"jcmd 0 GC.*" - Running GC Diagnostic Commands

Java Tools Tutorials - Herong's Tutorial Examples

∟"jcmd 0 GC.*" - Running GC Diagnostic Commands

This section provides a tutorial example on how to use 'jcmd 0 GC.*' commands to get heap information, run GC or generate heap dump file, of a given JVM.

Similar to VM.* diagnostic commands, I can run GC.* diagnostic commands the LongSleep JVM:

herong> jcmd LongSleep GC.heap_info
1928:
 garbage-first heap   total 61440K, used 1024K [0x00000000c4000000, ...)
  region size 1024K, 2 young (2048K), 0 survivors (0K)
 Metaspace used 4971K, capacity 5040K, committed 5120K, reserved 1056768K
  class space used 406K, capacity 447K, committed 512K, reserved 1048576K

herong> jcmd LongSleep GC.finalizer_info
1928:
No instances waiting for finalization found

herong> jcmd LongSleep GC.class_stats
1928:
Index Super InstBytes KlassBytes Bytecodes ... ClassName
    1    -1    169552        472         0 ... [B
    2    -1     92200        472         0 ... [Ljava.lang.Object;
    3    51     90384        640      5437 ... java.lang.Class
    4    51     76736        552       149 ... java.util.HashMap$Node
    5    51     76224        584      4108 ... java.lang.String
...

herong> jcmd LongSleep GC.class_histogram
1928:
 num     #instances         #bytes  class name (module)
-------------------------------------------------------
   1:          3273         169552  [B (java.base@10.0.1)
   2:          1373          92200  [Ljava.lang.Object; (java.base@10.0.1)
   3:           739          90384  java.lang.Class (java.base@10.0.1)
   5:          3176          76224  java.lang.String (java.base@10.0.1)
   8:            16          42448  [C (java.base@10.0.1)
...

herong> jcmd LongSleep GC.heap_dump LongSleep.heap
1928:
Heap dump file created

herong> jcmd LongSleep GC.run
1928:
Command executed successfully