{lmsg}和\textit{mmsg-npof2}的调谐广播设计性能提高了2%~ 54%.
英文摘要：
The efficiency and scalability of MPI collective operations, in particular the broadcast operation, plays an integral part in high performance computing applications. MPICH, as one of the contemporary widely-used MPI software stacks, implements the broadcast operation based on point-to-point operation. Depending on the parameters, such as message size and process count, the library chooses to use different algorithms, as for instance binomial dissemination, recursive-doubling exchange or ring all-to-all broadcast (allgather). However, the existing broadcast design in latest release of MPICH does not provide good performance for large messages (\textit{lmsg}) or medium messages with non-power-of-two process counts (\textit{mmsg-npof2}) due to the inner suboptimal ring allgather algorithm. In this paper, based on the native broadcast design in MPICH, we propose a tuned broadcast approach with bandwidth-saving in mind catering to the case of \textit{lmsg} and \textit{mmsg-npof2}. Several comparisons of the native and tuned broadcast designs are made for different data sizes and program sizes on Cray XC40 cluster. The results show that the performance of the tuned broadcast design can get improved by a range from 2\% to 54\% for \textit{lmsg} and \textit{mmsg-npof2} in terms of user-level testing.
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