Android中内存占用的含义：（VSS，PSS，RSS，USS）
在eng版本的android手机上，可以执行：procrank
会列出来VSS，PSS，RSS，USS的占用信息。
- VSS:Virtual Set Size:how virtual memory associated witth process.（【共享】进程所占的虚拟内存是多少，跟物理内存没有多大关心。）
- PSS：Proportional Set Size.Take the RSS number but evenly distribute shared pages among the sharing process.For example, if 3 process are sharing 3MB,each process gets 1MB in PSS.(【共享】按比例被N个进程瓜分的内存)
- RSS：Resident Set Size.how much physical pages allocated for the process.Pages shared between processes are counted multiple times.(【共享】进程所占的物理内存，可被其他进程共享)
- USS,Also know as Private Dirty,which is basically the amount of RAM inside the process can not be paged to disk (it is not backed by the same data on disk),and is not shared with any other process.(【私有】独占的物理内存，不可分享)