"-XX:+PrintTenuringDistribution" - Tunuring Distribution

Java 8 GC Tutorials - Herong's Tutorial Examples

∟"-XX:+PrintTenuringDistribution" - Tunuring Distribution

This section describes the garbage collection logging option, '-XX:+PrintTenuringDistribution', which prints the tenuring distribution of objects in the survivor space after each Minor GC.

In this tutorial, let's try the "-XX:+PrintTenuringDistribution" 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:+PrintTenuringDistribution -XX:+PrintHeapAtGC \
   -XX:+PrintGCDetails -Xloggc:gc.log GarbageCollection

Step/TotalMemory/FreeMemory/UsedMemory:
1   2031616   698408   1333208
2   3874816   1472824   2401992
3   3874816   424944   3449872
4   8970240   4441352   4528888
5   8970240   3358312   5611928
...
(Ctrl-C)

herong> more gc.log

{Heap before GC invocations=0 (full 0):
 def new generation   total 960K, used 878K [...
  eden space 896K,  98% used [...
  from space 64K,   0% used [...
  to   space 64K,   0% used [...
 tenured generation   total 1024K, used 0K [...
   the space 1024K,   0% used [...
 Metaspace used 1555K, capacity 2242K, committed 2368K, reserved 4480K
0.104: [GC (Allocation Failure) 0.104: [DefNew
Desired survivor size 32768 bytes, new threshold 1 (max 15)
- age   1:      65536 bytes,      65536 total
   -- First group of objects with age of 1 identified

: 878K->64K(960K), 0.0010261 secs] 878K->756K(1984K), 0.0011489 secs]
Heap after GC invocations=1 (full 0):
 def new generation   total 960K, used 64K [...
  eden space 896K,   0% used [...
  from space 64K, 100% used [...
  to   space 64K,   0% used [...
 tenured generation   total 1024K, used 692K [...
   the space 1024K,  67% used [...
 Metaspace used 1555K, capacity 2242K, committed 2368K, reserved 4480K
}

{Heap before GC invocations=1 (full 0):
 def new generation   total 960K, used 865K [...
  eden space 896K,  89% used [...
  from space 64K, 100% used [...
  to   space 64K,   0% used [...
 tenured generation   total 1024K, used 692K [...
   the space 1024K,  67% used [...
 Metaspace used 1555K, capacity 2242K, committed 2368K, reserved 4480K
1.109: [GC (Allocation Failure) 1.109: [DefNew
Desired survivor size 32768 bytes, new threshold 1 (max 15)
- age   1:      65536 bytes,      65536 total
   -- Second group of objects with age of 1 identified
   -- The previous group has been promoted to the "tenure"

: 865K->64K(960K), 0.0013277 secs]
   1.111: [Tenured: 1476K->1539K(1540K), 0.0013836 secs]
   1557K->1540K(2500K), ...
Heap after GC invocations=2 (full 1):
 def new generation   total 1216K, used 0K [...
  eden space 1088K,   0% used [...
  from space 128K,   0% used [...
  to   space 128K,   0% used [...
 tenured generation   total 2568K, used 1539K [...
   the space 2568K,  59% used [...
 Metaspace used 1555K, capacity 2242K, committed 2368K, reserved 4480K
}
...

The "-XX:+PrintTenuringDistribution" prints two messages after each minor GC:

"Desired survivor size 32768 bytes, new threshold 1 (max 15)" - The garbage collector suggests to set the survivor space to 32K based on analysis done in this minor GC. It also sets the tenuring age to 1. So any objects in the survivor space older than age of 1 will be promoted the "tenured" generation in the next round minor GC.
"- age 1: 65536 bytes, 65536 total" - The garbage collector prints a list of object age distributions. This line says 64K objects are age of 1.