Terms

• VSS - Virtual Set Size 虛擬耗用記憶體（包含共享庫佔用的記憶體）
• RSS - Resident Set Size 實際使用實體記憶體（包含共享庫佔用的記憶體）
• PSS - Proportional Set Size 實際使用的實體記憶體（比例分配共享庫佔用的記憶體）
• USS - Unique Set Size 程序獨自佔用的實體記憶體（不包含共享庫佔用的記憶體）

一般來說記憶體佔用大小有如下規律：VSS >= RSS >= PSS >= USS

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

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 leaks in a process.

For systems that have Python available, there is also a nice tool called smem that will report memory statistics including all of these categories.

# procrankprocrankPID      Vss        Rss           Pss         Uss cmdline481   31536K   30936K   14337K    9956K system_server475   26128K   26128K   10046K    5992K zygote526   25108K   25108K    9225K    5384K android.process.acore523   22388K   22388K    7166K    3432K com.android.phone574   21632K   21632K    6109K    2468K com.android.settings521   20816K   20816K    6050K    2776K jp.co.omronsoft.openwnn474    3304K    3304K    1097K     624K /system/bin/mediaserver37     304K     304K     289K     288K /sbin/adbd29     720K     720K     261K     212K /system/bin/rild601     412K     412K     225K     216K procrank   1     204K     204K     185K     184K /init35     388K     388K     182K     172K /system/bin/qemud284     384K     384K     160K     148K top27     376K     376K     148K     136K /system/bin/vold261     332K     332K     123K    112K logcat33     396K     396K     105K      80K /system/bin/keystore32     316K     316K     100K      88K /system/bin/installd269     328K     328K      95K      72K /system/bin/sh26     280K     280K      93K     84K /system/bin/servicemanager45     304K     304K      91K      80K /system/bin/qemu-props34     324K     324K      91K     68K /system/bin/sh260     324K     324K      91K      68K /system/bin/sh600     324K     324K      91K     68K /system/bin/sh25     308K     308K      88K      68K /system/bin/sh28     232K     232K      67K      60K /system/bin/debuggerd#