Android Memory Usage

The memory of an Android system is managed by several different allocators, in several different pools.

System Memory
You can examine the system's view of the memory on the machine, by examining /proc/meminfo.

If you use 'ddms', you can see a summary of the memory used on the machine, by the system and by the different executing processes. Click on the SysInfo tab, and select "Memory Usage" in the box on the upper left of the pane.

Here's a screenshot:



Note that you can get the numbers for each process by hovering your mouse over a particular pie slice. Numbers are shown in K and percentages.

Process Memory
You can see an individual process' memory usage by examining /proc/ /status

Details about memory usage are in
 * /proc/ /statm
 * /proc/ /maps
 * /proc/ /smaps

The 'top' command will show VSS and RSS.

Also, see ddms info above.

procrank
procrank will show you a quick summary of process memory utilization. By default, it shows Vss, Rss, Pss and Uss, and sorts by Vss. However, you can control the sorting order.

procrank source is included in system/extras/procrank, and the binary is located in /system/xbin on an android device.


 * Vss = virtual set size
 * Rss = resident set size
 * Pss = proportional set size
 * Uss = unique set size

In general, the two numbers you want to watch are the Pss and Uss (Vss and Rss are generally worthless, because they don't accurately reflect a process's usage of pages shared with other processes.)

to the overall memory load.
 * Uss is the set of pages that are unique to a process. This is the amount of memory that would be freed if the application was terminated right now.
 * Pss is the amount of memory shared with other processes, accounted in a way that the amount is divided evenly between the processes that share it. This is memory that would not be released if the process was terminated, but is indicative of the amount that this process is "contributing"

You can also use procrank to view the working set size of each process, and to reset the working set size counters.

Here is procrank's usage: Usage: procrank [ -W ] [ -v | -r | -p | -u | -h ] -v Sort by VSS. -r Sort by RSS. -p Sort by PSS. -u Sort by USS. (Default sort order is PSS.) -R Reverse sort order (default is descending). -w Display statistics for working set only. -W Reset working set of all processes. -h Display this help screen.
 * 1) procrank -h

And here is some sample output: PID     Vss      Rss      Pss      Uss  cmdline 1217  36848K   35648K   17983K   13956K  system_server 1276  32200K   32200K   14048K   10116K  android.process.acore 1189  26920K   26920K    9293K    5500K  zygote 1321  20328K   20328K    4743K    2344K  android.process.media 1356  20360K   20360K    4621K    2148K  com.android.email 1303  20184K   20184K    4381K    1724K  com.android.settings 1271  19888K   19888K    4297K    1764K  com.android.inputmethod.latin 1332  19560K   19560K    3993K    1620K  com.android.alarmclock 1187   5068K    5068K    2119K    1476K  /system/bin/mediaserver 1384    436K     436K     248K     236K  procrank 1    212K     212K     200K     200K  /init 753    572K     572K     171K     136K  /system/bin/rild 748    340K     340K     163K     152K  /system/bin/sh 751    388K     388K     156K     140K  /system/bin/vold 1215    148K     148K     136K     136K  /sbin/adbd 757    352K     352K     117K      92K  /system/bin/dbus-daemon 760    404K     404K     104K      80K  /system/bin/keystore 759    312K     312K     102K      88K  /system/bin/installd 749    288K     288K      96K      84K  /system/bin/servicemanager 752    244K     244K      71K      60K  /system/bin/debuggerd
 * 1) procrank

In this example, it shows that the native daemons and programs are an order of magnitude smaller than the Dalvik-based services and programs. Also, even the smallest Dalvik program requires about 1.5 meg (Uss) to run.

smem tool
You can see very detailed per-process or systemwide memory information with smem.

See Using smem on Android

Dalvik Heap
The Dalvik heap is preloaded with classes and data by zygote (loading over 1900 classes as of Android version 2.2). When zygote forks to start an android application, the new application gets a copy-on-write mapping of this heap. As Dan Borstein says below, this helps with memory reduction as well as application startup time.

Dalvik, like virtual machines for many other languages, does garbage collection on the heap. There appears to be a separate thread (called the HeapWorker) in each VM process that performs the garbage collection actions. (See toolbox ps -t) [need more notes on the garbage collection]

Dan Borstein said this about heap sharing :  It's used in Android to amortize the RAM footprint of the large amount of effectively-read-only data (technically writable but rarely actually written) associated with common library classes across all active VM processes. 1000+ classes get preloaded by the system at boot time, and each class consumes at least a little heap for itself, including often pointing off to a constellation of other objects. The heap created by the preloading process gets shared copy-on-write with each spawned VM process (but again doesn't in practice get written much). This saves hundreds of kB of dirty unpageable RAM per process and also helps speed up process startup.

[INFO NEEDED: how to show dalvik heap info?]