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O'Reilly - Java Performance Tuning. Underlying JDK Improvements - Object Creation - Strings - Exceptions, Casts, and Variables - Loops and Switches - Reader Converter7. Sorting - Threading - 3 -. Appropriate Data Structures and Algorithms - Distributed Computing - When to Optimize - Underlying Operating System and Network Improvements - Further Resources - Making those first interpreted programs run fast enough was a huge challenge for many developers.
Since then, Java performance has improved enormously, and any Java program can now be made torun fast enough provided you avoid the main performance pitfalls. This book provides all the details a developer needs to performance-tune any type of Java program. I give step-by-step instructions on all aspects of the performance-tuning process, right from earlyconsiderations such as setting goals, measuring performance, and choosing a compiler, to detailedexamples on using profiling tools and applying the results to tune the code.
This is not an entrylevelbook about Java , but you do not need any previous tuning knowledge to benefit from readingit. Many of the tuning techniques presented in this book lead to an increased maintenance cost, so theyshould not be applied arbitrarily. Change your code only when a bottleneck has been identified, andnever change the design of your application for minor performance gains.
Contents of This BookChapter 1 gives general guidelines on how to tune. If you do not yet have a tuning strategy, thischapter provides a methodical tuning process. Chapter 2 covers the tools you need to use while tuning. Chapter 4 through Chapter 12 cover various techniques you can apply to Java code.
Chapter 12looks at tuning techniques specific to distributed applications. Chapter 13 steps back from the low-level code-tuning techniques examined throughout most of thebook and considers tuning at all other stages of the development process. Chapter 14 is a quick look at some operating system-level tuning techniques. Each chapter has a performance tuning checklist at its end.
Use these lists to ensure that you havenot missed any core tuning techniques while you are tuning. I have shown the time variation across different VMs for many of the tests. However, yourmain focus should be on the effects that tuning has on any one VM, as this identifies the usefulnessof a tuning technique. Differences between VMs are interesting, but are only indicative and need tobe verified for your specific application. Where I have shown the results of timed tests, the VMversions I have used are Version 1.
Despite many later improvements throughout the JDK, the- 5 -. The default is with a JIT, and this is the mode usedfor all measurements in the book. Java 2 VMs have more work to do than priorVMs because of additional features such as Reference objects, and 1. The default is with a JIT, and thisis the mode used for measurements labeled "1.
The default is the HotSpot technology, and this is the mode I'veused for measurements labeled simply "1. Because HotSpot optimizationsfrequently do not kick in until after the program has run for a little while, I sometimes showmeasurements labeled "HotSpot 2nd Run.
To be put on the mailing list or request a catalog, sendemail to:info oreilly. This book wouldhave been considerably poorer without her improvements in clarity and consistency throughout. Iam also very grateful to Mike Loukides and Kirk Pepperdine for the enormously helpful assistance Ireceived from them while writing this book.
Their many notes have helped to make this book muchclearer and complete. They identified several errors and added good advice that makesthis book more useful. I am, of course, responsible for the final text of this book, including any erroors tthat rremain. Chapter 1. IntroductionThe trouble with doing something right the first time is that nobody appreciates how difficult it was. Part of this perception is pureassumption: many people assume that if a program is not compiled, it must be slow.
Part of thisperception is based in reality: many early applets and applications were slow, because ofnonoptimal coding, initially unoptimized Java Virtual Machines VMs , and the overheads of thelanguage. In earlier versions of Java , you had to struggle hard and compromise a lot to make a Java application run quickly. More recently, there have been fewer reasons why an application should beslow. The VM technology and Java development tools have progressed to the point where a Java application or applet, servlet, etc.
With good designs and byfollowing good coding practices and avoiding bottlenecks, applications usually run fast enough. However, the truth is that the first and even several subsequent versions of a program written inany language are often slower than expected, and the reasons for this lack of performance are notalways clear to the developer.
This book shows you why a particular Java application might be running slower than expected, andsuggests ways to avoid or overcome these pitfalls and improve the performance of your application. In this book I've gathered several years of tuning experiences in one place. I hope you will find ituseful in making your Java application, applet, servlet, and component run as fast as you need. Throughout the book I use the generic words "application" and "program" to cover Java applications, applets, servlets, beans, libraries, and really any use of Java code.
Where a techniquecan be applied only to some subset of these various types of Java programs, I say so. Otherwise, thetechnique applies across all types of Java programs. This question is always asked as soon as the first tests are timed: "Where is the time going? I didnot expect it to take this long. In the same way the first version of the code is likely to have bugs that needfixing, it is also rarely as fast as it can be. Fortunately, performance tuning is usually easier thandebugging.
When debugging, you have to fix bugs throughout the code; in performance tuning , youcan focus your effort on the few parts of the application that are the bottlenecks. The longer answer? Well, it's true that there are overheads in the Java runtime system, mainly dueto its virtual machine layer that abstracts Java away from the underlying hardware.
It's also true thatthere are overheads from Java 's dynamic nature. These overhead s can cause a Java application torun slower than an equivalent application written in a lower-level language just as a C program isgenerally slower than the equivalent program written in assembler. Java 's advantages—namely, itsplatform-independence, memory management, powerful exception checking, built-inmultithreading, dynamic resource loading, and security checks—add costs in terms of aninterpreter, garbage collector, thread monitors, repeated disk and network accessing, and extraruntime checks.
For example, hierarchical method invocation requires an extra computation for every method call,because the runtime system has to work out which of the possible methods in the hierarchy is theactual target of the call.
Most modern CPU s are designed to be optimized for fixed call and branchtargets and do not perform as well when a significant percentage of calls need to be computed onthe fly. On the other hand, good object-oriented design actually encourages many small methodsand significant polymorphism in the method hierarchy. Compiler inlining is another frequently usedtechnique that can significantly improve compiled code. However, this technique cannot be applied- 8 -.
Of course, the same Java language features that cause these overheads may be the features thatpersuaded you to use Java in the first place. The important thing is that none of these overheadsslows the system down too much. Naturally, "too much" is different depending on the application,and the users of the application usually make this choice. But the key point with Java is that a goodround of performance tuning normally makes your application run as fast as you need it to run.
There are already plenty of nontrivial Java applications, applets, and servlets that run fast enough toshow that Java itself is not too slow. So if your application is not running fast enough, chances arethat it just needs tuning.
Your target is to get a better score lower time than the last score after each attempt. You areplaying with, not against, the computer, the programmer, the design and architecture, the compiler,and the flow of control. Your opponents are time, competing applications, budgetary restrictions,etc. You can complete this list better than I can for your particular situation. I once attended a customer who wanted to know if there was a "go faster" switch somewhere that hecould just turn on and make the whole application go faster.
Of course, he was not really expectingone, but checked just in case he had missed a basic option somewhere. There isn't such a switch, but very simple techniques sometimes provide the equivalent. Techniquesinclude switching compilers , turning on optimizations, using a different runtime VM, finding twoor three bottlenecks in the code or architecture that have simple fixes, and so on.
I have seen all ofthese give huge improvements to applications, sometimes a fold speedup. Order-of-magnitudespeedups are typical for the first round of performance tuning. If your application is CPU -bound, you need to concentrate your efforts on the code, looking forbottlenecks, inefficient algorithms, too many short-lived objects object creation and garbagecollection are CPU-intensive operations , and other problems, which I will cover in this book.
If your application is hitting system-memory limits, it may be paging sections in and out of mainmemory. In this case, the problem may be caused by too many objects, or even just a few largeobjects, being erroneously held in memory; by too many large arrays being allocated frequentlyused in buffered applications ; or by the design of the application, which may need to bereexamined to reduce its running memory footprint.
Inthis case, you need to look at exactly what you are doing to the disks that is slowing the application:first identify the operations, then determine the problems, and finally eliminate or change these toimprove the situation. For example, one program I know of went through web server logs and did reverse lookups on theIP addresses. The first version of this program was very slow.
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Банк данных снова был в безопасности. В комнате творилось нечто невообразимое. Техники обнимали друг друга, подбрасывая вверх длинные полосы распечаток. Бринкерхофф обнимал Мидж. Соши заливалась слезами. - Джабба, - спросил Фонтейн, - много они похитили.
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Она не могла больше ждать. Пора. Она должна немедленно поговорить со Стратмором. Сьюзан осторожно приоткрыла дверь и посмотрела на глянцевую, почти зеркальную стену шифровалки. Узнать, следит ли за ней Хейл, было невозможно.
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Сел. Поднял посверкивающую полуавтоматическую беретту и нацелил ее на дверь, а потом опустил себе на колени. - Сьюзан, - сказал он торжественно.
У нас ничего такого не случалось. - Вот. - Она едва заметно подмигнула.
Ему удалось проскользнуть внутрь в последнюю секунду перед тем, как дверь закрылась. Человек улыбнулся: охота становилась интересной. Беккер здесь… Я чувствую, что .
Она должна помочь ему найти ключ в компьютере Хейла. Стратмор пока не сказал ей, что этот ключ представляет для него отнюдь не только академический интерес. Он думал, что сможет обойтись без ее участия - принимая во внимание ее склонность к самостоятельности - и сам найдет этот ключ, но уже столкнулся с проблемами, пытаясь самостоятельно запустить Следопыта. Рисковать еще раз ему не хотелось.
Между шифровалкой и стоянкой для машин не менее дюжины вооруженных охранников. - Я не такой дурак, как вы думаете, - бросил Хейл. - Я воспользуюсь вашим лифтом.
Чего вы от меня хотите. Беккер задумался: Я бы хотел, чтобы ты как следует вымыл голову, научился говорить по-человечески и нашел себе работу. Но решил, что хочет от этого парня слишком многого.