Garbage First (G1) Collector

Java GC Tutorials - Herong's Tutorial Examples

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

This chapter provides tutorial notes and example codes on the Garbage First (G1) Collector specified by the '+XX:+UseG1GC' JVM option. Topics include introduction the G1 Collector; G1 log message format; Stop-The-World behavior of G1 Collector.

These sections are omitted from this Web preview version. To view the full content, see information on how to obtain the full version this book.

What Is Garbage First (G1) Collector

Garbage First GC Log Message Format

Maximum Logging of All Phases on Normal Young GC

Maximum Logging on Concurrent Start Young GC

Maximum Logging of All Phases on Old GC

Maximum Logging of All Phases on Full GC

"-XX:ParallelGCThreads=6" - G1 Parallel Threads

"-XX:ConcGCThreads=5" - Old GC Concurrent Threads

"-XX:G1HeapRegionSize=n" - G1 Region Size

G1 Humongous Allocation - Objects Too Large

"-Xlog:gc+age=trace" - Object Age

"-XX:+MaxTenuringThreshold=0" - Skip G1 Survivors

Takeaways:

The Garbage First (G1) Collector, -XX:+UseG1GC, improves the performance of the Concurrent Mark-Sweep (CMS) Collector by dividing the Heap area into regions and working on regions mostly filled with garbage first.
G1 Young (or Minor) GC is performed with application stopped and with multiple parallel threads.
G1 Old (or Major) GC has 7 phases: Clear Claimed Marks, Scan Root Regions, Mark, Remark, Rebuild Remembered Sets, Cleanup, Cleanup for Next Mark.
3 of 7 G1 Old GC phases, Clear Claimed Marks, Remark, Cleanup, are paused phases performed with application stopped and with multiple parallel threads.
4 of 7 G1 Old GC phases, Scan Root Regions, Mark, Rebuild Remembered Sets, Cleanup for Next Mark, are concurrent phases performed with application, Young/Full GC running and with multiple concurrent threads.
G1 Full GC is triggered when an Old GC is running too long and the heap is running out of space.
G1 Full GC is performed with application stopped and with multiple parallel threads.
"-XX:G1HeapRegionSize=n" can be used to specify the size of regions used by the G1 collector. The default region size is 1024K.
Any object that is more than half a region size is considered a "Humongous object". Such an object is allocated directly in the old generation into "Humongous regions". A Humongous region is a contiguous set of regions.

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"

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