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

Java GC Tutorials - Herong's Tutorial Examples

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

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

In the first test, let's try the "-Xlog:gc" garbage collection logging option to see what we will get in the log file.

herong> java -Xms2m -Xmx64m -Xlog:gc: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)

herong> more gc.log
[0.013s][info][gc] Using G1
[0.028s][info][gc] Periodic GC disabled
[0.056s][info][gc] GC(0) Pause Young (Normal) (G1 Evacuation Pause) ...
[0.118s][info][gc] GC(1) Pause Young (Normal) (G1 Evacuation Pause) ...
[0.154s][info][gc] GC(2) Pause Young (Normal) (G1 Evacuation Pause) ...
[0.211s][info][gc] GC(3) Pause Young (Normal) (G1 Evacuation Pause) ...
[0.246s][info][gc] GC(4) Pause Young (Normal) (G1 Evacuation Pause) ...
[0.296s][info][gc] GC(5) Pause Young (Normal) (G1 Evacuation Pause) ...
[0.320s][info][gc] GC(6) Pause Young (Normal) (G1 Evacuation Pause) ...
[0.333s][info][gc] GC(7) Pause Young (Normal) (G1 Evacuation Pause) ...
[0.349s][info][gc] GC(8) Pause Young (Normal) (G1 Evacuation Pause) ...
[0.356s][info][gc] GC(9) Pause Young (Normal) (G1 Evacuation Pause) ...
[0.371s][info][gc] GC(10) Pause Young (Normal) (G1 Evacuation Pause) ...
[0.388s][info][gc] GC(11) Pause Young (Normal) (G1 Evacuation Pause) ...
[0.398s][info][gc] GC(12) Pause Young (Normal) (G1 Evacuation Pause) ...
[0.412s][info][gc] GC(13) Pause Young (Normal) (G1 Evacuation Pause) ...
[0.478s][info][gc] GC(14) Pause Young (Normal) (G1 Evacuation Pause) ...
[0.500s][info][gc] GC(15) Pause Young (Normal) (G1 Evacuation Pause) ...
[0.503s][info][gc] GC(16) Pause Young (Concurrent Start) (G1 Evacuati...
[0.503s][info][gc] GC(17) Concurrent Cycle
[0.505s][info][gc] GC(18) Pause Young (Normal) (G1 Evacuation Pause) ...
[0.508s][info][gc] GC(19) Pause Young (Normal) (G1 Evacuation Pause) ...
[0.510s][info][gc] GC(17) Pause Remark 14M->14M(27M) 0.830ms
[0.513s][info][gc] GC(20) Pause Young (Normal) (G1 Evacuation Pause) ...
[0.515s][info][gc] GC(17) Pause Cleanup 18M->18M(27M) 0.025ms
[0.515s][info][gc] GC(17) Concurrent Cycle 12.548ms
[0.519s][info][gc] GC(21) Pause Young (Prepare Mixed) (G1 Evacuation ...
[0.522s][info][gc] GC(22) Pause Young (Mixed) (G1 Evacuation Pause)  ...
[0.523s][info][gc] GC(23) Pause Young (Concurrent Start) (G1 Evacuat ...
[0.524s][info][gc] GC(24) Concurrent Cycle
[0.528s][info][gc] GC(25) Pause Young (Normal) (G1 Evacuation Pause) ...
[0.533s][info][gc] GC(26) Pause Young (Normal) (G1 Evacuation Pause) ...
[0.536s][info][gc] GC(27) Pause Young (Normal) (G1 Evacuation Pause) ...
[0.538s][info][gc] GC(24) Pause Remark 34M->34M(59M) 0.934ms         ...
[0.540s][info][gc] GC(28) Pause Young (Normal) (G1 Evacuation Pause) ...
[0.543s][info][gc] GC(24) Pause Cleanup 38M->38M(61M) 0.050ms
[0.545s][info][gc] GC(24) Concurrent Cycle 21.163ms
...

The output shows that the JVM is using the G1 as the default garbage collector. The default logging level is INFO.

JVM supports multiple garbage collectors and each garbage collector uses different log message format. I will explain how to read log messages later when we have better understandings on how each garbage collector works.