How to troubleshoot GC run

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Memory setting is very important for a java application.If properly set it will give significant clue when there is a performance issue from jvm side.

Whenever there is a memory issue occurring in a java application first thing that comes in our mind is whether the memory setting given is sufficient.

We specify memory configuration with -mx argument while running the java process.

-Investigate JVM memory usage in case of out of memory

-Xmx<max_memory_value>

Garbage collection logging flag:

• -XX:+PrintGC or -verbose:gc 
• -XX:+PrintGCDetails
• -XX:+PrintGCTimeStamps and -XX:+PrintGCDateStamps
• -Xloggc //-Xloggc:<file>

How to debug GC run:

Log diagnostic information after each garbage collector run

Run application with -verbose:gc 

 [GC 69068K->56180K(116544K), 0.0054920 secs]

 [GC 69068K->56180K(116544K), 0.0054920 secs] 

 // Young generation usage before 69068K

  and after run 56180K

         Size of JVM heap  116544K

  [Full GC 99119K->50845K(116544K), 0.0137490 secs] //

  The above shows full garbage collection. When application is having frequent full GC running means there is a problem.

  And even after full GC run memory utilisation is not going down that needs to be addressed on priority.

  

 How to get heapdump?

1. Using jmap

jmap -F -dump:format=b,file=heap.bin <process_id>

2. Getting a heap dump on OutOfMemoryError

java -Xmx64m  -XX:+HeapDumpOnOutOfMemoryError  -XX:HeapDumpPath=/tmp/heap.bin  

How to read Heap dump?

I. Using jhat 

jhat heap.bin

Access to http://localhost:7000/ to explore the heap dump

II. Using visual tool

  jvisualvm

   upload the heap dump

Manageable Flags:
All the above flags that w start with “PrintGC” belong to the “manageable category.
 eg. The verbose:gc option can be enabled at run time using java management API or JVM TI.Jconsole can also be used to enable this option as it is complaint with JMX extension.

  

More about JVM diagnostic :http://www.oracle.com/technetwork/java/javase/clopts-139448.html