97 Things Every Java Programmer Should Know

Chapter 29. Garbage Collection Is Your Friend

Holly Cummins

Poor old garbage collection. One of the unsung heroes of Java, often blamed, rarely praised. Before Java made garbage collection mainstream, programmers had little choice but to track all the memory they’d allocated manually, and deallocate it once nothing was using it anymore. This is hard. Even with discipline, manual deallocation is a frequent cause of memory leaks (if too late) and crashes (if too early).

Java GC (garbage collection) is often thought of as a necessary cost, and “reduce time spent in GC” is common performance guidance. However, modern garbage collection can be faster than malloc/free, and time spent in GC can speed everything up. Why? Garbage collectors do more than memory deallocation: they also handle the allocation of memory and the arrangement of objects in memory. A good memory management algorithm can make allocation efficient by reducing fragmentation and contention. It can also boost throughput and lower response times by rearranging objects.

Why does the location of an object in memory affect application performance? A high proportion of a program’s execution time is spent stalled in hardware, waiting for memory access. Heap access is geologically slow compared to instruction processing, so modern computers use caches. When an object is fetched into a processor’s cache, its neighbors are also ...