Even Faster Web Sites

Chapter 1. Understanding Ajax Performance

Douglas Crockford

Premature optimization is the root of all evil.

—Donald Knuth

Trade-offs

The design and construction of a computer program can involve thousands of decisions, each representing a trade-off. In difficult decisions, each alternative has significant positive and negative consequences. In trading off, we hope to obtain a near optimal good while minimizing the bad. Perhaps the ultimate trade-off is:

I want to go to heaven, but I don’t want to die.

More practically, the Project Triangle:

Fast. Good. Cheap. Pick Two.

predicts that even under ideal circumstances, it is not possible to obtain fast, good, and cheap. There must be a trade-off.

In computer programs, we see time versus memory trade-offs in the selection of algorithms. We also see expediency or time to market traded against code quality. Such trades can have a large impact on the effectiveness of incremental development.

Every time we touch the code, we are trading off the potential of improving the code against the possibility of injecting a bug. When we look at the performance of programs, we must consider all of these trade-offs.

Principles of Optimization

When looking at optimization, we want to reduce the overall cost of the program. Typically, this cost is the perceived execution time of the program, although we could optimize on other factors. We then should focus on the parts of the program that contribute most significantly to its cost.

For example, suppose that by profiling ...