What Is Parallel Collector

Java GC Tutorials - Herong's Tutorial Examples

∟Parallel Collector - "+XX:+UseParallelGC"

∟What Is Parallel Collector

This section describes the Parallel Collector, which uses the combination of the Parallel Scavenge (PS) Collector of Young generation and the Parallel Old Collector of Tenured generation, so both minor GC and major GC are performed in multiple threads parallelly in a stop-the-world fashion.

What Is Parallel Collector? Parallel Collector is actually a combination of two garbage collectors:

The Parallel Scavenge (PS) Collector of Young generation - It uses the 3-area (Eden, Survivor Space "From" and Survivor Space "To") algorithm parallelly with multiple threads in a stop-the-world fashion.
The Parallel Old Collector of Tenured generation - It uses the "Mark-Sweep-Compact" algorithm parallelly with multiple threads in a stop-the-world fashion.

Main features of the Parallel Collector:

Parallel Collector runs serially with the application. That is, the Java application execution is halted while garbage collection is taking place.
Parallel Collector runs with multiple threads. So it is able take advantages of multi-processor computer systems.
Parallel Collector can be specified using the "-XX:+UseParallelGC", or "-XX:+UseParallelOldGC" JVM option.
Parallel Collector improves the performance compare to the Serial Collector on multi-processor computer systems because it is running multiple threads.
Parallel Collector may decrease the performance compare to the Serial Collector on single-processor computer systems because it needs extra time to start and manage multiple threads.

The Parallel Collector is also called Throughput Collector because it reduces GC time to improve the throughput of the application. It's a good choice for backend applications.