mongodb io交换比例高的问题

[chenchongqi]

swap增高，换页频繁。

查了一下资料觉得比较有可能,即mongodb的mmap内存映射被虚拟机回收：
 http://blog.boxedice.com/2011/06/15/mongodb-cpu-cores-and-vmmemctl/

The thing to note here is in relation to MongoDB memory mapping files which may appear unused 
but actually be in use. If memory is paged out then this can affect performance. Since we 
solved the problem by adjusting the VM CPU configuration

这个后来经李思亮和刘海超手动实验证明，虚拟机确实会回收超过一段时间不用的内存映射，降低mongodb每个线程的内存，降低客户端链接数都可以减少这种状况，要彻底避免目前唯有修改虚拟机参数或者直接运行在物理机上才可以。