Android内存之VSS/RSS/PSS/USS

Terms

• VSS - Virtual Set Size 虚拟耗用内存（包含共享库占用的内存）
• RSS - Resident Set Size 实际使用物理内存（包含共享库占用的内存）
• PSS - Proportional Set Size 实际使用的物理内存（比例分配共享库占用的内存）
• 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）

一般来说内存占用大小有如下规律：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. 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 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.

# 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
#