Where to Optimize

Once you have determined that you have a performance issue you need to determine where in the code the problems occur. You can do this by running a profiler. A profiler is a program that will let you run your program, and it will tell you how much time is spent in each function, and how many times they are run. gprof is the standard GNU/Linux profiling tool, but a discussion of using profilers is outside the scope of this text. After running a profiler, you can determine which functions are called the most or have the most time spent in them. These are the ones you should focus your optimization efforts on.

If a program only spends 1 % of its time in a given function, then no matter how much you speed it up you will only achieve a maximum of a 1% overall speed improvement. However, if a program spends 20% of its time in a given function, then even minor improvements to that functions speed will be noticeable. Therefore, profiling gives you the information you need to make good choices about where to spend your programming time.

In order to optimize functions, you need to understand in what ways they are being called and used. The more you know about how and when a function is called, the better position you will be in to optimize it appropriately.

There are two main categories of optimization - local optimizations and global optimizations. Local optimizations consist of optimizations that are either hardware specific - such as the fastest way to perform a given computation - or program-specific - such as making a specific piece of code perform the best for the most often-occuring case. Global optimization consist of optimizations which are structural. For example, if you were trying to find the best way for three people in different cities to meet in St. Louis, a local optimization would be finding a better road to get there, while a global optimization would be to decide to teleconference instead of meeting in person. Global optimization often involves restructuring code to avoid performance problems, rather than trying to find the best way through them.