title: Is this explanation about VSS/RSS/PSS/USS accurate?
body: <p>I read an explanation about VSS/RSS/PSS/USS:</p>
  
  <hr>
  
  <p>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.</p>
  
  <p>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.</p>
  
  <p>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.</p>
  
  <p>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.</p>
  
  <p>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.</p>
  
  <p>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.</p>
  
  <p>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.</p>
  
  <p>For systems that have Python available, there is also a nice tool called smem that will report memory statistics including all of these categories.</p>
  
  <pre><code># 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
  #
  </code></pre>
  
  <hr>
  
  <p>But I cannot find the original of this article, and I would like to know whether this explanation is accurate. </p>