Chapter 6. Garbage Collection
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 to minimize the impact of GC 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 per-
spective of its log output, which 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 GC 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.
Understanding the Throughput Collector
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.