| page.title=Designing for Performance |
| @jd:body |
| |
| <div id="qv-wrapper"> |
| <div id="qv"> |
| |
| <h2>In this document</h2> |
| <ol> |
| <li><a href="#intro">Introduction</a></li> |
| <li><a href="#optimize_judiciously">Optimize Judiciously</a></li> |
| <li><a href="#object_creation">Avoid Creating Unnecessary Objects</a></li> |
| <li><a href="#myths">Performance Myths</a></li> |
| <li><a href="#prefer_static">Prefer Static Over Virtual</a></li> |
| <li><a href="#internal_get_set">Avoid Internal Getters/Setters</a></li> |
| <li><a href="#use_final">Use Static Final For Constants</a></li> |
| <li><a href="#foreach">Use Enhanced For Loop Syntax</a></li> |
| <li><a href="#package_inner">Consider Package Instead of Private Access with Inner Classes</a></li> |
| <li><a href="#avoidfloat">Use Floating-Point Judiciously</a> </li> |
| <li><a href="#library">Know And Use The Libraries</a></li> |
| <li><a href="#native_methods">Use Native Methods Judiciously</a></li> |
| <li><a href="#closing_notes">Closing Notes</a></li> |
| </ol> |
| |
| </div> |
| </div> |
| |
| <p>An Android application will run on a mobile device with limited computing |
| power and storage, and constrained battery life. Because of |
| this, it should be <em>efficient</em>. Battery life is one reason you might |
| want to optimize your app even if it already seems to run "fast enough". |
| Battery life is important to users, and Android's battery usage breakdown |
| means users will know if your app is responsible draining their battery.</p> |
| |
| <p>Note that although this document primarily covers micro-optimizations, |
| these will almost never make or break your software. Choosing the right |
| algorithms and data structures should always be your priority, but is |
| outside the scope of this document.</p> |
| |
| <a name="intro" id="intro"></a> |
| <h2>Introduction</h2> |
| |
| <p>There are two basic rules for writing efficient code:</p> |
| <ul> |
| <li>Don't do work that you don't need to do.</li> |
| <li>Don't allocate memory if you can avoid it.</li> |
| </ul> |
| |
| <h2 id="optimize_judiciously">Optimize Judiciously</h2> |
| |
| <p>This document is about Android-specific micro-optimization, so it assumes |
| that you've already used profiling to work out exactly what code needs to be |
| optimized, and that you already have a way to measure the effect (good or bad) |
| of any changes you make. You only have so much engineering time to invest, so |
| it's important to know you're spending it wisely. |
| |
| <p>(See <a href="#closing_notes">Closing Notes</a> for more on profiling and |
| writing effective benchmarks.) |
| |
| <p>This document also assumes that you made the best decisions about data |
| structures and algorithms, and that you've also considered the future |
| performance consequences of your API decisions. Using the right data |
| structures and algorithms will make more difference than any of the advice |
| here, and considering the performance consequences of your API decisions will |
| make it easier to switch to better implementations later (this is more |
| important for library code than for application code). |
| |
| <p>(If you need that kind of advice, see Josh Bloch's <em>Effective Java</em>, |
| item 47.)</p> |
| |
| <p>One of the trickiest problems you'll face when micro-optimizing an Android |
| app is that your app is pretty much guaranteed to be running on multiple |
| hardware platforms. Different versions of the VM running on different |
| processors running at different speeds. It's not even generally the case |
| that you can simply say "device X is a factor F faster/slower than device Y", |
| and scale your results from one device to others. In particular, measurement |
| on the emulator tells you very little about performance on any device. There |
| are also huge differences between devices with and without a JIT: the "best" |
| code for a device with a JIT is not always the best code for a device |
| without.</p> |
| |
| <p>If you want to know how your app performs on a given device, you need to |
| test on that device.</p> |
| |
| <a name="object_creation"></a> |
| <h2>Avoid Creating Unnecessary Objects</h2> |
| |
| <p>Object creation is never free. A generational GC with per-thread allocation |
| pools for temporary objects can make allocation cheaper, but allocating memory |
| is always more expensive than not allocating memory.</p> |
| |
| <p>If you allocate objects in a user interface loop, you will force a periodic |
| garbage collection, creating little "hiccups" in the user experience. The |
| concurrent collector introduced in Gingerbread helps, but unnecessary work |
| should always be avoided.</p> |
| |
| <p>Thus, you should avoid creating object instances you don't need to. Some |
| examples of things that can help:</p> |
| |
| <ul> |
| <li>If you have a method returning a string, and you know that its result |
| will always be appended to a StringBuffer anyway, change your signature |
| and implementation so that the function does the append directly, |
| instead of creating a short-lived temporary object.</li> |
| <li>When extracting strings from a set of input data, try |
| to return a substring of the original data, instead of creating a copy. |
| You will create a new String object, but it will share the char[] |
| with the data. (The trade-off being that if you're only using a small |
| part of the original input, you'll be keeping it all around in memory |
| anyway if you go this route.)</li> |
| </ul> |
| |
| <p>A somewhat more radical idea is to slice up multidimensional arrays into |
| parallel single one-dimension arrays:</p> |
| |
| <ul> |
| <li>An array of ints is a much better than an array of Integers, |
| but this also generalizes to the fact that two parallel arrays of ints |
| are also a <strong>lot</strong> more efficient than an array of (int,int) |
| objects. The same goes for any combination of primitive types.</li> |
| <li>If you need to implement a container that stores tuples of (Foo,Bar) |
| objects, try to remember that two parallel Foo[] and Bar[] arrays are |
| generally much better than a single array of custom (Foo,Bar) objects. |
| (The exception to this, of course, is when you're designing an API for |
| other code to access; in those cases, it's usually better to trade |
| good API design for a small hit in speed. But in your own internal |
| code, you should try and be as efficient as possible.)</li> |
| </ul> |
| |
| <p>Generally speaking, avoid creating short-term temporary objects if you |
| can. Fewer objects created mean less-frequent garbage collection, which has |
| a direct impact on user experience.</p> |
| |
| <a name="avoid_enums" id="avoid_enums"></a> |
| <a name="myths" id="myths"></a> |
| <h2>Performance Myths</h2> |
| |
| <p>Previous versions of this document made various misleading claims. We |
| address some of them here.</p> |
| |
| <p>On devices without a JIT, it is true that invoking methods via a |
| variable with an exact type rather than an interface is slightly more |
| efficient. (So, for example, it was cheaper to invoke methods on a |
| <code>HashMap map</code> than a <code>Map map</code>, even though in both |
| cases the map was a <code>HashMap</code>.) It was not the case that this |
| was 2x slower; the actual difference was more like 6% slower. Furthermore, |
| the JIT makes the two effectively indistinguishable.</p> |
| |
| <p>On devices without a JIT, caching field accesses is about 20% faster than |
| repeatedly accesssing the field. With a JIT, field access costs about the same |
| as local access, so this isn't a worthwhile optimization unless you feel it |
| makes your code easier to read. (This is true of final, static, and static |
| final fields too.) |
| |
| <a name="prefer_static" id="prefer_static"></a> |
| <h2>Prefer Static Over Virtual</h2> |
| |
| <p>If you don't need to access an object's fields, make your method static. |
| Invocations will be about 15%-20% faster. |
| It's also good practice, because you can tell from the method |
| signature that calling the method can't alter the object's state.</p> |
| |
| <a name="internal_get_set" id="internal_get_set"></a> |
| <h2>Avoid Internal Getters/Setters</h2> |
| |
| <p>In native languages like C++ it's common practice to use getters (e.g. |
| <code>i = getCount()</code>) instead of accessing the field directly (<code>i |
| = mCount</code>). This is an excellent habit for C++, because the compiler can |
| usually inline the access, and if you need to restrict or debug field access |
| you can add the code at any time.</p> |
| |
| <p>On Android, this is a bad idea. Virtual method calls are expensive, |
| much more so than instance field lookups. It's reasonable to follow |
| common object-oriented programming practices and have getters and setters |
| in the public interface, but within a class you should always access |
| fields directly.</p> |
| |
| <p>Without a JIT, direct field access is about 3x faster than invoking a |
| trivial getter. With the JIT (where direct field access is as cheap as |
| accessing a local), direct field access is about 7x faster than invoking a |
| trivial getter. This is true in Froyo, but will improve in the future when |
| the JIT inlines getter methods.</p> |
| |
| <a name="use_final" id="use_final"></a> |
| <h2>Use Static Final For Constants</h2> |
| |
| <p>Consider the following declaration at the top of a class:</p> |
| |
| <pre>static int intVal = 42; |
| static String strVal = "Hello, world!";</pre> |
| |
| <p>The compiler generates a class initializer method, called |
| <code><clinit></code>, that is executed when the class is first used. |
| The method stores the value 42 into <code>intVal</code>, and extracts a |
| reference from the classfile string constant table for <code>strVal</code>. |
| When these values are referenced later on, they are accessed with field |
| lookups.</p> |
| |
| <p>We can improve matters with the "final" keyword:</p> |
| |
| <pre>static final int intVal = 42; |
| static final String strVal = "Hello, world!";</pre> |
| |
| <p>The class no longer requires a <code><clinit></code> method, |
| because the constants go into static field initializers in the dex file. |
| Code that refers to <code>intVal</code> will use |
| the integer value 42 directly, and accesses to <code>strVal</code> will |
| use a relatively inexpensive "string constant" instruction instead of a |
| field lookup. (Note that this optimization only applies to primitive types and |
| <code>String</code> constants, not arbitrary reference types. Still, it's good |
| practice to declare constants <code>static final</code> whenever possible.)</p> |
| |
| <a name="foreach" id="foreach"></a> |
| <h2>Use Enhanced For Loop Syntax</h2> |
| |
| <p>The enhanced for loop (also sometimes known as "for-each" loop) can be used |
| for collections that implement the Iterable interface and for arrays. |
| With collections, an iterator is allocated to make interface calls |
| to hasNext() and next(). With an ArrayList, a hand-written counted loop is |
| about 3x faster (with or without JIT), but for other collections the enhanced |
| for loop syntax will be exactly equivalent to explicit iterator usage.</p> |
| |
| <p>There are several alternatives for iterating through an array:</p> |
| |
| <pre> static class Foo { |
| int mSplat; |
| } |
| Foo[] mArray = ... |
| |
| public void zero() { |
| int sum = 0; |
| for (int i = 0; i < mArray.length; ++i) { |
| sum += mArray[i].mSplat; |
| } |
| } |
| |
| public void one() { |
| int sum = 0; |
| Foo[] localArray = mArray; |
| int len = localArray.length; |
| |
| for (int i = 0; i < len; ++i) { |
| sum += localArray[i].mSplat; |
| } |
| } |
| |
| public void two() { |
| int sum = 0; |
| for (Foo a : mArray) { |
| sum += a.mSplat; |
| } |
| } |
| </pre> |
| |
| <p><strong>zero()</strong> is slowest, because the JIT can't yet optimize away |
| the cost of getting the array length once for every iteration through the |
| loop.</p> |
| |
| <p><strong>one()</strong> is faster. It pulls everything out into local |
| variables, avoiding the lookups. Only the array length offers a performance |
| benefit.</p> |
| |
| <p><strong>two()</strong> is fastest for devices without a JIT, and |
| indistinguishable from <strong>one()</strong> for devices with a JIT. |
| It uses the enhanced for loop syntax introduced in version 1.5 of the Java |
| programming language.</p> |
| |
| <p>To summarize: use the enhanced for loop by default, but consider a |
| hand-written counted loop for performance-critical ArrayList iteration.</p> |
| |
| <p>(See also <em>Effective Java</em> item 46.)</p> |
| |
| <a name="package_inner" id="package_inner"></a> |
| <h2>Consider Package Instead of Private Access with Private Inner Classes</h2> |
| |
| <p>Consider the following class definition:</p> |
| |
| <pre>public class Foo { |
| private class Inner { |
| void stuff() { |
| Foo.this.doStuff(Foo.this.mValue); |
| } |
| } |
| |
| private int mValue; |
| |
| public void run() { |
| Inner in = new Inner(); |
| mValue = 27; |
| in.stuff(); |
| } |
| |
| private void doStuff(int value) { |
| System.out.println("Value is " + value); |
| } |
| }</pre> |
| |
| <p>The key things to note here are that we define a private inner class |
| (<code>Foo$Inner</code>) that directly accesses a private method and a private |
| instance field in the outer class. This is legal, and the code prints "Value is |
| 27" as expected.</p> |
| |
| <p>The problem is that the VM considers direct access to <code>Foo</code>'s |
| private members from <code>Foo$Inner</code> to be illegal because |
| <code>Foo</code> and <code>Foo$Inner</code> are different classes, even though |
| the Java language allows an inner class to access an outer class' private |
| members. To bridge the gap, the compiler generates a couple of synthetic |
| methods:</p> |
| |
| <pre>/*package*/ static int Foo.access$100(Foo foo) { |
| return foo.mValue; |
| } |
| /*package*/ static void Foo.access$200(Foo foo, int value) { |
| foo.doStuff(value); |
| }</pre> |
| |
| <p>The inner class code calls these static methods whenever it needs to |
| access the <code>mValue</code> field or invoke the <code>doStuff</code> method |
| in the outer class. What this means is that the code above really boils down to |
| a case where you're accessing member fields through accessor methods. |
| Earlier we talked about how accessors are slower than direct field |
| accesses, so this is an example of a certain language idiom resulting in an |
| "invisible" performance hit.</p> |
| |
| <p>If you're using code like this in a performance hotspot, you can avoid the |
| overhead by declaring fields and methods accessed by inner classes to have |
| package access, rather than private access. Unfortunately this means the fields |
| can be accessed directly by other classes in the same package, so you shouldn't |
| use this in public API.</p> |
| |
| <a name="avoidfloat" id="avoidfloat"></a> |
| <h2>Use Floating-Point Judiciously</h2> |
| |
| <p>As a rule of thumb, floating-point is about 2x slower than integer on |
| Android devices. This is true on a FPU-less, JIT-less G1 and a Nexus One with |
| an FPU and the JIT. (Of course, absolute speed difference between those two |
| devices is about 10x for arithmetic operations.)</p> |
| |
| <p>In speed terms, there's no difference between <code>float</code> and |
| <code>double</code> on the more modern hardware. Space-wise, <code>double</code> |
| is 2x larger. As with desktop machines, assuming space isn't an issue, you |
| should prefer <code>double</code> to <code>float</code>.</p> |
| |
| <p>Also, even for integers, some chips have hardware multiply but lack |
| hardware divide. In such cases, integer division and modulus operations are |
| performed in software — something to think about if you're designing a |
| hash table or doing lots of math.</p> |
| |
| <a name="library" id="library"></a> |
| <h2>Know And Use The Libraries</h2> |
| |
| <p>In addition to all the usual reasons to prefer library code over rolling |
| your own, bear in mind that the system is at liberty to replace calls |
| to library methods with hand-coded assembler, which may be better than the |
| best code the JIT can produce for the equivalent Java. The typical example |
| here is <code>String.indexOf</code> and friends, which Dalvik replaces with |
| an inlined intrinsic. Similarly, the <code>System.arraycopy</code> method |
| is about 9x faster than a hand-coded loop on a Nexus One with the JIT.</p> |
| |
| <p>(See also <em>Effective Java</em> item 47.)</p> |
| |
| <a name="native_methods" id="native_methods"></a> |
| <h2>Use Native Methods Judiciously</h2> |
| |
| <p>Native code isn't necessarily more efficient than Java. For one thing, |
| there's a cost associated with the Java-native transition, and the JIT can't |
| optimize across these boundaries. If you're allocating native resources (memory |
| on the native heap, file descriptors, or whatever), it can be significantly |
| more difficult to arrange timely collection of these resources. You also |
| need to compile your code for each architecture you wish to run on (rather |
| than rely on it having a JIT). You may even have to compile multiple versions |
| for what you consider the same architecture: native code compiled for the ARM |
| processor in the G1 can't take full advantage of the ARM in the Nexus One, and |
| code compiled for the ARM in the Nexus One won't run on the ARM in the G1.</p> |
| |
| <p>Native code is primarily useful when you have an existing native codebase |
| that you want to port to Android, not for "speeding up" parts of a Java app.</p> |
| |
| <p>(See also <em>Effective Java</em> item 54.)</p> |
| |
| <a name="closing_notes" id="closing_notes"></a> |
| <h2>Closing Notes</h2> |
| |
| <p>One last thing: always measure. Before you start optimizing, make sure you |
| have a problem. Make sure you can accurately measure your existing performance, |
| or you won't be able to measure the benefit of the alternatives you try.</p> |
| |
| <p>Every claim made in this document is backed up by a benchmark. The source |
| to these benchmarks can be found in the <a href="http://code.google.com/p/dalvik/source/browse/#svn/trunk/benchmarks">code.google.com "dalvik" project</a>.</p> |
| |
| <p>The benchmarks are built with the |
| <a href="http://code.google.com/p/caliper/">Caliper</a> microbenchmarking |
| framework for Java. Microbenchmarks are hard to get right, so Caliper goes out |
| of its way to do the hard work for you, and even detect some cases where you're |
| not measuring what you think you're measuring (because, say, the VM has |
| managed to optimize all your code away). We highly recommend you use Caliper |
| to run your own microbenchmarks.</p> |
| |
| <p>You may also find |
| <a href="{@docRoot}guide/developing/debugging/debugging-tracing.html">Traceview</a> useful |
| for profiling, but it's important to realize that it currently disables the JIT, |
| which may cause it to misattribute time to code that the JIT may be able to win |
| back. It's especially important after making changes suggested by Traceview |
| data to ensure that the resulting code actually runs faster when run without |
| Traceview. |