Garbage Collectors

Java has four types of garbage collectors,

1. Serial Garbage Collector
2. Parallel Garbage Collector
3. CMS Garbage Collector
4. G1 Garbage Collector

Each of these four types has its own advantages and disadvantages. Most importantly, we the programmers can choose the type of garbage collector to be used by the JVM. We can choose them by passing the choice as JVM argument. Each of these types differ largely and can provide completely different application performance. It is critical to understand each of these types of garbage collectors and use it rightly based on the application.

1. Serial Garbage Collector

Serial garbage collector works by holding all the application threads. It is designed for the single-threaded environments. It uses just a single thread for garbage collection. The way it works by freezing all the application threads while doing garbage collection may not be suitable for a server environment. It is best suited for simple command-line programs.

Turn on the -XX:+UseSerialGC JVM argument to use the serial garbage collector.

2. Parallel Garbage Collector

Parallel garbage collector is also called as throughput collector. It is the default garbage collector of the JVM. Unlike serial garbage collector, this uses multiple threads for garbage collection. Similar to serial garbage collector this also freezes all the application threads while performing garbage collection.

3. CMS Garbage Collector

Concurrent Mark Sweep (CMS) garbage collector uses multiple threads to scan the heap memory to mark instances for eviction and then sweep the marked instances. CMS garbage collector holds all the application threads in the following two scenarios only,

1. while marking the referenced objects in the tenured generation space.
2. if there is a change in heap memory in parallel while doing the garbage collection.

In comparison with parallel garbage collector, CMS collector uses more CPU to ensure better application throughput. If we can allocate more CPU for better performance then CMS garbage collector is the preferred choice over the parallel collector.

Turn on the XX:+USeParNewGC JVM argument to use the CMS garbage collector.

4. G1 Garbage Collector

G1 garbage collector is used for large heap memory areas. It separates the heap memory into regions and does collection within them in parallel. G1 also does compacts the free heap space on the go just after reclaiming the memory. But CMS garbage collector compacts the memory on stop the world (STW) situations. G1 collector prioritizes the region based on most garbage first.

Turn on the –XX:+UseG1GC JVM argument to use the G1 garbage collector

Garbage Collection JVM Options

Following are the key JVM options that are related to Java garbage collection.

Type of Garbage Collector to run

Option	Description
-XX:+UseSerialGC	Serial Garbage Collector
-XX:+UseParallelGC	Parallel Garbage Collector
-XX:+UseConcMarkSweepGC	CMS Garbage Collector
-XX:ParallelCMSThreads=	CMS Collector – number of threads to use
-XX:+UseG1GC	G1 Gargbage Collector

GC Optimization Options

Option	Description
-Xms	Initial heap memory size
-Xmx	Maximum heap memory size
-Xmn	Size of Young Generation
-XX:PermSize	Initial Permanent Generation size
-XX:MaxPermSize	Maximum Permanent Generation size

Example Usage of JVM GC Options

java -Xmx12m -Xms3m -Xmn1m -XX:PermSize=20m -XX:MaxPermSize=20m -XX:+UseSerialGC -jar java-application.jar