Item 37: Optimize judiciously

Effective Java™: Programming Language Guide > 7. General Programming > Item 37: Optimize judiciously

< BACK CONTINUE > Item 37: Optimize judiciously 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: More computing sins are committed in the name of efficiency (without necessarily achieving it) than for any other single reason-including blind stupidity. --William A. Wulf [Wulf72] We should forget about small efficiencies, say about 97% of the time: premature optimization is the root of all evil. --Donald E. Knuth [Knuth74] We follow two rules in the matter of optimization: Rule 1. Don't do it. Rule 2 (for experts only). Don't do it yet-that is, not until you have a perfectly clear and unoptimized solution. --M. A. Jackson [Jackson75] 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. Don't sacrifice sound architectural principles for performance. Strive to write good programs rather than fast ones. If a good program is not fast enough, its architecture will allow it to be optimized. Good programs embody the principle of information hiding: Where possible, they localize design decisions within individual modules, so individual decisions can be changed without affecting the remainder of the system (Item 12). This does not 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 nearly 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 should think about performance during the design process. Strive to avoid design decisions that limit performance.� 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. Consider the performance consequences of your API design decisions.� Making a public type mutable may require a lot of needless defensive copying (Item 24). 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 14). 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 34). The effects of API design on performance are very real. Consider the getSize method in the java.awt.Component class. The decision that this performance-critical method was to return a Dimension instance, coupled with the decision that Dimension instances are mutable, forces any implementation of this method to allocate a new Dimension instance on every invocation. Even though, as of release 1.3, allocating small objects is relatively inexpensive, allocating millions of objects needlessly can do real harm to performance. In this case, several alternatives existed. Ideally, Dimension should have been immutable (Item 13); alternatively, the getSize method could have been replaced by two methods returning the individual primitive components of a Dimension 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 getSize method and still suffers the performance consequences of the original API design decisions. Luckily, it is generally the case that good API design is consistent with good performance. It is a very bad idea to warp an API to achieve good performance. 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 it causes will be with you for life. Once you've carefully designed your program and produced a clear, concise, and well-structured implementation, then it may be time to consider optimization, assuming you're not already satisfied with the performance of the program. 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: Measure performance before and after each attempted optimization. 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 reveals that programs spend 80 percent of their time in 20 percent of their code. Profiling tools can help you decide where to focus your optimization efforts. Such tools give you run-time 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's 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 Java 2 SDK comes with a simple profiler, and several more sophisticated profiling tools are available commercially. The need to measure the effects of optimization is even greater on the Java platform than on more traditional platforms, as the Java programming language does not have a strong performance model. 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 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. Not only is the performance model ill-defined, but it varies from JVM implementation to JVM implementation and from release to release. If you will be running your program on multiple JVM implementations, 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. To summarize, do not strive to write fast programs-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. < BACK CONTINUE >