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 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.

Table of Contents

 About This Book

 Heap Memory Area and Size Control

 JVM Garbage Collection Logging

 Introduction of Garbage Collectors

 Serial Collector - "+XX:+UseSerialGC"

 Parallel Collector - "+XX:+UseParallelGC"

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

What Is Concurrent Mark-Sweep (CMS) Collector

 Concurrent Mark-Sweep Collector GC Log Message Format

 Reduce Stop-The-World with Concurrent Mark and Sweep

 Parallel New (ParNew) Collector for Minor GC

 ParNew Collector - Tenuring Age Distribution

 Maximium Logging of All Phases on Young GC

 "-XX:ParallelGCThreads=6" - Young GC Parallel Threads

 "-XX:MaxTenuringThreshold=0" - Tenuring Objects Immediately

 "-XX:CMSInitiatingOccupancyFraction=20" - Initiate CMS

 Maximium Logging of All Phases on Old GC

 Maximium Logging of All Phases on Full GC

 "-XX:ParallelGCThreads=6" - Old GC Parallel Threads

 "-XX:ConcGCThreads=3" - Old GC Concurrent Threads

 Garbage First (G1) Collector - "+XX:+UseG1GC"

 The Z Garbage Collector (ZGC) - "+XX:+UseZGC"

 Object References and Garbage Collection

 Garbage Collection Performance Test Program

 Performance Tests on Serial Collector

 Performance Tests on Parallel collector

 Performance Tests on Concurrent collector

 Performance Tests on G1 collector

 Garbage Collection Performance Test Summary

 References

 Full Version in PDF/EPUB