+ The Android Runtime (ART) and Dalvik virtual machine use + paging + and memory-mapping + (mmapping) to manage memory. This means that any memory an app + modifies—whether by allocating + new objects or touching mmapped pages—remains resident in RAM and + cannot be paged out. The only way to release memory from an app is to release + object references that the app holds, 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. +
+ ++ This page explains how Android manages app processes and memory + allocation. For more information about how to manage memory more efficiently + in your app, see + Manage Your App's Memory. +
+ + + ++ A managed memory environment, like the ART or Dalvik virtual machine, + keeps track of each memory allocation. Once it determines + that a piece of memory is no longer being used by the program, + it frees it back to the heap, without any intervention from the programmer. + The mechanism for reclaiming unused memory + within a managed memory environment + is known as garbage collection. Garbage collection has two goals: + find data objects in a program that cannot be accessed in the future; and + reclaim the resources used by those objects. +
+ ++ Android’s memory heap is a generational one, meaning that there are + different buckets of allocations that it tracks, + based on the expected life and size of an object being allocated. + For example, recently allocated objects belong in the Young generation. + When an object stays active long enough, it can be promoted + to an older generation, followed by a permanent generation. +
+ ++ Each heap generation has its own dedicated upper limit on the amount + of memory that objects there can occupy. Any time a generation starts + to fill up, the system executes a garbage collection + event in an attempt to free up memory. The duration of the garbage collection + depends on which generation of objects it's collecting + and how many active objects are in each generation. +
+ ++ Even though garbage collection can be quite fast, it can still + affect your app's performance. You don’t generally control + when a garbage collection event occurs from within your code. + The system has a running set of criteria for determining when to perform + garbage collection. When the criteria are satisfied, + the system stops executing the process and begins garbage collection. If + garbage collection occurs in the middle of an intensive processing loop + like an animation or during music playback, it can increase processing time. + This increase can potentially push code execution in your app past the + recommended 16ms threshold for efficient and smooth frame rendering. +
+ ++ Additionally, your code flow may perform kinds of work that + force garbage collection events to occur + more often or make them last longer-than-normal. + For example, if you allocate multiple objects in the + innermost part of a for-loop during each frame of an alpha + blending animation, you might pollute your memory heap with a + lot of objects. + In that circumstance, the garbage collector executes multiple garbage + collection events and can degrade the performance of your app. +
+ ++ For more general information about garbage collection, see + Garbage collection. +
+ + + ++ 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: +
+ +.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 .so files.
+ + 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 + Investigating Your RAM Usage. +
+ + + ++ The Dalvik heap is constrained to a + single virtual memory range for each app process. 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. +
+ ++ 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—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 + Investigating Your RAM Usage + guide. +
+ ++ 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. However, + the system can still reduce physical memory used by the heap. + 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. + +
+ + + ++ 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 can receive an {@link java.lang.OutOfMemoryError}. +
+ ++ In some cases, you might want to query the + system to determine exactly how much heap space you + have available on the current device—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 method returns an integer indicating the number of + megabytes available for your app's heap. +
+ + + ++ When users switch between apps, + Android keeps apps that + are not foreground—that is, not visible to the user or running a + foreground service like music playback— + in a least-recently used (LRU) cache. + For example, when a user first launches an app, + a process is created for it; but when the user + leaves the app, that process does not quit. + The system keeps the process cached. If + the user later returns to the app, the system reuses the process, thereby + making the app switching faster. +
+ ++ If your app has a cached process and it retains memory + that it currently does not need, + then your app—even while the user is not using it— + affects the system's + overall performance. As the system runs low on memory, + it kills processes in the LRU cache + beginning with the process least recently used. The system also + accounts for processes that hold onto the most memory + and can terminate them to free up RAM. +
+ ++ Note: When the system begins killing processes in the + LRU cache, it primarily works bottom-up. The system also considers which + processes consume more memory and thus provide the system + more memory gain if killed. + 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. +
+ ++ For more information about how processes are cached while + not running in the foreground and how + Android decides which ones + can be killed, see the + Processes and Threads + guide. +
\ No newline at end of file diff --git a/docs/html/training/articles/memory.jd b/docs/html/training/articles/memory.jd index de7af589aefd0..885112168e252 100644 --- a/docs/html/training/articles/memory.jd +++ b/docs/html/training/articles/memory.jd @@ -1,4 +1,4 @@ -page.title=Managing Your App's Memory +page.title=Manage Your App's Memory page.tags=ram,low memory,OutOfMemoryError,onTrimMemory page.article=true @jd:body @@ -9,732 +9,586 @@ page.article=trueRandom-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.
- -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.
- -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 Investigating Your RAM Usage.
- - - - -Android does not offer swap space for memory, but it does use paging and memory-mapping -(mmapping) to manage memory. This means that any memory you modify—whether by allocating -new objects or touching mmapped pages—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.
- - -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:
-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 Investigating Your RAM Usage.
- - -Here are some facts about how Android allocates then reclaims memory from your app:
- -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}.
- -In some cases, you might want to query the system to determine exactly how much heap space you -have available on the current device—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 -Check how much memory you should use.
- - -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 not quit. The system keeps the process cached, so if -the user later returns to the app, the process is reused for faster app switching.
- -If your app has a cached process and it retains memory that it currently does not need, -then your app—even while the user is not using it—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.
- -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 Processes and Threads guide.
- - - - -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.
- -You should apply the following techniques while designing and implementing your app to make it -more memory efficient.
- - -If your app needs a service -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.
- -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.
- -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 -Running in a Background Service -.
- -Leaving a service running when it’s not needed is one of the worst memory-management -mistakes 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.
- - -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.
- -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.
- - -Notice that your app receives the {@link android.content.ComponentCallbacks2#onTrimMemory -onTrimMemory()} callback with {@link android.content.ComponentCallbacks2#TRIM_MEMORY_UI_HIDDEN} -only when all the UI components 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 back from another activity in your app, your UI resources are -still available to resume the activity quickly.
- - - -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()}:
- -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.
-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).
-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.
-Also, when your app process is currently cached, you may receive one of the following -levels from {@link android.content.ComponentCallbacks2#onTrimMemory onTrimMemory()}:
-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.
-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.
-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.
- -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.
- -Note: 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.
- - - -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}.
- -In very special situations, you can request a larger heap size by setting the {@code largeHeap}
-attribute to "true" in the manifest {@code
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). Never request a -large heap simply because you've run out of memory and you need a quick fix—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.
- -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.
- - -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 (increase2) in memory needed, -because both the X and Y dimensions increase.
- -Note: 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.
- -For more tips about working with bitmaps, read Managing Bitmap Memory.
- - -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 autobox -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.
- - - -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:
-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.
- - -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.
- - -Protocol -buffers are a language-neutral, platform-neutral, extensible mechanism designed by Google for -serializing structured data—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.
- -For more information, see the "Nano version" section in the protobuf readme.
- - - -Using a dependency injection framework such as Guice or -RoboGuice 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.
- - -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.
- -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. ProGuard 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.
- -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.
- - -A variety of information about optimizing your app's overall performance is available -in other documents listed in Best Practices -for Performance. 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.
- -You should also read about optimizing -your UI with the layout debugging tools and take advantage of -the optimization suggestions provided by the lint tool.
- - -The ProGuard 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.
- - -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 zipalign 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.
- -Note: Google Play Store does not accept APK files that -are not zipaligned.
- - -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 Investigating Your RAM Usage.
- - - - -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 most apps should not run multiple processes, as it can easily -increase—rather than decrease—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.
- - -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.
- -You can specify a separate process for each app component by declaring the {@code android:process} 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):
- --<service android:name=".PlaybackService" - android:process=":background" /> -- -
Your process name should begin with a colon (':') to ensure that the process remains private to -your app.
- -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.
- --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 -- -
Note: More information about how to read this output is provided -in Investigating -Your RAM Usage. The key data here is the Private Dirty and Private -Clean 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.
- -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:
- --** 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 -- -
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.
- -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 you should not use enums), 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.
- -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.
- - - - - - - - - - - + ++ 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 both the Android Runtime (ART) and Dalvik virtual machine perform + routine garbage collection, this does not mean you can ignore + when and where your app allocates and releases memory. + You still need to avoid + introducing memory leaks, usually caused by holding onto + object references in static member variables, and + release any {@link java.lang.ref.Reference} objects at the appropriate + time as defined by + lifecycle callbacks. +
+ ++ This page explains how you can + proactively reduce memory usage within your app. + 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 memory in a running app, read + Tools for analyzing RAM usage. + For more detailed information about how the Android Runtime and Dalvik + virtual machine manage memory, see the + Overview of Android Memory Management. +
+ + + ++ The Android framework, Android Studio, and Android SDK + can help you analyze and adjust your app's memory usage. + The Android framework + exposes several APIs that allow your app to reduce its memory usage + dynamically during runtime. Android Studio and the Android SDK + contain several tools that allow you to investigate how your + app uses memory. +
+ + + ++ Before you can fix the memory usage problems in your app, you first need + to find them. Android Studio and the Android SDK include several tools + for analyzing memory usage in your app: +
For more information about how to use the DDMS tool, see + Using DDMS. +
+ -+ For more information about how to use Memory Monitor tool, see + Viewing Heap Updates. +
++ For more information about how to use the Traceview viewer, see + Profiling with Traceview and dmtracedump. +
++ For more information about how to use the Allocation Tracker tool, see + Allocation Tracker Walkthrough. +
+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.
+ -There are many other options you can use with the activity command to analyze further details of
-your app's state—use adb shell dumpsys activity -h for help on its use.
+ An Android device can run with varying amounts of free memory + depending on the physical amount of RAM on the device and how the user + operates it. The system broadcasts signals to indicate when it is under + memory pressure, and apps should listen for these signals and adjust + their memory usage as appropriate. +
+ + + You can use the {@link android.content.ComponentCallbacks2} API + to listen for these signals and then adjust your memory + usage in response to app lifecycle + or device events. The + {@link android.content.ComponentCallbacks2#onTrimMemory onTrimMemory()} + method allows your app to listen for memory related events when the app runs + in the foreground (is visible) and when it runs in the background. + + ++ To listen for these events, implement the {@link + android.content.ComponentCallbacks2#onTrimMemory onTrimMemory()} + callback in your {@link android.app.Activity} + classes, as shown in the following code snippet. +
+ +
+import android.content.ComponentCallbacks2;
+// Other import statements ...
+
+public class MainActivity extends AppCompatActivity
+ implements ComponentCallbacks2 {
+
+ // Other activity code ...
+
+ /**
+ * Release memory when the UI becomes hidden or when system resources become low.
+ * @param level the memory-related event that was raised.
+ */
+ public void onTrimMemory(int level) {
+
+ // Determine which lifecycle or system event was raised.
+ switch (level) {
+
+ case ComponentCallbacks2.TRIM_MEMORY_UI_HIDDEN:
+
+ /*
+ Release any UI objects that currently hold memory.
+
+ The user interface has moved to the background.
+ */
+
+ break;
+
+ case ComponentCallbacks2.TRIM_MEMORY_RUNNING_MODERATE:
+ case ComponentCallbacks2.TRIM_MEMORY_RUNNING_LOW:
+ case ComponentCallbacks2.TRIM_MEMORY_RUNNING_CRITICAL:
+
+ /*
+ Release any memory that your app doesn't need to run.
+
+ The device is running low on memory while the app is running.
+ The event raised indicates the severity of the memory-related event.
+ If the event is TRIM_MEMORY_RUNNING_CRITICAL, then the system will
+ begin killing background processes.
+ */
+
+ break;
+
+ case ComponentCallbacks2.TRIM_MEMORY_BACKGROUND:
+ case ComponentCallbacks2.TRIM_MEMORY_MODERATE:
+ case ComponentCallbacks2.TRIM_MEMORY_COMPLETE:
+
+ /*
+ Release as much memory as the process can.
+
+ The app is on the LRU list and the system is running low on memory.
+ The event raised indicates where the app sits within the LRU list.
+ If the event is TRIM_MEMORY_COMPLETE, the process will be one of
+ the first to be terminated.
+ */
+
+ break;
+
+ default:
+ /*
+ Release any non-critical data structures.
+
+ The app received an unrecognized memory level value
+ from the system. Treat this as a generic low-memory message.
+ */
+ break;
+ }
+ }
+}
+
+
++ The + {@link android.content.ComponentCallbacks2#onTrimMemory onTrimMemory()} + callback was added in Android 4.0 (API level 14). For earlier versions, + 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. +
+ + + ++ To allow multiple running processes, Android sets a hard limit + on the heap size alloted 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, the system throws an {@link java.lang.OutOfMemoryError}. +
+ ++ To avoid running out of memory, you can to query the system to determine + how much heap space you have available on the current device. + You can query the system for this figure by calling + {@link android.app.ActivityManager#getMemoryInfo(android.app.ActivityManager.MemoryInfo) getMemoryInfo()}. + This returns an + {@link android.app.ActivityManager.MemoryInfo } object that provides + information about the device's + current memory status, including available memory, total memory, and + the memory threshold—the memory level below which the system begins + to kill processes. The + {@link android.app.ActivityManager.MemoryInfo } class also exposes a simple + boolean field, + {@link android.app.ActivityManager.MemoryInfo#lowMemory } + that tells you whether the device is running low on memory. +
+ ++ The following code snippet shows an example of how you can use the + {@link android.app.ActivityManager#getMemoryInfo(android.app.ActivityManager.MemoryInfo) getMemoryInfo()}. + method in your application. +
+ +
+public void doSomethingMemoryIntensive() {
+
+ // Before doing something that requires a lot of memory,
+ // check to see whether the device is in a low memory state.
+ ActivityManager.MemoryInfo memoryInfo = getAvailableMemory();
+
+ if (!memoryInfo.lowMemory) {
+ // Do memory intensive work ...
+ }
+}
+
+// Get a MemoryInfo object for the device's current memory status.
+private ActivityManager.MemoryInfo getAvailableMemory() {
+ ActivityManager activityManager = (ActivityManager) this.getSystemService(ACTIVITY_SERVICE);
+ ActivityManager.MemoryInfo memoryInfo = new ActivityManager.MemoryInfo();
+ activityManager.getMemoryInfo(memoryInfo);
+ return memoryInfo;
+}
+
+
+
+
++ Some Android features, Java classes, and code constructs tend to + use more memory than others. You can minimize how + much memory your app uses by choosing more efficient alternatives in + your code. +
+ + + ++ Leaving a service running when it’s not needed is + one of the worst memory-management + mistakes an Android app can make. If your app needs a + service + to perform work in the background, do not keep it running unless + it needs to run a job. Remember to stop your service when it has completed + its task. Otherwise, you can inadvertently cause a memory leak. +
+ ++ When you start a service, the system prefers to always keep the process + for that service running. This behavior + makes services processes very expensive + because the RAM used by a service remains unavailable to other processes. + 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. +
+ ++ You should generally avoid use of persistent services because of + the on-going demands they place on available memory. Instead, we + recommend that you use an alternative implementation + such as {@llink android.app.job.JobScheduler}. For more information about + how to use {@llink android.app.job.JobScheduler} to schedule background + processes, see + Background Optimizations. +
+ If you must use a service, 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 + Running in a Background Service. +
+ + + ++ Some of the classes provided by the programming language are not optimized for + use on mobile devices. For example, the generic + {@link java.util.HashMap} implementation can be quite memory + inefficient because it needs a separate entry object for every mapping. +
+ ++ The Android framework includes several optimized data containers, including + {@link android.util.SparseArray}, {@link android.util.SparseBooleanArray}, + and {@link android.support.v4.util.LongSparseArray}. + For example, the {@link android.util.SparseArray} classes are more + efficient because they avoid the system's need to + autobox + the key and sometimes value (which creates yet another object or + two per entry). +
+ ++ If necessary, you can always switch to raw arrays for a really lean data + structure. +
+ + + ++ Developers often 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. +
+ ++ For example, enums often require more than twice as much memory as static + constants. You should strictly avoid using enums on Android. +
+ + + ++ Protocol buffers + are a language-neutral, platform-neutral, extensible mechanism + designed by Google for serializing structured data—similar to 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 + can cause many kinds of problems in your app such as + increased RAM use, significant APK size increase, and slower execution. +
+ ++ For more information, see the "Nano version" section in the + protobuf readme. +
+ + + ++ As mentioned previously, garbage collections events don't normally affect + your app's performance. However, many garbage collection events that occur + over a short period of time can quickly eat up your frame time. The more time + that the system spends on garbage collection, the less time it has to do + other stuff like rendering or streaming audio. +
+ ++ Often, memory churn can cause a large number of + garbage collection events to occur. In practice, memory churn describes the + number of allocated temporary objects that occur in a given amount of time. +
+ +
+ For example, you might allocate multiple temporary objects within a
+ for loop. Or you might create new
+ {@link android.graphics.Paint} or {@link android.graphics.Bitmap}
+ objects inside the
+ {@link android.view.View#onDraw(android.graphics.Canvas) onDraw()}
+ function of a view.
+ In both cases, the app creates a lot of objects quickly at high volume.
+ These can quickly consume all the available memory in the young generation,
+ forcing a garbage collection event to occur.
+
+ Of course, you need to find the places in your code where + the memory churn is high before you can fix them. Use the tools discussed in + Analyze your RAM usage +
+ ++ Once you identify the problem areas in your code, try to reduce the number of + allocations within performance critical areas. Consider moving things out of + inner loops or perhaps moving them into a + Factory + based allocation structure. +
+ + + ++ Some resources and libraries within your code can gobble up memory without + you knowing it. Overall size of your APK, including third-party libraries + or embedded resources, can affect how much memory your app consumes. You can + improve your app's memory consumption by removing any redundant, unnecessary, + or bloated components, resources, or libraries from your code. +
+ + + ++ You can significantly reduce your app's memory usage by reducing the overall + size of your app. Bitmap size, resources, animation frames, and third-party + libraries can all contribute to the size of your APK. + Android Studio and the Android SDK provide multiple tools + to help you reduce the size of your resources and external dependencies. +
+ ++ For more information about how to reduce your overall APK size, see + Reduce APK Size. +
+ + + ++ Dependency injection framework such as + Guice + or + RoboGuice + can simplify the code you write and provide an adaptive environment + that's useful for testing and other configuration changes. However, dependency + frameworks aren't always optimized for mobile devices. +
+ ++ For example, these frameworks tend to initialize processes by + scanning your code for annotations. This which can require significant + amounts of your code to be mapped into RAM unnecessarily. The system + allocates these mapped pages into clean memory so Android can drop them; yet + that can't happen until the pages have remained in memory for a long period + of time. +
+ ++ If you need to use a dependency injection framework in your app, consider + using + Dagger + instead. For example, Dagger does not use reflection to scan your app's code. + Dagger's strict implementation means that it can be used in Android apps + without needlessly increasing memory usage. +
+ + + ++ External library code is often not written for mobile environments and + can be inefficient when used + for work on a mobile client. When you decide to use an + external library, you may need to optimize that library for mobile devices. + 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. +
+ ++ Even some mobile-optimized libraries can cause problems due to differing + implementations. For example, one library may use nano protobufs + while another uses micro protobufs, resulting in two different protobuf + implementations in your app. This can happen with different + implementations of logging, analytics, image loading frameworks, + caching, and many other things you don't expect. +
+ ++ Although ProGuard can + help to remove APIs and resources with the right flags, it can't remove a + library's large internal dependencies. The features that you want in these + libraries may require lower-level dependencies. 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. +
+ ++ Also avoid using a shared library for just one or two features out of dozens. + You don't want to pull in a large amount of code and overhead that + you don't even use. When you consider whether to use a library, look for + an implementation that strongly matches what you need. Otherwise, you might + decide to create your own implementation. +
---> diff --git a/docs/html/training/training_toc.cs b/docs/html/training/training_toc.cs index d0dccba64d357..39ca6fb405e34 100644 --- a/docs/html/training/training_toc.cs +++ b/docs/html/training/training_toc.cs @@ -1887,6 +1887,12 @@ results." on a variety of mobile devices." >Managing Your App's Memory +