Memory Management and Garbage Collectors

JVM Tutorials - Herong's Tutorial Examples - Version 4.23, by Dr. Herong Yang

JVM Tutorials - Herong's Tutorial Examples

∟Memory Management and Garbage Collectors

This chapter provides tutorial notes and example codes on memory management and garbage collectors. Topics include JVM memory management guidelines; OutOfMemoryError exception; Garbage Collection (GC); Young and Tenured Heap regions; Eden and Survivor spaces; Mark-Sweep-Compact algorithm; serial, parallel, concurrent and regionalized collectors; GC details logging.

Memory Management General Rules

Java Exception: "java.lang.OutOfMemoryError: Java heap space"

OutOfMemoryError Comparison of HotSpot and JRockit

Garbage Collection Demonstration

JVM Memory Manager - Garbage Collector

Generational Garbage Collection in HotSpot

Young Generation Collection - Minor Collection

Tenured Generation Collection - Full Collection

HotSpot Default Garbage Collector - Serial Collector

"-XX:+PrintGCDetails" - Garbage Collection Logging

GC Log Messages on GarbageCollection.java

Serial, Parallel, Concurrent, and Regionalized Collectors

Parallel Collector GC Log Message Format

Parallel Compacting Collector GC Log Message Format

Concurrent Mark-Sweep Collector GC Log Message Format

Garbage First GC Log Message Format

Conclusions:

JVM raises OutOfMemoryError exception when the Java application requires more memory than its given maximum limit.
OutOfMemoryError exceptions happen mostly in the Heap data area.
The Heap area memory management is done by the garbage collector, which allocates and deallocates data objects automatically for the Java application. 's garbage collector automatically collects dead objects to free up used memory.
HotSpot JVM Heap area is divided into 2 generations: Young Generation and Tenured (Old) Generation.
The Young Generation is divided into 3 areas: Eden, Survivor Space 'From' and Survivor Space 'To'.
When the Eden area is full, a Minor Garbage Collection (GC) is performed, which moves live objects from the Eden area to the Survivor Space 'To' area. It also empties the Survivor Space 'From' area by moving "young" live objects to the Survivor Space 'To' area and promoting "old" live objects to the Tenured Generation.
When the Tenured Generation is full, a Full Garbage Collection (GC) is performed, which follows a 3-step, Mark-Sweep-Compact (MSC), process to deallocate dead objects and defragment free space.
The Serial Collector, -XX:+UseSerialGC, performs both Minor and Full GC in a stop-the-world fashion and serially using 1 CPU only.
The Parallel Collector, -XX:+UseParallelGC, improves the performance of the Serial Collector by performing Minor GC parallelly using multiple CPUs.
The Parallel Compacting Collector, -XX:+UseParallelOldGC, improves the performance of the Parallel Collector by performing Full GC parallelly using multiple CPUs.
The Concurrent Mark-Sweep (CMS) Collector, -XX:+UseConcMarkSweepGC, improves the performance of the Parallel Compacting Collector by performing Full GC concurrently while the Java application is running.
The Garbage First (G1) Collector, -XX:+UseG1GC, improves the performance of the Concurrent Mark-Sweep (CMS) Collector by dividing the Heap area into regions and working on regions mostly filled with garbage first.
The -XX:+PrintGCDetails option generates GC detail log messages helping us to know how well the garbage collector is doing its job.