"-XX:+PrintGCDetails" - Option for Detailed GC Messages

Java 8 GC Tutorials - Herong's Tutorial Examples

∟"-XX:+PrintGCDetails" - Option for Detailed GC Messages

This section describes the garbage collection logging option, '-XX:+PrintGCDetails', which prints a long detailed message with more details after each garbage collection is done.

In the second test, let's try the "-XX:+PrintGCDetails" garbage collection logging option to see what we will get in the log file:

herong> \progra~1\java\jdk1.8.0\bin\java -Xms2m -Xmx64m -XX:+PrintGCDetails \
   -Xloggc: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
Java HotSpot(TM) Client VM (25.45-b02) for windows-x86
   JRE (1.8.0_45-b15), built on Apr 30 2015 12:31:00
Memory: 4k page, physical 3931768k(1445688k free),
   swap 7861660k(3346560k free)
CommandLine flags: -XX:InitialHeapSize=2097152 -XX:MaxHeapSize=67108864
   -XX:+PrintGC -XX:+PrintGCDetails
   -XX:+PrintGCTimeStamps -XX:-UseLargePagesIndividualAllocation
0.100: [GC (Allocation Failure)
   0.100: [DefNew: 889K->64K(960K), 0.0016722 secs]
   889K->755K(1984K), 0.0017865 secs]
1.095: [GC (Allocation Failure)
   1.095: [DefNew: 865K->64K(960K), 0.0040666 secs]
   1.099: [Tenured: 1476K->1540K(1540K), 0.0084915 secs]
   1557K->1540K(2500K), [Metaspace: 1555K->1555K(4480K)], 0.0279583 secs]
2.116: [GC (Allocation Failure)
   2.116: [DefNew: 1061K->127K(1216K), 0.0059764 secs]
   2601K->2580K(3784K), 0.0061521 secs]
3.116: [GC (Allocation Failure)
   3.116: [DefNew: 1171K->127K(1216K), 0.0060344 secs]
   3.122: [Tenured: 3492K->3604K(3604K), 0.0044445 secs]
   3624K->3620K(4820K), [Metaspace: 1556K->1556K(4480K)], 0.0107617 secs]
...

Heap
 def new generation   total 19648K, used 13200K [...
  eden space 17472K,  75% used [...
  from space 2176K,   0% used [...
  to   space 2176K,   0% used [...
 tenured generation   total 43712K, used 32479K [...
   the space 43712K,  74% used [...
 Metaspace used 1561K, capacity 2242K, committed 2368K, reserved 4480K

Notes on the output:

Three JVM options: "-XX:+PrintGC", "-XX:+PrintGCTimeStamps" and "-XX:-UseLargePagesIndividualAllocation" were added by default.
Each GC log message did have more detailed information. I will explain how to read log messages later when we have better understandings on how each garbage collector works.
A summary of the heap memory area usage was reported at the end.