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@ -42,188 +42,39 @@ outside the scope of this document.</p>
<h2 id="optimize_judiciously">Optimize Judiciously</h2>
<p>As you get started thinking about how to design your application, and as
you write it, consider
the cautionary points about optimization that Josh Bloch makes in his book
<em>Effective Java</em>. Here's "Item 47: Optimize Judiciously", excerpted from
the latest edition of the book with permission. Although Josh didn't have
Android application development in mind when writing this section &mdash; for
example, the <code style="color:black">java.awt.Component</code> class
referenced is not available in Android, and Android uses the
Dalvik VM, rather than a standard JVM &mdash; his points are still valid. </p>
<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.
<blockquote>
<p>(See <a href="#closing_notes">Closing Notes</a> for more on profiling and
writing effective benchmarks.)
<p>There are three aphorisms concerning optimization that everyone should know.
They are perhaps beginning to suffer from overexposure, but in case you aren't
yet familiar with them, here they are:</p>
<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).
<div style="padding-left:3em;padding-right:4em;">
<p>(If you need that kind of advice, see Josh Bloch's <em>Effective Java</em>,
item 47.)</p>
<p style="margin-bottom:.5em;">More computing sins are committed in the name of
efficiency (without necessarily achieving it) than for any other single
reason&mdash;including blind stupidity.</p>
<p>&mdash;William A. Wulf <span style="font-size:80%;"><sup>1</sup></span></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 style="margin-bottom:.5em;">We should forget about small efficiencies, say
about 97% of the time: premature optimization is the root of all evil. </p>
<p>&mdash;Donald E. Knuth <span style="font-size:80%;"><sup>2</sup></span></p>
<p style="margin-bottom:.5em;">We follow two rules in the matter of optimization:</p>
<ul style="margin-bottom:0">
<li>Rule 1. Don't do it.</li>
<li>Rule 2 (for experts only). Don't do it yet &mdash; that is, not until you have a
perfectly clear and unoptimized solution. </li>
</ul>
<p>&mdash;M. A. Jackson <span style="font-size:80%;"><sup>3</sup></span></p>
</div>
<p>All of these aphorisms predate the Java programming language by two decades.
They tell a deep truth about optimization: it is easy to do more harm than good,
especially if you optimize prematurely. In the process, you may produce software
that is neither fast nor correct and cannot easily be fixed.</p>
<p>Don't sacrifice sound architectural principles for performance.
<strong>Strive to write good programs rather than fast ones.</strong> If a good
program is not fast enough, its architecture will allow it to be optimized. Good
programs embody the principle of <em>information hiding</em>: where possible,
they localize design decisions within individual modules, so individual
decisions can be changed without affecting the remainder of the system (Item
13).</p>
<p>This does <em>not</em> mean that you can ignore performance concerns until
your program is complete. Implementation problems can be fixed by later
optimization, but pervasive architectural flaws that limit performance can be
impossible to fix without rewriting the system. Changing a fundamental facet of
your design after the fact can result in an ill-structured system that is
difficult to maintain and evolve. Therefore you must think about performance
during the design process.</p>
<p><strong>Strive to avoid design decisions that limit performance.</strong> The
components of a design that are most difficult to change after the fact are
those specifying interactions between modules and with the outside world. Chief
among these design components are APIs, wire-level protocols, and persistent
data formats. Not only are these design components difficult or impossible to
change after the fact, but all of them can place significant limitations on the
performance that a system can ever achieve.</p>
<p><strong>Consider the performance consequences of your API design
decisions.</strong> Making a public type mutable may require a lot of needless
defensive copying (Item 39). Similarly, using inheritance in a public class
where composition would have been appropriate ties the class forever to its
superclass, which can place artificial limits on the performance of the subclass
(Item 16). As a final example, using an implementation type rather than an
interface in an API ties you to a specific implementation, even though faster
implementations may be written in the future (Item 52).</p>
<p>The effects of API design on performance are very real. Consider the <code
style="color:black">getSize</code> method in the <code
style="color:black">java.awt.Component</code> class. The decision that this
performance-critical method was to return a <code
style="color:black">Dimension</code> instance, coupled with the decision that
<code style="color:black">Dimension</code> instances are mutable, forces any
implementation of this method to allocate a new <code
style="color:black">Dimension</code> instance on every invocation. Even though
allocating small objects is inexpensive on a modern VM, allocating millions of
objects needlessly can do real harm to performance.</p>
<p>In this case, several alternatives existed. Ideally, <code
style="color:black">Dimension</code> should have been immutable (Item 15);
alternatively, the <code style="color:black">getSize</code> method could have
been replaced by two methods returning the individual primitive components of a
<code style="color:black">Dimension</code> object. In fact, two such methods
were added to the Component API in the 1.2 release for performance reasons.
Preexisting client code, however, still uses the <code
style="color:black">getSize</code> method and still suffers the performance
consequences of the original API design decisions.</p>
<p>Luckily, it is generally the case that good API design is consistent with
good performance. <strong>It is a very bad idea to warp an API to achieve good
performance.</strong> The performance issue that caused you to warp the API may
go away in a future release of the platform or other underlying software, but
the warped API and the support headaches that come with it will be with you for
life.</p>
<p>Once you've carefully designed your program and produced a clear, concise,
and well-structured implementation, <em>then</em> it may be time to consider
optimization, assuming you're not already satisfied with the performance of the
program.</p>
<p>Recall that Jackson's two rules of optimization were "Don't do it," and "(for
experts only). Don't do it yet." He could have added one more: <strong>measure
performance before and after each attempted optimization.</strong> You may be
surprised by what you find. Often, attempted optimizations have no measurable
effect on performance; sometimes, they make it worse. The main reason is that
it's difficult to guess where your program is spending its time. The part of the
program that you think is slow may not be at fault, in which case you'd be
wasting your time trying to optimize it. Common wisdom says that programs spend
80 percent of their time in 20 percent of their code.</p>
<p>Profiling tools can help you decide where to focus your optimization efforts.
Such tools give you runtime information, such as roughly how much time each
method is consuming and how many times it is invoked. In addition to focusing
your tuning efforts, this can alert you to the need for algorithmic changes. If
a quadratic (or worse) algorithm lurks inside your program, no amount of tuning
will fix the problem. You must replace the algorithm with one that is more
efficient. The more code in the system, the more important it is to use a
profiler. It's like looking for a needle in a haystack: the bigger the haystack,
the more useful it is to have a metal detector. The JDK comes with a simple
profiler and modern IDEs provide more sophisticated profiling tools.</p>
<p>The need to measure the effects of attempted optimization is even greater on
the Java platform than on more traditional platforms, because the Java
programming language does not have a strong <em>performance model</em>. The
relative costs of the various primitive operations are not well defined. The
"semantic gap" between what the programmer writes and what the CPU executes is
far greater than in traditional statically compiled languages, which makes it
very difficult to reliably predict the performance consequences of any
optimization. There are plenty of performance myths floating around that turn
out to be half-truths or outright lies.</p>
<p>Not only is Java's performance model ill-defined, but it varies from JVM
implementation to JVM implementation, from release to release, and from
processor to processor. If you will be running your program on multiple JVM
implementations or multiple hardware platforms, it is important that you measure
the effects of your optimization on each. Occasionally you may be forced to make
trade-offs between performance on different JVM implementations or hardware
platforms.</p>
<p>To summarize, do not strive to write fast programs &mdash; strive to write
good ones; speed will follow. Do think about performance issues while you're
designing systems and especially while you're designing APIs, wire-level
protocols, and persistent data formats. When you've finished building the
system, measure its performance. If it's fast enough, you're done. If not,
locate the source of the problems with the aid of a profiler, and go to work
optimizing the relevant parts of the system. The first step is to examine your
choice of algorithms: no amount of low-level optimization can make up for a poor
choice of algorithm. Repeat this process as necessary, measuring the performance
after every change, until you're satisfied.</p>
<p>&mdash;Excerpted from Josh Bloch's <em>Effective Java</em>, Second Ed.
(Addison-Wesley, 2008).</em></p>
<p style="font-size:80%;margin-bottom:0;"><sup>1</sup> Wulf, W. A Case Against
the GOTO. <em>Proceedings of the 25th ACM National
Conference</em> 2 (1972): 791–797.</p>
<p style="font-size:80%;margin-bottom:0;"><sup>2</sup> Knuth, Donald. Structured
Programming with go to Statements. <em>Computing
Surveys 6</em> (1974): 261–301.</p>
<p style="font-size:80%"><sup>3</sup> Jackson, M. A. <em>Principles of Program
Design</em>, Academic Press, London, 1975.
ISBN: 0123790506.</p>
</blockquote>
<p>One of the trickiest problems you'll face when micro-optimizing Android
apps is that the "if you will be running your program on ... multiple hardware
platforms" clause above is always true. And it's not even generally the case
that you can say "device X is a factor F faster/slower than device Y".
This is especially true if one of the devices is the emulator, or one of the
devices has a JIT. If you want to know how your app performs on a given device,
you need to test it on that device. Drawing conclusions from the emulator is
particularly dangerous, as is attempting to compare JIT versus non-JIT
performance: the performance <em>profiles</em> can differ wildly.</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 Objects</h2>
@ -566,3 +417,11 @@ 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/tools/traceview.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.