top指令输出的VIRT, RES 和SHR有什么区别

VIRT 意味着进程虚拟空间的大小, 是真实使用的内存,加上映射进程自己使用的内存(如, X server使用显卡内存), 加上映射磁盘文件使用的内存(主要是加载共享程序库文件), 加上与其他进程共享的内存. VIRT代表进程当前时刻有多少内存可以访问.

RES 意味驻留内存大小, 是当前进程真正占用物理内存的精确反映. (直接与%MEM列相对应.) RES始终要比VIRT小, 因为多数程序依赖C库文件.

SHR 表示VIRT里有多少其实是共享部分(库文件使用的内存). 关系到库文件里, 并不是整个的库文件会驻留. 如, 如果程序仅用到了库文件里的少数函数, 整个库文件会映射并被计算到VIRT和SHR里, 但只有库文件包含用到的函数那部分真正加载到内存并被计算到RES里.

The difference among VIRT, RES, and SHR in top output

VIRT stands for the virtual size of a process, which is the sum of memory it is actually using, memory it has mapped into itself (for instance the video card’s RAM for the X server), files on disk that have been mapped into it (most notably shared libraries), and memory shared with other processes. VIRT represents how much memory the program is able to access at the present moment.

RES stands for the resident size, which is an accurate representation of how much actual physical memory a process is consuming. (This also corresponds directly to the %MEM column.) This will virtually always be less than the VIRT size, since most programs depend on the C library.

SHR indicates how much of the VIRT size is actually sharable (memory or libraries). In the case of libraries, it does not necessarily mean that the entire library is resident. For example, if a program only uses a few functions in a library, the whole library is mapped and will be counted in VIRT and SHR, but only the parts of the library file containing the functions being used will actually be loaded in and be counted under RES.