The right way to optimize

The Hotspot Virtual Machine

This article is pretty old. But still there are some points there.

The right way to optimize

Of course, the basic principles of optimizing any kind of software apply to Java. Java programmers who live by these principles have little cause to choose performance tweaks over good designs, except in rare cases. Unfortunately, not every Java programmer subscribes to these principles.

The fundamental principle of optimization is: Don't optimize until you know you have a problem. As Donald Knuth once said, "Premature optimization is the root of all evil." In general, you should forget about optimization and just create good quality designs and clear code. After you get your well-designed program working, if you then find that its performance is lacking, that is the time to optimize.

Another basic principle is: Measure the program before and after your optimization efforts. If you find that a particular effort did not make a significant improvement in the program's performance, then revert back to the original, clear code.

A third principle takes into account that most programs spend 80 to 90 percent of their time executing 10 to 20 percent of the code: You should profile the program to isolate the code that really matters to performance (that 10 to 20 percent), and just focus your optimization efforts there.

Once you isolate the time-critical areas of your program, you should first try to devise a better algorithm, use APIs in a smarter way, or use standard code optimization techniques such as strength reduction, common sub-expression elimination, code motion, and loop unrolling. Only as a last resort should you sacrifice good object-oriented, thread-safe design and maintainable code in the name of performance.