Log Message Types from Serial Collector

This section describes various types of log messages generated from the Serial Collector with the '-Xlog:gc*=debug' option.

If you run your application using the Serial Collector (-XX:+UseSerialGC) with the "-Xlog:gc*=debug" logging option, you may see the following types of log messages:

1. Heap initial allocation - Reported when JVM starts:

[debug][gc,heap] Minimum heap 2097152  Initial heap 2097152
   Maximum heap 67108864

2. Young generation heap reduction message - Reported as part of a Young GC process:

[info][gc,heap] GC(0) DefNew: 1024K->128K(1152K)

3. Tenured generation heap expansion message - Reported as part of a Young GC process:

[info][gc,heap] GC(0) Tenured: 0K->418K(768K)

4. Start/end of a Young GC:

[info][gc,start] GC(0) Pause Young (Allocation Failure)
...
[info][gc] GC(0) Pause Young (Allocation Failure) 1M->0M(1M) 2.341ms

5. Object aging threshold - Reported as part of a Young GC:

[debug][gc,age] GC(0) Desired survivor size 65536 bytes,
                       new threshold 1 (max threshold 15)

6. Metaspace reduction - Reported as part of a Young GC:

[0.059s][info ][gc,metaspace] GC(0) Metaspace: 328K->328K(1056768K)

7. GC execution time - Reported as part of a Young or Full GC:

[info][gc,cpu] GC(0) User=0.00s Sys=0.00s Real=0.00s

8. Heap snapshot report before GC - Reported before a Young GC or Full GC process:

[debug][gc,heap] GC(0) Heap before GC invocations=0 (full 0):
                        def new generation total 1152K, used 1024K [...
[debug][gc,heap] GC(0)   eden space 1024K, 100% used [...
[debug][gc,heap] GC(0)   from space 128K,   0% used [...
[debug][gc,heap] GC(0)   to   space 128K,   0% used [...
[debug][gc,heap] GC(0)  tenured generation   total 768K, used 0K [...
[debug][gc,heap] GC(0)    the space 768K,   0% used [...
[debug][gc,heap] GC(0)  Metaspace
                          used 328K, capacity 4515K, committed 4864K,
						  reserved 1056768K
[debug][gc,heap] GC(0)   class space
                          used 30K, capacity 398K, committed 512K,
						  reserved 1048576K

9. Heap snapshot report after GC - Reported before a Young GC or Full GC process:

[debug][gc,heap] GC(0) Heap after GC invocations=1 (full 0):
                        def new generation total 1152K, used 128K [...
[debug][gc,heap] GC(0)   eden space 1024K,   0% used [...
[debug][gc,heap] GC(0)   from space 128K, 100% used [...
[debug][gc,heap] GC(0)   to   space 128K,   0% used [...
[debug][gc,heap] GC(0)  tenured generation   total 768K, used 418K [...
[debug][gc,heap] GC(0)    the space 768K,  54% used [...
[debug][gc,heap] GC(0)  Metaspace
                          used 328K, capacity 4515K, committed 4864K,
						  reserved 1056768K
[debug][gc,heap] GC(0)   class space
                          used 30K, capacity 398K, committed 512K,
						  reserved 1048576K

10. Start/end of a Full GC - Reported at the beginning of a Full GC:

[info ][gc,start] GC(2) Pause Full (Allocation Failure)
...
[info ][gc] GC(2) Pause Full (Allocation Failure) 0M->0M(2M) 15004100.942ms

11. Mark phase of a Full GC:

[info][gc,phases,start] GC(2) Phase 1: Mark live objects
...
[info][gc,phases] GC(2) Phase 1: Mark live objects 15004095.903ms

12. New address calculation phase of a Full GC:

[info][gc,phases,start] GC(2) Phase 2: Compute new object addresses
[info][gc,phases] GC(2) Phase 2: Compute new object addresses 2.754ms

13. Adjust pointers phase of a Full GC:

[info][gc,phases,start] GC(2) Phase 3: Adjust pointers
[info][gc,phases] GC(2) Phase 3: Adjust pointers 1.896ms

14. Move objects phase of a Full GC:

[info][gc,phases,start] GC(2) Phase 4: Move objects
[info][gc,phases] GC(2) Phase 4: Move objects 0.067ms

Table of Contents

 About This Book

 Heap Memory Area and Size Control

 JVM Garbage Collection Logging

 Introduction of Garbage Collectors

Serial Collector - "+XX:+UseSerialGC"

 What Is Serial Collector

 GC Log Message Format for Serial Collector

 GC Log Message Examples of Serial Collector

Log Message Types from Serial Collector

 Serial Collector Stops Application for Minor/Major GC

 Usage Report on Heap Memory Areas

 Default NewRatio - Old vs. New Generation

 "-XX:NewRatio" - Ratio of Tenured and "new" Generation

 "-XX:SurvivorRatio" - Ratio of Eden and Survivor Space

 Serial GC Tracing - Tight Heap

 Serial GC Tracing - Tight Heap (Part 2)

 Serial GC Tracing - Tight Heap (Part 3)

 Serial GC Tracing - Plenty of Heap

 Serial GC Tracing - Aged Live Objects

 Serial GC Tracing - Tenuring Threshold

 "-XX:TargetSurvivorRatio" - Second Tenuring Condition

 Serial GC Tracing - Tenuring Threshold Controlled

 "-XX:+NeverTenure" and "-XX:+AlwaysTenure" Working

 Minor GC Triggering Condition of Serial Collector

 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