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

Java 8 GC Tutorials - Herong's Tutorial Examples

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

This chapter provides tutorial notes and example codes on the Concurrent Mark-Sweep Collector specified by the '+XX:+UseConcMarkSweepGC' JVM option. Topics include introduction to Concurrent Mark-Sweep (CMS) Collector; CMS Collector log message format; demonstration of reduction of stop-the-world by running parts of Mark and Sweep step concurrently with the application.

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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

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

"-XX:CMSInitiatingOccupancyFraction=20" - Initiate CMS

"concurrent mode failure" - CMS Cancelled

"-XX:PrintCMSStatistics=1" - Print CMS Threads Stats

CMS GC Phases and Threads

"-XX:ParallelGCThreads=2" - CMS Parallel Threads

"-XX:ConcGCThreads=3" - CMS Concurrent Threads

Takeaways:

The Concurrent Mark-Sweep (CMS) Collector, -XX:+UseConcMarkSweepGC, actually uses the Parallel New (ParNew) Collector of the Young generation and the Concurrent Mark Sweep (CMS) Collector of the Tenured generation.
The CMS Collector uses "ParNew" to represent Young Generation GC in log messages.
The CMS Collector uses "CMS-concurrent-*" to represent Tenured Generation GC in log messages.
The CMS Collector performs Major GC with multiple threads concurrently without stopping the application most of the time.
Application is stopped by the CMS Collector for Initial Mark and Final Mark steps of Major GC.
Application is stopped by the CMS Collector for Minor GC.
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.
ParNew collector is used to perform Young generation GC when "-XX:+UseConcMarkSweepGC" option is used.
ParNew maintains a Tenuring Age Distribution list to determine object promotion to Tenured generation.
Tenured generation occupancy percentage initiates the CMS GC.
CMS Concurrent mode may fail, if Tenured generation gets fully occupied before it can finish the GC process.
The CMS collector does uses multiple threads for some phases in the GC process.
The number of threads in CMS parallel phases can be controlled by the "-XX:ParallelGCThreads=n" option.
The number of threads in 1 CMS concurrent phase, Mark phase, can be controlled by the "-XX:ConcGCThreads=n" option.

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"

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

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

Archived Tutorials

References

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