When O’Reilly first approached me about writing a book on Java performance tuning, I was unsure. Java performance, I thought—aren’t we done with that? Yes, I still work on performance of Java (and other) applications on a daily basis, but I like to think that I spend most of my time dealing with algorithmic inefficiences and external system bottlenecks rather than on anything directly related to Java tuning.

A moment’s reflection convinced me that I was (as usual) kidding myself. It is certainly true that end-to-end system performance takes up a lot of my time, and that I sometimes come across code that uses an algorithm when it could use one with O(log N) performance. Still, it turns out that every day, I think about GC performance, or the performance of the JVM compiler, or how to get the best performance from Java Enterprise Edition APIs.

That is not to minimize the enormous progress that has been made in the performance of Java and JVMs over the past 15-plus years. When I was a Java evangelist at Sun during the late 1990s, the only real “benchmark” available was CaffeineMark 2.0 from Pendragon software. For a variety of reasons, the design of that benchmark quickly limited its value; yet in its day, we were fond of telling everyone that Java 1.1.8 performance was eight times faster than Java 1.0 performance based on that benchmark. And that was true—Java 1.1.8 had an actual just-in-time compiler, where Java 1.0 was pretty much completely interpreted.

Then standards committees began to develop more rigorous benchmarks, and Java performance began to be centered around them. The result was a continuous improvement in all areas of the JVM—garbage collection, compilations, and within the APIs. That process continues today, of course, but one of the interesting facts about performance work is that it gets successively harder. Achieving an eightfold increase in performance by introducing a just-in-time compiler was a straightforward matter of engineering, and even though the compiler continues to improve, we’re not going to see an improvement like that again. Parallelizing the garbage collector was a huge performance improvement, but more recent changes have been more incremental.

This is a typical process for applications (and the JVM itself is just another application): in the beginning of a project, it’s easy enough to find archictural changes (or code bugs) which, when addressed, yield huge performance improvements. In a mature application, finding such performance improvements is quite rare.

That precept was behind my original concern that, to a large extent, the engineering world might be done with Java performance. A few things convinced me I was wrong. First is the number of questions I see daily about how this or that aspect of the JVM performs under certain circumstances. New engineers come to Java all the time, and JVM behavior remains complex enough in certain areas that a guide to how it operates is still beneficial. Second is that environmental changes in computing seem to have altered the performance concerns that engineers face today.

What’s changed in the past few years is that performance concerns have become bifurcated. On the one hand, very large machines capabable of running JVMs with very large heaps are now commonplace. The JVM has moved to address those concerns with a new garbage collector (G1), which—as a new technology—requires a little more hand-tuning than traditional collectors. At the same time, cloud computing has renewed the importance of small, single-CPU machines: you can go to Oracle or Amazon or a host of other companies and very cheaply rent a single CPU machine to run a small application server. (You’re not actually getting a single-CPU machine: you’re getting a virtual OS image on a very large machine, but the virtual OS is limited to using a single CPU. From the perspective of Java, that turns out to be the same as single-CPU machine.) In those environments, correctly managing small amounts of memory turns out to be quite important.

The Java platform also continues to evolve. Each new edition of Java provides new language features and new APIs that improve the productivity of developers—if not always the performance of their applications. Best practice use of these language features can help to differentiate between an application that sizzles, and one that plods along. And the evolution of the platform brings up interesting performance questions: there is no question that using JSON to exchange information between two programs is much simpler than coming up with a highly optimized proprietary protocol. Saving time for developers is a big win—but making sure that productivity win comes with a performance win (or at least breaks even) is the real goal.

Who Should (and Shouldn’t) Read This Book

This book is designed for performance engineers and developers who are looking to understand how various aspects of the JVM and the Java APIs impact performance.

If it is late Sunday night, your site is going live Monday morning, and you’re looking for a quick fix for performance issues, this is not the book for you.

If you are new to performance analysis and are starting that analysis in Java, then this book can help you. Certainly my goal is to provide enough information and context that novice engineers can understand how to apply basic tuning and performance principles to a Java application. However, system analysis is a very broad field. There are a number of excellent resources for system analysis in general (and those principles of course apply to Java), and in that sense, this book will hopefully be a useful companion to those texts.

At a fundamental level, though, making Java go really fast requires a deep understanding about how the JVM (and Java APIs) actually work. There are literally hundreds of Java tuning flags, and tuning the JVM has to be more than an approach of blindly trying them and seeing what works. Instead, my goal is to provide some very detailed knowledge about what the JVM and APIs are doing, with the hope that if you understand how those things work, you’ll be able to look at the specific behavior of an application and understand why it is performing badly. Understanding that, it becomes a simple (or at least simpler) task to get rid of undesirable (badly performing) behavior.

One interesting aspect to Java performance work is that developers often have a very different background than engineers in a performance or QA group. I know developers who can remember thousands of obscure method signatures on little-used Java APIs but who have no idea what the flag -Xmn means. And I know testing engineers who can get every last ounce of performance from setting various flags for the garbage collector but who could barely write a suitable “Hello, World” program in Java.

Java performance covers both of these areas: tuning flags for the compiler and garbage collector and so on, and best-practice uses of the APIs. So I assume that you have a good understanding of how to write programs in Java. Even if your primary interest is not in the programming aspects of Java, I do spent a fair amount of time discussing programs, including the sample programs used to provide a lot of the data points in the examples.

Still, if your primary interest is in the performance of the JVM itself—meaning how to alter the behavior of the JVM without any coding—then large sections of this book should still be beneficial to you. Feel free to skip over the coding parts and focus in on the areas that interest you. And maybe along the way, you’ll pick up some insight into how Java applications can affect JVM performance and start to suggest changes to developers so they can make your performance-testing life easier.

Conventions Used in This Book

The following typographical conventions are used in this book:

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This element signifies a tip or suggestion.


This element signifies a general note.


This element indicates a warning or caution.

Using Code Examples

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I would like to thank everyone who helped me as I worked on this book. In many ways, this book is an accumulation of knowledge gained over my past 15 years in the Java Performance Group at Sun Microsystems and Oracle, so the list of people who have provided positive input into this book is quite broad. To all the engineers I have worked with during that time, and particularly to those who patiently answered my random questions over the past year, thank you!

I would especially like to thank Stanley Guan, Azeem Jiva, Kim LiChong, Deep Singh, Martijn Verburg, and Edward Yue Shung Wong for their time reviewing draft copies and providing valuable feedback. I am sure that they were unable to find all my errors, though the material here is greatly improved by their input.

The production staff at O’Reilly was as always very helpful, and thanks to my editor Meg Blanchette for all your encouragement during the process. Finally, I must thank my husband James for putting up with the long nights and those weekend dinners where I was in a continual state of distraction.

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