MongoDB副本节点replWriterThreadCount参数的细节问题

Question

现在有几台做了读写分离的MongoDB服务器，在配置副本节点的时候注意到一个参数replWriterThreadCount（用于并行应用复制操作的最大线程数）。

这里面有个细节不是很明白，假设在机器是8核的情况下，mongo默认配置replWriterThreadCount的是16，那么其实现的时候会按照CPU最大的核数还是会按照配置的线程数进行opLog重放？

Answer 1

最终还是找来源代码，找到了答案。

源代码链接

具体细节，在最后一段：

std::unique_ptr<ThreadPool> makeReplWriterPool() {
    // Reduce content pinned in cache by single oplog batch on small machines by reducing the number
    // of threads of ReplWriter to reduce the number of concurrent open WT transactions.
    if (replWriterThreadCount < replWriterMinThreadCount) {
        LOGV2_FATAL_NOTRACE(
            5605400,
            "replWriterMinThreadCount must be less than or equal to replWriterThreadCount",
            "replWriterMinThreadCount"_attr = replWriterMinThreadCount,
            "replWriterThreadCount"_attr = replWriterThreadCount);
    }
    auto numberOfThreads =
        std::min(replWriterThreadCount, 2 * static_cast<int>(ProcessInfo::getNumAvailableCores()));
    return makeReplWriterPool(numberOfThreads);
}

解释一下就是replWriterThreadCount与2倍的CPU核心数（IO密集型线程池预设大小）取最小值。