Chapter 5 examined the general behavior of all garbage collectors, including JVM flags that apply universally to all GC algorithms: how to select heap sizes, generation sizes, logging, and so on.
The basic tunings of garbage collection suffice for many circumstances. When they do not, it is time to examine the specific operation of the GC algorithm in use to determine how its parameters can be changed in order to minimize the impact of garbage collection on the application.
The key information needed to tune an individual collector is the data from the GC log when that collector is enabled. This chapter starts, then, by looking at each algorithm from the perspective of its log output—that allows us to understand how the GC algorithm works, and how it can be adjusted to work better. Each section then includes tuning information to achieve that better performance.
There are a few unusual cases that impact the performance of all garbage collection algorithms—allocation of very large objects, objects that are neither short- nor long-lived, and so on. Those cases are covered at the end of this chapter.
We’ll start by looking at the individual garbage collectors, beginning with the throughput collector. The throughput collector has two basic operations: it collects the young generation, and it collects the old generation.
Figure 6-1 shows the heap before and after a young collection.