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diff --git a/docs/html/guide/practices/performance.jd b/docs/html/guide/practices/performance.jd new file mode 100644 index 0000000..078999b --- /dev/null +++ b/docs/html/guide/practices/performance.jd @@ -0,0 +1,410 @@ +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> + +<p>Note that if you're using ProGuard, you can have the best +of both worlds because ProGuard can inline accessors for you.</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>If you do need to use native code, you should read our +<a href="{@docRoot}guide/practices/jni.html">JNI Tips</a>.</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}tools/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. |