What Is Concurrent Mark-Sweep (CMS) Collector

Java GC Tutorials - Herong's Tutorial Examples

∟Concurrent Mark-Sweep (CMS) Collector - "+XX:+UseConcMarkSweepGC"

∟What Is Concurrent Mark-Sweep (CMS) Collector

This section describes the Concurrent Mark-Sweep (CMS) Collector, which attempts to reduce application pause times of Major GC by using separate garbage collector threads to trace the reachable objects concurrently with the execution of the application threads.

What Is Concurrent Mark-Sweep (CMS) Collector? Concurrent Mark-Sweep (CMS) Collector is actually a combination of two garbage collectors:

The Parallel New (ParNew) 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. It also has an internal 'callback' that allows an old generation collector to operate on the objects it collects
The Concurrent Mark Sweep (CMS) Collector of Tenured generation - It uses the "Mark-Sweep-Compact" algorithm with multiple threads concurrently with the application. A small portion is still running in a stop-the-world fashion.

The CMS Collector can be invoked using the "-XX:+UseConcMarkSweepGC" JVM option.

During each Major GC cycle, the CMS collector pauses all the application threads for a brief period at the beginning of the collection and again toward the middle of the collection. The second pause tends to be the longer of the two pauses. Multiple threads are used to do the collection work during both pauses. The remainder of the collection (including most of the tracing of live objects and sweeping of unreachable objects is done with one or more garbage collector threads that run concurrently with the application.

Typically applications that have a relatively large set of long-lived data (a large tenured generation) and run on machines with two or more processors tend to benefit from the use of this collector.

The CMS collector is also called Low-Latency Collector, because it reduces the application pause time. But it may slowdown the application, because the GC is running concurrently with the application. It's a good choice for interactive applications like Websites.