Java Performance Tuning

Faster VMs

VM runtimes and Java compilers vary enormously over time and across vendors. More and more optimizations are finding their way into both VMs and compilers. Many possible compiler optimizations are considered in later sections of this chapter. In this section I focus on VM optimizations.

VM Speed Variations

Different VMs have different running characteristics. Some VMs are intended purely for development and are highly suboptimal in terms of performance. These VMs may have huge inefficiencies, even in such basic operations as casting between different numeric types. One development VM I used had this behavior; it provided the foundation of an excellent development environment (actually my preferred environment), but was all but useless for performance testing, as any data type manipulation other than with ints or booleans produced highly varying and misleading times.

It is important to run any tests involving timing or profiling in the same VM you plan to run the application. You should test your application in the current “standard” VMs if your target environment is not fully defined.

There is, of course, nothing much you can do about speeding up any one VM (short of upgrading the CPUs). But you should be aware of the different VMs available, whether or not you control the deployment environment of your application. If you control the target environment, you can choose your VM appropriately. If you do not control the environment on which your application runs, remember ...