Evans Benjamin J. - Optimizing Java: practical techniques for improving JVM application performance

• 2018-04-12: First Release

How do you define performance?

Most developers, when asked about the performance of their application, will assume some measure of speed is requested. Something like transactions per second, or gigabytes of data processedgetting a lot of work done in the shortest amount of time possible. If youre an application architect, you may measure performance in broader metrics. You may be more concerned about resource utilization than straight-line execution. You might pay more attention to the performance of connections between services than of the services themselves. If you make business decisions for your company, application performance will probably not be measured in time as often as it is measured in dollars. You may argue with developers and architects about resource allocation, weighing the cost of devops against the time it takes to do the companys work.

And regardless of which role you identify with, all these metrics are important.

I started out developing Java applications in 1996. I had just moved from my first job writing AppleScript CGIs for the University of Minnesotas business school to maintaining server-side Perl applications for the web development team. Java was very new thenthe first stable version, 1.0.2, was released earlier that yearand I was tasked with finding something useful to build.

Back in those days, the best way to get performance out of a Java application was to write it in some other language. This was before Java had a Just-in-Time (JIT) compiler, before parallel and concurrent garbage collectors, and long before the server side would become dominated by Java technology. But many of us wanted to use Java, and we developed all sorts of tricks to make our code run well. We wrote gigantic methods to avoid method dispatch overhead. We pooled and reused objects because garbage collection was slow and disruptive. We used lots of global state and static methods. We wrote truly awful Java code, but it workedfor a little while.

In 1999, things started to change.

After years struggling to use Java for anything demanding speed, JIT technologies started to reach us. With compilers that could inline methods, the number of method calls became less important than breaking up our giant monoliths into smaller pieces. We gleefully embraced object-oriented design, splitting our methods into tiny chunks and wrapping interfaces around everything. We marveled at how every release of Java would run things just a little bit better, because we were writing good Java code and the JIT compiler loved it. Java soared past other technologies on the server, leading us to build larger and more complex apps with richer abstractions.

At the same time, garbage collectors were rapidly improving. Now the overhead of pooling would very frequently overshadow the cost of allocation. Many garbage collectors offered multithreaded operation, and we started to see low-pause, nearly concurrent GCs that stayed out of our applications way. The standard practice moved toward a carefree creating and throwing away of objects with the promise that a sufficiently smart GC would eventually make it all OK. And it workedfor a little while.

The problem with technology is that it always invalidates itself. As JIT and GC technologies have improved, the paths to application performance have become tricky to navigate. Even though JVMs can optimize our code and make objects almost free, the demands of applications and users continue to grow.

Some of the time, maybe even most of the time, the good coding patterns prevail: small methods inline properly, interface and type checks become inexpensive, native code produced by the JIT compiler is compact and efficient. But other times we need to hand-craft our code, dial back abstractions and architecture in deference to the limitations of the compiler and CPU. Some of the time, objects really are free and we can ignore the fact that were consuming memory bandwidth and GC cycles. Other times were dealing with terabyte-scale (or larger) datasets that put stress on even the best garbage collectors and memory subsystems.

The answer to the performance question these days is to know your tools. And frequently, that means knowing not just how Java the language works, but also how JVM libraries, memory, the compiler, GCs, and the hardware your apps run on are interacting. In my work on the JRuby project, Ive learned an immutable truth about the JVM: theres no single solution for all performance problems, but for all performance problems there are solutions. The trick is finding those solutions and piecing together the ones that meet your needs best. Now you have a secret weapon in these performance battles: the book you are about to read.

Turn the page, friends, and discover the wealth of tools and techniques available to you. Learn how to balance application design with available resources. Learn how to monitor and tune the JVM. Learn how to make use of the latest Java technologies that are more efficient than old libraries and patterns. Learn how to make Java fly.