- VSS
VSS（Virtual Set Size）是进程向系统申请的虚拟内存（包含共享库内存总数），即单个进程全部可访问的地址空间，其大小可能包括还尚未在内存中驻留的部分。 - RSS
RSS（Resident Set Size）是进程在 RAM 中实际保存的总内存（包含共享库占用的共享内存总数）。即单个进程实际占用内存大小，RSS 可能会产生误导，因为包含了共享库占用的共享内存总数。然而实际上一个共享库仅会被加载到内存中一次，无论被多少个进程使用。所以，RSS 不能准确的表示单个进程的内存占用情况。 - PSS
PSS（Proportional Set Size）是单个进程运行时实际占用的物理内存（包含比例分配共享库占用的内存）。对比 RSS 来说，PSS 中的共享库内存是按照比例计算的。一个共享库有 N 个进程使用，那么该库比例分配给 PSS 的大小为：1/N；
PSS 明确的表示了单个进程在系统总内存中的实际使用量。 - USS:
USS （Unique Set Size）进程独自占用的物理内存（不包含共享库占用的内存）表示一个进程本身占用的内存空间大小，不包含其它任何成分，这是表示进程内存大小的最好方式！
USS是一个非常有用的数字，因为它表示运行特定进程的真正内存增量成本。当进程被终止时，USS是实际返回到系统的总内存。 USS是判断进程中的内存泄漏时最值得注意的数字。
Overview
The aim of this post is to provide information that will assist in interpreting memory reports from various tools so the true memory usage for Linux processes and the system can be determined.
Android has a tool called procrank (/system/xbin/procrank), which lists out the memory usage of Linux processes in order from highest to lowest usage. The sizes reported per process are VSS, RSS, PSS, and USS.
For the sake of simplicity in this description, memory will be expressed in terms of pages, rather than bytes. Linux systems like ours manage memory in 4096 byte pages at the lowest level.
VSS (reported as VSZ from ps) is the total accessible address space of a process.This size also includes memory that may not be resident in RAM like mallocs that have been allocated but not written to. VSS is of very little use for determing real memory usage of a process.
RSS is the total memory actually held in RAM for a process.RSS can be misleading, because it reports the total all of the shared libraries that the process uses, even though a shared library is only loaded into memory once regardless of how many processes use it. RSS is not an accurate representation of the memory usage for a single process.
PSS differs from RSS in that it reports the proportional size of its shared libraries, i.e. if three processes all use a shared library that has 30 pages, that library will only contribute 10 pages to the PSS that is reported for each of the three processes. PSS is a very useful number because when the PSS for all processes in the system are summed together, that is a good representation for the total memory usage in the system. When a process is killed, the shared libraries that contributed to its PSS will be proportionally distributed to the PSS totals for the remaining processes still using that library. In this way PSS can be slightly misleading, because when a process is killed, PSS does not accurately represent the memory returned to the overall system.
USS is the total private memory for a process, i.e. that memory that is completely unique to that process.USS is an extremely useful number because it indicates the true incremental cost of running a particular process. When a process is killed, the USS is the total memory that is actually returned to the system. USS is the best number to watch when initially suspicious of memory leaksin a process.
For systems that have Python available, there is also a nice tool calledsmem that will report memory statistics including all of these categories.
# procrank
procrank
PID Vss Rss Pss Uss cmdline
481 31536K 30936K 14337K 9956K system_server
475 26128K 26128K 10046K 5992K zygote
526 25108K 25108K 9225K 5384K android.process.acore
523 22388K 22388K 7166K 3432K com.android.phone
574 21632K 21632K 6109K 2468K com.android.settings
521 20816K 20816K 6050K 2776K jp.co.omronsoft.openwnn
474 3304K 3304K 1097K 624K /system/bin/mediaserver
37 304K 304K 289K 288K /sbin/adbd
29 720K 720K 261K 212K /system/bin/rild
601 412K 412K 225K 216K procrank
1 204K 204K 185K 184K /init
35 388K 388K 182K 172K /system/bin/qemud
284 384K 384K 160K 148K top
27 376K 376K 148K 136K /system/bin/vold
261 332K 332K 123K 112K logcat
33 396K 396K 105K 80K /system/bin/keystore
32 316K 316K 100K 88K /system/bin/installd
269 328K 328K 95K 72K /system/bin/sh
26 280K 280K 93K 84K /system/bin/servicemanager
45 304K 304K 91K 80K /system/bin/qemu-props
34 324K 324K 91K 68K /system/bin/sh
260 324K 324K 91K 68K /system/bin/sh
600 324K 324K 91K 68K /system/bin/sh
25 308K 308K 88K 68K /system/bin/sh
28 232K 232K 67K 60K /system/bin/debuggerd
