xref: /frameworks/base/docs/html/training/articles/memory.jd

page.title=Managing Your App's Memory
page.tags=ram,low memory,OutOfMemoryError,onTrimMemory
page.article=true
@jd:body


<div id="tb-wrapper">
<div id="tb">

<h2>In this document</h2>
<ol class="nolist">
  <li><a href="#Android">How Android Manages Memory</a>
    <ol>
      <li><a href="#SharingRAM">Sharing Memory</a></li>
      <li><a href="#AllocatingRAM">Allocating and Reclaiming App Memory</a></li>
      <li><a href="#RestrictingMemory">Restricting App Memory</a></li>
      <li><a href="#SwitchingApps">Switching Apps</a></li>
    </ol>
  </li>
  <li><a href="#YourApp">How Your App Should Manage Memory</a>
    <ol>
      <li><a href="#Services">Use services sparingly</a></li>
      <li><a href="#ReleaseMemoryAsUiGone">Release memory when your user interface becomes hidden</a></li>
      <li><a href="#ReleaseMemoryAsTight">Release memory as memory becomes tight</a></li>
      <li><a href="#CheckHowMuchMemory">Check how much memory you should use</a></li>
      <li><a href="#Bitmaps">Avoid wasting memory with bitmaps</a></li>
      <li><a href="#DataContainers">Use optimized data containers</a></li>
      <li><a href="#Overhead">Be aware of memory overhead</a></li>
      <li><a href="#Abstractions">Be careful with code abstractions</a></li>
      <li><a href="#NanoProto">Use nano protobufs for serialized data</a></li>
      <li><a href="#DependencyInjection">Avoid dependency injection frameworks</a></li>
      <li><a href="#ExternalLibs">Be careful about using external libraries</a></li>
      <li><a href="#OverallPerf">Optimize overall performance</a></li>
      <li><a href="#Proguard">Use ProGuard to strip out any unneeded code</a></li>
      <li><a href="#Zipalign">Use zipalign on your final APK</a></li>
      <li><a href="#AnalyzeRam">Analyze your RAM usage</a></li>
      <li><a href="#MultipleProcesses">Use multiple processes</a></li>
    </ol>
  </li>
</ol>
<h2>See Also</h2>
<ul>
  <li><a href="{@docRoot}tools/debugging/debugging-memory.html">Investigating Your RAM Usage</a>
  </li>
</ul>

</div>
</div>


<p>Random-access memory (RAM) is a valuable resource in any software development environment, but
it's even more valuable on a mobile operating system where physical memory is often constrained.
Although Android's Dalvik virtual machine performs routine garbage collection, this doesn't allow
you to ignore when and where your app allocates and releases memory.</p>

<p>In order for the garbage collector to reclaim memory from your app, you need to avoid
introducing memory leaks (usually caused by holding onto object references in global members) and
release any {@link java.lang.ref.Reference} objects at the appropriate time (as defined by
lifecycle callbacks discussed further below). For most apps, the Dalvik garbage collector takes
care of the rest: the system reclaims your memory allocations when the corresponding objects leave
the scope of your app's active threads.</p>

<p>This document explains how Android manages app processes and memory allocation, and how you can
proactively reduce memory usage while developing for Android. For more information about general
practices to clean up your resources when programming in Java, refer to other books or online
documentation about managing resource references. If you’re looking for information about how to
analyze your app’s memory once you’ve already built it, read <a
href="{@docRoot}tools/debugging/debugging-memory.html">Investigating Your RAM Usage</a>.</p>




<h2 id="Android">How Android Manages Memory</h2>

<p>Android does not offer swap space for memory, but it does use <a href=
"http://en.wikipedia.org/wiki/Paging" class="external-link">paging</a> and <a href=
"http://en.wikipedia.org/wiki/Memory-mapped_files" class="external-link">memory-mapping</a>
(mmapping) to manage memory. This means that any memory you modify&mdash;whether by allocating
new objects or touching mmapped pages&mdash;remains resident in RAM and cannot be paged out.
So the only way to completely release memory from your app is to release object references you may
be holding, making the memory available to the garbage collector. That is with one exception:
any files mmapped in without modification, such as code, can be paged out of RAM if the system
wants to use that memory elsewhere.</p>


<h3 id="SharingRAM">Sharing Memory</h3>

<p>In order to fit everything it needs in RAM, Android tries to share RAM pages across processes. It
can do so in the following ways:</p>
<ul>
<li>Each app process is forked from an existing process called Zygote.
The Zygote process starts when the system boots and loads common framework code and resources
(such as activity themes). To start a new app process, the system forks the Zygote process then
loads and runs the app's code in the new process. This allows most of the RAM pages allocated for
framework code and resources to be shared across all app processes.</li>

<li>Most static data is mmapped into a process. This not only allows that same data to be shared
between processes but also allows it to be paged out when needed. Example static data include:
Dalvik code (by placing it in a pre-linked {@code .odex} file for direct mmapping), app resources
(by designing the resource table to be a structure that can be mmapped and by aligning the zip
entries of the APK), and traditional project elements like native code in {@code .so} files.</li>

<li>In many places, Android shares the same dynamic RAM across processes using explicitly allocated
shared memory regions (either with ashmem or gralloc). For example, window surfaces use shared
memory between the app and screen compositor, and cursor buffers use shared memory between the
content provider and client.</li>
</ul>

<p>Due to the extensive use of shared memory, determining how much memory your app is using requires
care. Techniques to properly determine your app's memory use are discussed in <a
href="{@docRoot}tools/debugging/debugging-memory.html">Investigating Your RAM Usage</a>.</p>


<h3 id="AllocatingRAM">Allocating and Reclaiming App Memory</h3>

<p>Here are some facts about how Android allocates then reclaims memory from your app:</p>

<ul>
<li>The Dalvik heap for each process is constrained to a single virtual memory range. This defines
the logical heap size, which can grow as it needs to (but only up to a limit that the system defines
for each app).</li>

<li>The logical size of the heap is not the same as the amount of physical memory used by the heap.
When inspecting your app's heap, Android computes a value called the Proportional Set Size (PSS),
which accounts for both dirty and clean pages that are shared with other processes&mdash;but only in an
amount that's proportional to how many apps share that RAM. This (PSS) total is what the system
considers to be your physical memory footprint. For more information about PSS, see the <a
href="{@docRoot}tools/debugging/debugging-memory.html#ViewingAllocations">Investigating Your
RAM Usage</a> guide.</li>

<li>The Dalvik heap does not compact the logical size of the heap, meaning that Android does not
defragment the heap to close up space. Android can only shrink the logical heap size when there
is unused space at the end of the heap. But this doesn't mean the physical memory used by the heap
can't shrink. After garbage collection, Dalvik walks the heap and finds unused pages, then returns
those pages to the kernel using madvise. So, paired allocations and deallocations of large
chunks should result in reclaiming all (or nearly all) the physical memory used. However,
reclaiming memory from small allocations can be much less efficient because the page used
for a small allocation may still be shared with something else that has not yet been freed.</li>
</ul>


<h3 id="RestrictingMemory">Restricting App Memory</h3>

<p>To maintain a functional multi-tasking environment, Android sets a hard limit on the heap size
for each app. The exact heap size limit varies between devices based on how much RAM the device
has available overall. If your app has reached the heap capacity and tries to allocate more
memory, it will receive an {@link java.lang.OutOfMemoryError}.</p>

<p>In some cases, you might want to query the system to determine exactly how much heap space you
have available on the current device&mdash;for example, to determine how much data is safe to keep in a
cache. You can query the system for this figure by calling {@link
android.app.ActivityManager#getMemoryClass()}. This returns an integer indicating the number of
megabytes available for your app's heap. This is discussed further below, under
<a href="#CheckHowMuchMemory">Check how much memory you should use</a>.</p>


<h3 id="SwitchingApps">Switching Apps</h3>

<p>Instead of using swap space when the user switches between apps, Android keeps processes that
are not hosting a foreground ("user visible") app component in a least-recently used (LRU) cache.
For example, when the user first launches an app, a process is created for it, but when the user
leaves the app, that process does <em>not</em> quit. The system keeps the process cached, so if
the user later returns to the app, the process is reused for faster app switching.</p>

<p>If your app has a cached process and it retains memory that it currently does not need,
then your app&mdash;even while the user is not using it&mdash;is constraining the system's
overall performance. So, as the system runs low on memory, it may kill processes in the LRU cache
beginning with the process least recently used, but also giving some consideration toward
which processes are most memory intensive. To keep your process cached as long as possible, follow
the advice in the following sections about when to release your references.</p>

<p>More information about how processes are cached while not running in the foreground and how
Android decides which ones
can be killed is available in the <a href="{@docRoot}guide/components/processes-and-threads.html"
>Processes and Threads</a> guide.</p>




<h2 id="YourApp">How Your App Should Manage Memory</h2>

<p>You should consider RAM constraints throughout all phases of development, including during app
design (before you begin development). There are many
ways you can design and write code that lead to more efficient results, through aggregation of the
same techniques applied over and over.</p>

<p>You should apply the following techniques while designing and implementing your app to make it
more memory efficient.</p>


<h3 id="Services">Use services sparingly</h3>

<p>If your app needs a <a href="{@docRoot}guide/components/services.html">service</a>
to perform work in the background, do not keep it running unless
it's actively performing a job. Also be careful to never leak your service by failing to stop it
when its work is done.</p>

<p>When you start a service, the system prefers to always keep the process for that service
running. This makes the process very expensive because the RAM used by the service can’t be used by
anything else or paged out. This reduces the number of cached processes that the system can keep in
the LRU cache, making app switching less efficient. It can even lead to thrashing in the system
when memory is tight and the system can’t maintain enough processes to host all the services
currently running.</p>

<p>The best way to limit the lifespan of your service is to use an {@link
android.app.IntentService}, which finishes
itself as soon as it's done handling the intent that started it. For more information, read
<a href="{@docRoot}training/run-background-service/index.html">Running in a Background Service</a>
.</p>

<p>Leaving a service running when it’s not needed is <strong>one of the worst memory-management
mistakes</strong> an Android app can make. So don’t be greedy by keeping a service for your app
running. Not only will it increase the risk of your app performing poorly due to RAM constraints,
but users will discover such misbehaving apps and uninstall them.</p>


<h3 id="ReleaseMemoryAsUiGone">Release memory when your user interface becomes hidden</h3>

<p>When the user navigates to a different app and your UI is no longer visible, you should
release any resources that are used by only your UI. Releasing UI resources at this time can
significantly increase the system's capacity for cached processes, which has a direct impact on the
quality of the user experience.</p>

<p>To be notified when the user exits your UI, implement the {@link
android.content.ComponentCallbacks2#onTrimMemory onTrimMemory()} callback in your {@link
android.app.Activity} classes. You should use this
method to listen for the {@link android.content.ComponentCallbacks2#TRIM_MEMORY_UI_HIDDEN} level,
which indicates your UI is now hidden from view and you should free resources that only your UI
uses.</p>


<p>Notice that your app receives the {@link android.content.ComponentCallbacks2#onTrimMemory
onTrimMemory()} callback with {@link android.content.ComponentCallbacks2#TRIM_MEMORY_UI_HIDDEN}
only when <em>all the UI components</em> of your app process become hidden from the user.
This is distinct
from the {@link android.app.Activity#onStop onStop()} callback, which is called when an {@link
android.app.Activity} instance becomes hidden, which occurs even when the user moves to
another activity in your app. So although you should implement {@link android.app.Activity#onStop
onStop()} to release activity resources such as a network connection or to unregister broadcast
receivers, you usually should not release your UI resources until you receive {@link
android.content.ComponentCallbacks2#onTrimMemory onTrimMemory(TRIM_MEMORY_UI_HIDDEN)}. This ensures
that if the user navigates <em>back</em> from another activity in your app, your UI resources are
still available to resume the activity quickly.</p>



<h3 id="ReleaseMemoryAsTight">Release memory as memory becomes tight</h3>

<p>During any stage of your app's lifecycle, the {@link
android.content.ComponentCallbacks2#onTrimMemory onTrimMemory()} callback also tells you when
the overall device memory is getting low. You should respond by further releasing resources based
on the following memory levels delivered by {@link android.content.ComponentCallbacks2#onTrimMemory
onTrimMemory()}:</p>

<ul>
<li>{@link android.content.ComponentCallbacks2#TRIM_MEMORY_RUNNING_MODERATE}
<p>Your app is running and not considered killable, but the device is running low on memory and the
system is actively killing processes in the LRU cache.</p>
</li>

<li>{@link android.content.ComponentCallbacks2#TRIM_MEMORY_RUNNING_LOW}
<p>Your app is running and not considered killable, but the device is running much lower on
memory so you should release unused resources to improve system performance (which directly
impacts your app's performance).</p>
</li>

<li>{@link android.content.ComponentCallbacks2#TRIM_MEMORY_RUNNING_CRITICAL}
<p>Your app is still running, but the system has already killed most of the processes in the
LRU cache, so you should release all non-critical resources now. If the system cannot reclaim
sufficient amounts of RAM, it will clear all of the LRU cache and begin killing processes that
the system prefers to keep alive, such as those hosting a running service.</p>
</li>
</ul>

<p>Also, when your app process is currently cached, you may receive one of the following
levels from {@link android.content.ComponentCallbacks2#onTrimMemory onTrimMemory()}:</p>
<ul>
<li>{@link android.content.ComponentCallbacks2#TRIM_MEMORY_BACKGROUND}
<p>The system is running low on memory and your process is near the beginning of the LRU list.
Although your app process is not at a high risk of being killed, the system may already be killing
processes in the LRU cache. You should release resources that are easy to recover so your process
will remain in the list and resume quickly when the user returns to your app.</p>
</li>

<li>{@link android.content.ComponentCallbacks2#TRIM_MEMORY_MODERATE}
<p>The system is running low on memory and your process is near the middle of the LRU list. If the
system becomes further constrained for memory, there's a chance your process will be killed.</p>
</li>

<li>{@link android.content.ComponentCallbacks2#TRIM_MEMORY_COMPLETE}
<p>The system is running low on memory and your process is one of the first to be killed if the
system does not recover memory now. You should release everything that's not critical to
resuming your app state.</p>

</li>
</ul>

<p>Because the {@link android.content.ComponentCallbacks2#onTrimMemory onTrimMemory()} callback was
added in API level 14, you can use the {@link android.content.ComponentCallbacks#onLowMemory()}
callback as a fallback for older versions, which is roughly equivalent to the {@link
android.content.ComponentCallbacks2#TRIM_MEMORY_COMPLETE} event.</p>

<p class="note"><strong>Note:</strong> When the system begins killing processes in the LRU cache,
although it primarily works bottom-up, it does give some consideration to which processes are
consuming more memory and will thus provide the system more memory gain if killed.
So the less memory you consume while in the LRU list overall, the better your chances are
to remain in the list and be able to quickly resume.</p>



<h3 id="CheckHowMuchMemory">Check how much memory you should use</h3>

<p>As mentioned earlier, each Android-powered device has a different amount of RAM available to the
system and thus provides a different heap limit for each app. You can call {@link
android.app.ActivityManager#getMemoryClass()} to get an estimate of your app's available heap in
megabytes. If your app tries to allocate more memory than is available here, it will receive an
{@link java.lang.OutOfMemoryError}.</p>

<p>In very special situations, you can request a larger heap size by setting the <a
href="{@docRoot}guide/topics/manifest/application-element.html#largeHeap">{@code largeHeap}</a>
attribute to "true" in the manifest <a
href="{@docRoot}guide/topics/manifest/application-element.html">{@code &lt;application&gt;}</a>
tag. If you do so, you can call {@link
android.app.ActivityManager#getLargeMemoryClass()} to get an estimate of the large heap size.</p>

<p>However, the ability to request a large heap is intended only for a small set of apps that can
justify the need to consume more RAM (such as a large photo editing app). <strong>Never request a
large heap simply because you've run out of memory</strong> and you need a quick fix&mdash;you
should use it only when you know exactly where all your memory is being allocated and why it must
be retained. Yet, even when you're confident your app can justify the large heap, you should avoid
requesting it to whatever extent possible. Using the extra memory will increasingly be to the
detriment of the overall user experience because garbage collection will take longer and system
performance may be slower when task switching or performing other common operations.</p>

<p>Additionally, the large heap size is not the same on all devices and, when running on
devices that have limited RAM, the large heap size may be exactly the same as the regular heap
size. So even if you do request the large heap size, you should call {@link
android.app.ActivityManager#getMemoryClass()} to check the regular heap size and strive to always
stay below that limit.</p>


<h3 id="Bitmaps">Avoid wasting memory with bitmaps</h3>

<p>When you load a bitmap, keep it in RAM only at the resolution you need for the current device's
screen, scaling it down if the original bitmap is a higher resolution. Keep in mind that an
increase in bitmap resolution results in a corresponding (increase<sup>2</sup>) in memory needed,
because both the X and Y dimensions increase.</p>

<p class="note"><strong>Note:</strong> On Android 2.3.x (API level 10) and below, bitmap objects
always appear as the same size in your app heap regardless of the image resolution (the actual
pixel data is stored separately in native memory). This makes it more difficult to debug the bitmap
memory allocation because most heap analysis tools do not see the native allocation. However,
beginning in Android 3.0 (API level 11), the bitmap pixel data is allocated in your app's Dalvik
heap, improving garbage collection and debuggability. So if your app uses bitmaps and you're having
trouble discovering why your app is using some memory on an older device, switch to a device
running Android 3.0 or higher to debug it.</p>

<p>For more tips about working with bitmaps, read <a
href="{@docRoot}training/displaying-bitmaps/manage-memory.html">Managing Bitmap Memory</a>.</p>


<h3 id="DataContainers">Use optimized data containers</h3>

<p>Take advantage of optimized containers in the Android framework, such as {@link
android.util.SparseArray}, {@link android.util.SparseBooleanArray}, and {@link
android.support.v4.util.LongSparseArray}. The generic {@link java.util.HashMap}
implementation can be quite memory
inefficient because it needs a separate entry object for every mapping. Additionally, the {@link
android.util.SparseArray} classes are more efficient because they avoid the system's need
to <acronym title=
"Automatic conversion from primitive types to object classes (such as int to Integer)"
>autobox</acronym>
the key and sometimes value (which creates yet another object or two per entry). And don't be
afraid of dropping down to raw arrays when that makes sense.</p>



<h3 id="Overhead">Be aware of memory overhead</h3>

<p>Be knowledgeable about the cost and overhead of the language and libraries you are using, and
keep this information in mind when you design your app, from start to finish. Often, things on the
surface that look innocuous may in fact have a large amount of overhead. Examples include:</p>
<ul>
<li>Enums often require more than twice as much memory as static constants. You should strictly
avoid using enums on Android.</li>

<li>Every class in Java (including anonymous inner classes) uses about 500 bytes of code.</li>

<li>Every class instance has 12-16 bytes of RAM overhead.</li>

<li>Putting a single entry into a {@link java.util.HashMap} requires the allocation of an
additional entry object that takes 32 bytes (see the previous section about <a
href="#DataContainers">optimized data containers</a>).</li>
</ul>

<p>A few bytes here and there quickly add up—app designs that are class- or object-heavy will suffer
from this overhead. That can leave you in the difficult position of looking at a heap analysis and
realizing your problem is a lot of small objects using up your RAM.</p>


<h3 id="Abstractions">Be careful with code abstractions</h3>

<p>Often, developers use abstractions simply as a "good programming practice," because abstractions
can improve code flexibility and maintenance. However, abstractions come at a significant cost:
generally they require a fair amount more code that needs to be executed, requiring more time and
more RAM for that code to be mapped into memory. So if your abstractions aren't supplying a
significant benefit, you should avoid them.</p>


<h3 id="NanoProto">Use nano protobufs for serialized data</h3>

<p><a href="https://developers.google.com/protocol-buffers/docs/overview">Protocol
buffers</a> are a language-neutral, platform-neutral, extensible mechanism designed by Google for
serializing structured data&mdash;think XML, but smaller, faster, and simpler. If you decide to use
protobufs for your data, you should always use nano protobufs in your client-side code. Regular
protobufs generate extremely verbose code, which will cause many kinds of problems in your app:
increased RAM use, significant APK size increase, slower execution, and quickly hitting the DEX
symbol limit.</p>

<p>For more information, see the "Nano version" section in the <a
href="https://android.googlesource.com/platform/external/protobuf/+/master/java/README.txt"
class="external-link">protobuf readme</a>.</p>



<h3 id="DependencyInjection">Avoid dependency injection frameworks</h3>

<p>Using a dependency injection framework such as <a
href="https://code.google.com/p/google-guice/" class="external-link">Guice</a> or
<a href="https://github.com/roboguice/roboguice" class="external-link">RoboGuice</a> may be
attractive because they can simplify the code you write and provide an adaptive environment
that's useful for testing and other configuration changes. However, these frameworks tend to perform
a lot of process initialization by scanning your code for annotations, which can require significant
amounts of your code to be mapped into RAM even though you don't need it. These mapped pages are
allocated into clean memory so Android can drop them, but that won't happen until the pages have
been left in memory for a long period of time.</p>


<h3 id="ExternalLibs">Be careful about using external libraries</h3>

<p>External library code is often not written for mobile environments and can be inefficient when used
for work on a mobile client. At the very least, when you decide to use an external library, you
should assume you are taking on a significant porting and maintenance burden to optimize the
library for mobile. Plan for that work up-front and analyze the library in terms of code size and
RAM footprint before deciding to use it at all.</p>

<p>Even libraries supposedly designed for use on Android are potentially dangerous because each
library may do things differently. For example, one library may use nano protobufs while another
uses micro protobufs. Now you have two different protobuf implementations in your app. This can and
will also happen with different implementations of logging, analytics, image loading frameworks,
caching, and all kinds of other things you don't expect. <a
href="{@docRoot}tools/help/proguard.html">ProGuard</a> won't save you here because these
will all be lower-level dependencies that are required by the features for which you want the
library. This becomes especially problematic when you use an {@link android.app.Activity}
subclass from a library (which
will tend to have wide swaths of dependencies), when libraries use reflection (which is common and
means you need to spend a lot of time manually tweaking ProGuard to get it to work), and so on.</p>

<p>Also be careful not to fall into the trap of using a shared library for one or two features out of
dozens of other things it does; you don't want to pull in a large amount of code and overhead that
you don't even use. At the end of the day, if there isn't an existing implementation that is a
strong match for what you need to do, it may be best if you create your own implementation.</p>


<h3 id="OverallPerf">Optimize overall performance</h3>

<p>A variety of information about optimizing your app's overall performance is available
in other documents listed in <a href="{@docRoot}training/best-performance.html">Best Practices
for Performance</a>. Many of these documents include optimizations tips for CPU performance, but
many of these tips also help optimize your app's memory use, such as by reducing the number of
layout objects required by your UI.</p>

<p>You should also read about <a href="{@docRoot}tools/debugging/debugging-ui.html">optimizing
your UI</a> with the layout debugging tools and take advantage of
the optimization suggestions provided by the <a
href="{@docRoot}tools/debugging/improving-w-lint.html">lint tool</a>.</p>


<h3 id="Proguard">Use ProGuard to strip out any unneeded code</h3>

<p>The <a href="{@docRoot}tools/help/proguard.html">ProGuard</a> tool shrinks,
optimizes, and obfuscates your code by removing unused code and renaming classes, fields, and
methods with semantically obscure names. Using ProGuard can make your code more compact, requiring
fewer RAM pages to be mapped.</p>


<h3 id="Zipalign">Use zipalign on your final APK</h3>

<p>If you do any post-processing of an APK generated by a build system (including signing it
with your final production certificate), then you must run <a
href="{@docRoot}tools/help/zipalign.html">zipalign</a> on it to have it re-aligned.
Failing to do so can cause your app to require significantly more RAM, because things like
resources can no longer be mmapped from the APK.</p>

<p class="note"><strong>Note:</strong> Google Play Store does not accept APK files that
are not zipaligned.</p>


<h3 id="AnalyzeRam">Analyze your RAM usage</h3>

<p>Once you achieve a relatively stable build, begin analyzing how much RAM your app is using
throughout all stages of its lifecycle. For information about how to analyze your app, read <a
href="{@docRoot}tools/debugging/debugging-memory.html">Investigating Your RAM Usage</a>.</p>




<h3 id="MultipleProcesses">Use multiple processes</h3>

<p>If it's appropriate for your app, an advanced technique that may help you manage your app's
memory is dividing components of your app into multiple processes. This technique must always be
used carefully and <strong>most apps should not run multiple processes</strong>, as it can easily
increase&mdash;rather than decrease&mdash;your RAM footprint if done incorrectly. It is primarily
useful to apps that may run significant work in the background as well as the foreground and can
manage those operations separately.</p>


<p>An example of when multiple processes may be appropriate is when building a music player that
plays music from a service for long period of time. If
the entire app runs in one process, then many of the allocations performed for its activity UI must
be kept around as long as it is playing music, even if the user is currently in another app and the
service is controlling the playback. An app like this may be split into two process: one for its
UI, and the other for the work that continues running in the background service.</p>

<p>You can specify a separate process for each app component by declaring the <a href=
"{@docRoot}guide/topics/manifest/service-element.html#proc">{@code android:process}</a> attribute
for each component in the manifest file. For example, you can specify that your service should run
in a process separate from your app's main process by declaring a new process named "background"
(but you can name the process anything you like):</p>

<pre>
&lt;service android:name=".PlaybackService"
         android:process=":background" />
</pre>

<p>Your process name should begin with a colon (':') to ensure that the process remains private to
your app.</p>

<p>Before you decide to create a new process, you need to understand the memory implications.
To illustrate the consequences of each process, consider that an empty process doing basically
nothing has an extra memory footprint of about 1.4MB, as shown by the memory information
dump below.</p>

<pre class="no-pretty-print">
adb shell dumpsys meminfo com.example.android.apis:empty

** MEMINFO in pid 10172 [com.example.android.apis:empty] **
                Pss     Pss  Shared Private  Shared Private    Heap    Heap    Heap
              Total   Clean   Dirty   Dirty   Clean   Clean    Size   Alloc    Free
             ------  ------  ------  ------  ------  ------  ------  ------  ------
  Native Heap     0       0       0       0       0       0    1864    1800      63
  Dalvik Heap   764       0    5228     316       0       0    5584    5499      85
 Dalvik Other   619       0    3784     448       0       0
        Stack    28       0       8      28       0       0
    Other dev     4       0      12       0       0       4
     .so mmap   287       0    2840     212     972       0
    .apk mmap    54       0       0       0     136       0
    .dex mmap   250     148       0       0    3704     148
   Other mmap     8       0       8       8      20       0
      Unknown   403       0     600     380       0       0
        TOTAL  2417     148   12480    1392    4832     152    7448    7299     148
</pre>

<p class="note"><strong>Note:</strong> More information about how to read this output is provided
in <a href="{@docRoot}tools/debugging/debugging-memory.html#ViewingAllocations">Investigating
Your RAM Usage</a>. The key data here is the <em>Private Dirty</em> and <em>Private
Clean</em> memory, which shows that this process is using almost 1.4MB of non-pageable RAM
(distributed across the Dalvik heap, native allocations, book-keeping, and library-loading),
and another 150K of RAM for code that has been mapped in to execute.</p>

<p>This memory footprint for an empty process is fairly significant and it can quickly
grow as you start doing work in that process. For
example, here is the memory use of a process that is created only to show an activity with some
text in it:</p>

<pre class="no-pretty-print">
** MEMINFO in pid 10226 [com.example.android.helloactivity] **
                Pss     Pss  Shared Private  Shared Private    Heap    Heap    Heap
              Total   Clean   Dirty   Dirty   Clean   Clean    Size   Alloc    Free
             ------  ------  ------  ------  ------  ------  ------  ------  ------
  Native Heap     0       0       0       0       0       0    3000    2951      48
  Dalvik Heap  1074       0    4928     776       0       0    5744    5658      86
 Dalvik Other   802       0    3612     664       0       0
        Stack    28       0       8      28       0       0
       Ashmem     6       0      16       0       0       0
    Other dev   108       0      24     104       0       4
     .so mmap  2166       0    2824    1828    3756       0
    .apk mmap    48       0       0       0     632       0
    .ttf mmap     3       0       0       0      24       0
    .dex mmap   292       4       0       0    5672       4
   Other mmap    10       0       8       8      68       0
      Unknown   632       0     412     624       0       0
        TOTAL  5169       4   11832    4032   10152       8    8744    8609     134
</pre>

<p>The process has now almost tripled in size, to 4MB, simply by showing some text in the UI. This
leads to an important conclusion: If you are going to split your app into multiple processes, only
one process should be responsible for UI. Other processes should avoid any UI, as this will quickly
increase the RAM required by the process (especially once you start loading bitmap assets and other
resources). It may then be hard or impossible to reduce the memory usage once the UI is drawn.</p>

<p>Additionally, when running more than one process, it's more important than ever that you keep your
code as lean as possible, because any unnecessary RAM overhead for common implementations are now
replicated in each process. For example, if you are using enums (though <a
href="#Overhead">you should not use enums</a>), all of
the RAM needed to create and initialize those constants is duplicated in each process, and any
abstractions you have with adapters and temporaries or other overhead will likewise be replicated.</p>

<p>Another concern with multiple processes is the dependencies that exist between them. For example,
if your app has a content provider that you have running in the default process which also hosts
your UI, then code in a background process that uses that content provider will also require that
your UI process remain in RAM. If your goal is to have a background process that can run
independently of a heavy-weight UI process, it can't have dependencies on content providers or
services that execute in the UI process.</p>










<!-- THE FOLLOWING IS OVERWHELMING AND NOT NECESSARY FOR MOST APPS, LEAVING OUT FOR NOW


<p>You can examine the dependencies between your processes with the command:</p>

<pre class="no-pretty-print">
adb shell dumpsys activity
</pre>

<p>This dumps various information about the Activity Manager's state, ending with a list of all
processes in their memory management order, including the reason each process is at its given
level. For example, below is a dump with the Music app in the foreground.</p>

<pre class="no-pretty-print">
ACTIVITY MANAGER RUNNING PROCESSES (dumpsys activity processes)
  Process LRU list (sorted by oom_adj):
    PERS # 4: adj=sys  /F  trm= 0 20674:system/1000 (fixed)
    PERS #39: adj=pers /F  trm= 0 20964:com.android.nfc/1027 (fixed)
    PERS # 2: adj=pers /F  trm= 0 20959:com.android.phone/1001 (fixed)
    PERS # 1: adj=pers /F  trm= 0 20779:com.android.systemui/u0a10057 (fixed)
    Proc #11: adj=fore /FA trm= 0 8663:com.google.android.music:ui/u0a10043 (top-activity)
    Proc #10: adj=fore /F  trm= 0 30881:com.google.android.music:main/u0a10043 (provider)
        com.google.android.music/.store.MusicContentProvider<=Proc{8663:com.google.android.music:ui/u0a10043}
    Proc # 6: adj=fore /F  trm= 0 21014:com.google.process.gapps/u0a10023 (provider)
        com.google.android.gsf/.settings.GoogleSettingsProvider<=Proc{20935:com.google.process.location/u0a10023}
    Proc #38: adj=vis  /F  trm= 0 21028:com.android.nfc:handover/1027 (service)
        com.android.nfc/.handover.HandoverService<=Proc{20964:com.android.nfc/1027}
    Proc # 7: adj=vis  /B  trm= 0 20935:com.google.process.location/u0a10023 (service)
        com.google.android.location/.GeocodeService<=Proc{20674:system/1000}
    Proc # 3: adj=vis  /F  trm= 0 21225:com.android.bluetooth/1002 (service)
        com.android.bluetooth/.hfp.HeadsetService<=Proc{20674:system/1000}
    Proc # 0: adj=vis  /F  trm= 0 20908:com.google.android.inputmethod.latin/u0a10035 (service)
        com.google.android.inputmethod.latin/com.android.inputmethod.latin.LatinIME<=Proc{20674:system/1000}
    Proc #34: adj=svc  /B  trm= 0 16765:com.google.android.apps.currents/u0a10012 (started-services)
    Proc #14: adj=svc  /B  trm= 0 21148:com.google.android.gms/u0a10023 (started-services)
    Proc #12: adj=home /B  trm= 0 20989:com.android.launcher/u0a10036 (home)
    Proc #37: adj=svcb /B  trm= 0 15194:com.google.android.apps.googlevoice/u0a10089 (started-services)
    Proc #17: adj=svcb /B  trm= 0 24537:android.process.media/u0a10016 (started-services)
    Proc #35: adj=bak  /B  trm= 0 16087:com.android.defcontainer/u0a10013 (service)
        com.android.defcontainer/.DefaultContainerService<=Proc{16050:com.android.settings/1000}
    Proc #16: adj=bak  /B  trm= 0 7334:com.google.android.gm/u0a10022 (bg-act)
    Proc #15: adj=bak  /B  trm= 0 22499:com.google.android.googlequicksearchbox/u0a10060 (bg-act)
    Proc # 9: adj=bak  /B  trm= 0 20856:com.google.android.gsf.login/u0a10023 (bg-empty)
    Proc #26: adj=bak+1/B  trm= 0 9923:com.android.mms/u0a10042 (bg-act)
    Proc #23: adj=bak+1/B  trm= 0 16721:com.android.chrome/u0a10010 (bg-act)
    Proc #22: adj=bak+1/B  trm= 0 17596:com.android.chrome:sandboxed_process0/u0a10010i33 (service)
        com.android.chrome/org.chromium.content.app.SandboxedProcessService0<=Proc{16721:com.android.chrome/u0a10010}
    Proc #19: adj=bak+1/B  trm= 0 17442:com.google.android.youtube/u0a10067 (bg-services)
    Proc #18: adj=bak+2/B  trm= 0 16740:com.google.android.apps.plus/u0a10052 (bg-empty)
    Proc #13: adj=bak+2/B  trm= 0 7707:com.android.musicfx/u0a10044 (bg-empty)
    Proc #36: adj=bak+3/B  trm= 0 16050:com.android.settings/1000 (bg-act)
    Proc #33: adj=bak+3/B  trm= 0 16863:com.android.dialer/u0a10015 (bg-act)
</pre>


<p class="note"><strong>Note:</strong> The exact details of what is shown here will vary across
platform versions as process management policies are tweaked and improved.</p>


<p>Details on the highlighted sections are:</p>

<ol>
<li>Foreground app: This is the current app running in the foreground -- it is in the "fore" memory
class because it is the top activity on the activity stack.</li>

<li>Persistent processes: These are processes that are part of the core system that must always be
running.</li>

<li>Dependent process: This shows how the Music app is using two processes. Its UI process has a
dependency on the "main" process (through a content provider). So while the UI process is in use,
the main process must also be kept around. This means the app's memory footprint is actually the
sum of both processes. You will have this kind of connection on a content provider any time you
have active calls into it or have unclosed cursors or file streams that came from it.</li>

<li>Visible processes: These are processes that count in some way as "visible" to the user. This
generally means that it is either something the user can literally see (such as a process hosting a
paused but visible activity that is behind a non-full-screen dialog) or is something the user might
notice if the process disappeared (such as a foreground service playing music). You should be
certain that any process you have running at the "visible" level is indeed critical to the user,
because they are very expensive to the overall RAM load.</li>

<li>Service processes: These are processes running long-term jobs in a service. This level of the
list is the start of less-critical processes, which the system has some freedom to kill if RAM is
needed elsewhere. These services are still quite expensive because they can be killed only
temporarily and the system tries to keep them running whenever possible.</li>

<li>Home process: A special slot for the process that hosts the current Home activity, to try to
prevent it from being killed as much as possible. Killing this process is much more damaging to the
user experience than killing other cached processes, because so much user interaction goes through
home.</li>

<li>Secondary service processes: These are services that have been running for a relatively long time
and so should be killed more aggressively when RAM is needed elsewhere.</li>

<li>Cached processes: These are cached processes held in the LRU cache, which allow for fast app
switching and component launching. These processes are not required and the system will kill them
as needed to reclaim memory. You will often see a process hosting a running service here—this is
part of a platform policy of allowing very long-running services to drop down into the LRU list and
eventually be killed. If the service should continue running (as defined by the {@link
android.app.Service#onStartCommand onStartCommand()} return value, such as {@link
android.app.Service#START_STICKY}), the the system eventually restarts it. This avoids issues with
such services having memory leaks that over time reduce the number of regular cached processes that
can be kept.</li>

</ol>

<p>This numbered list of processes is essentially the LRU list of processes that the framework
provides to the kernel to help it determine which processes it should kill as it needs more RAM.
The kernel's out of memory killer will generally begin from the bottom of this list, killing the
last process and working its way up. It may not do it in exactly this order, as it can also take
into consideration other factors such as the relative RAM footprint of processes to some degree.</p>

<p>There are many other options you can use with the activity command to analyze further details of
your app's state&mdash;use <code>adb shell dumpsys activity -h</code> for help on its use.</p>

-->