"-Xlog:gc=trace" - Lowest GC Logging Level: TRACE

This section describes the garbage collection logging option, '-Xlog:gc=trace', which prints out log messages from the GC channel at the TRACE level, which is lowest logging level.

In the second test, let's try the "-Xlog:gc=trace" garbage collection logging option to get more logging messages to the lowest level of TRACE in the log file:

C:\herong>java -Xms2m -Xmx64m -Xlog:gc=trace:gc.log GarbageCollection

Step/TotalMemory/FreeMemory/UsedMemory:
1   2031616   698848   1332768
2   3874816   1473392   2401424
3   3874816   425688   3449128
4   8970240   4442016   4528224
...
(Ctrl-C)

C:\herong>type gc.log
[0.005s][trace][gc] MarkStackSize: 4096k  MarkStackSizeMax: 16384k
[0.013s][debug][gc] ConcGCThreads: 2 offset 16
[0.013s][debug][gc] ParallelGCThreads: 8
[0.013s][debug][gc] Initialize mark stack with 4096 chunks, maximum 16384
[0.014s][info ][gc] Using G1
[0.032s][info ][gc] Periodic GC disabled
...
[0.584s][info ][gc] GC(34) Pause Young (Normal) (G1 Evacuation Pause) ...
[0.587s][info ][gc] GC(35) Pause Young (Normal) (G1 Evacuation Pause) ...
[0.590s][trace][gc] GC(36) Restored 3175 marks                        ...
[0.590s][info ][gc] GC(36) To-space exhausted                         ...
[0.590s][info ][gc] GC(36) Pause Young (Normal) (G1 Evacuation Pause) ...
[0.590s][info ][gc] GC(27) Pause Cleanup 63M->63M(64M) 0.065ms        ...
[0.593s][trace][gc] GC(37) Restored 10478 marks                       ...
[0.593s][info ][gc] GC(37) To-space exhausted                         ...
[0.593s][info ][gc] GC(37) Pause Young (Prepare Mixed) (G1 Evacuation ...
[0.594s][debug][gc] GC(38) Clear Next Bitmap 0.261ms                  ...
[0.610s][trace][gc] GC(38) Restored 11390 marks                       ...
[0.611s][info ][gc] GC(38) Pause Full (G1 Evacuation Pause) 63M->34M(6...
[0.611s][info ][gc] GC(27) Concurrent Cycle 56.572ms
...

Obviously, "-Xlog:gc=trace" option is generating more logging messages. So you should use "=trace" all the time when troubleshooting any GC issues.

Table of Contents

 About This Book

 Heap Memory Area and Size Control

JVM Garbage Collection Logging

 Garbage Collection Demo Program

 Garbage Collection Logging Options

 "-Xlog:gc" - Default GC Logging Level: INFO

"-Xlog:gc=trace" - Lowest GC Logging Level: TRACE

 "-Xlog:gc+heap=trace" - GC+HEAP Log Messages

 "-Xlog:gc*=trace" - Maximum GC Logging

 "-XX:+PrintFlagsFinal" - Print JVM Options

 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