Java Threads, Second Edition

Multiprocessor Scaling

Scaling is a term that is sometimes overused. It can apply to how many applications a computer can execute simultaneously, how many disks can be written to simultaneously, or how many cream cheese bagel orders can be processed by the local bagel shop’s crew. When the output cannot be increased no matter how many resources are added, this limit is generally the value used to specify what something scales to. If the oven cannot produce more bagels per hour, it does not matter how many people are added to the assembly line: the rate of bagels cannot exceed the rate produced by the oven. The scaling limit can also be controlled by many other factors, such as the rate that the cream cheese can be produced, the size of the refrigerators, or even by the suppliers for the bagel shop.

In this chapter, when we refer to the scalability of a multithreaded application, we are referring to the limit on the number of processors we can add and still obtain an acceleration. Adding more than this limit will not make the application run faster. Obviously, how an application scales depends on many factors: the operating system, the Java virtual machine implementation, the browser or application server, and the Java application itself. The best an application can scale will be based on the scalability limits of all of these factors.

For perfect CPU-bound programs in a perfect world, we could expect perfect scaling: adding a second CPU would halve the amount of time that it takes ...