Heap & thread dump

About Heap and Thread dump in java, JVM parameter and Best practice

1. Heap & Thread Dump

2. In this session
•What is Thread/Heap dump?
•When to generate ?
•How to generate ?
•Best Practices and JVM Options
•Ref and suggestion links to understand more this topic

3. What is Thread dump?
•A Thread Dump is a snapshot taken at a given time which provides you with
a complete listing of all created Java Threads.
•Like:
•Servlet Request
•Job (scheduler) etc.
•Each Thread found gives you information such as:

4. •Thread name: Name of the Thread we/vendor can specify thread name at a time
of the creating. E.g: we can configure quartz-scheduler thread or by default quartz
scheduler name looks like “instanceName+ ‘_QuartzSchedulerThread’ ”
•Thread type & priority ex: daemon prio=3 where daemon meaning their Threads
are running in background
•Java Thread ID ex: tid=0x000001111a51a811 ** This is the Java Thread Id
obtained via java.lang.Thread.getId() and usually implemented as an auto-
incrementing long 1..n**
•Java Thread State and detail ex: waiting for monitor entry [0xfffffffea5afb000]
java.lang.Thread.State: BLOCKED (on object monitor)
•And lot more…

5. What is Heap dump?
•A heapdump is a snapshot of JVM memory. it shows the live objects on the
heap along with references between objects. It is used to determine memory
usage patterns and memory leak suspects.
•Java Heap can be categorized into three generations: -
•Young
•Tenured
•Permanent.

6. •Young generation has Eden Space and two survivor spaces. All newly created
objects are kept in Eden Space and moved to one of the survivor space after it
survives few gc’s. From one survivor space it moves to another. At an instant one
of the survivor space is completely free. From the survivor space, objects are
moved to the tenured generations. Permanent holds class data, such as describing
classes and methods
•What is a Memory Leak Memory leak.
When objects are not removed from the heap even when they are not required to be
present in the heap. This can be due to application code, application server code,
database driver or third party jar files.

7. When to generate Thread/Heap Dump ?
•Very frequent FULL GC
•Lesser memory is reclaimed on each Full GC
•Available free heap keeps on decreasing over time
•Response time decreases
Troubleshoot
•Gather GC Information by adding the following JVM flags
-Xloggc=filename -XX:+PrintGCTimeStamps -XX:+PrintGCDetails XX:-HeapDumpOnOutOfMemoryError

8. How to generate Thread/Heap Dump?
•Generate Thread Dump
•By using jstack
•Ex: jstack {pid} > stack-trace.log
(Ref: http://docs.oracle.com/javase/7/docs/technotes/tools/share/jstack.html#options)
•By using jvisualvm
•After connecting to the java process one can find Threads tab ->
Thread Dump button

9. •Generate Heap Dump
•By using Jmap
•Ex: jmap -dump:format=b,file=<file name>.hprof <process id>
(Ref: http://docs.oracle.com/javase/7/docs/technotes/tools/share/jmap.html)
•By using Visualvm
•After connecting to the java process One can see Monitor tab->Heap
Dump button
•By adding JVM parameter
•-XX:+HeapDumpOnOutOfMemoryError -XX:HeapDumpPath=$CATALINA_BASE/logs

10. Best Practices and JVM Options
•Keep a watch on heap usage and use alert mechanisms.
•Keep a watch on production logs
•Free up unnecessary objects.
•Close all connections in the finally block
•Don’t keep too much data in session.
•Exception handling

11. Type Option
Operation mode-sever
Entire heap sizeSpecify the same value for -Xms and -Xmx.
New area size
-XX:NewRatio: value of 2 to 4
-XX:NewSize=? –XX:MaxNewSize=?. Also good to specify NewSize instead of NewRatio.
Perm size
-XX:PermSize=256 m -XX:MaxPermSize=256 m. Specify the value to an extent not to cause any trouble in the operation because it
does not affect the performance.
GC log
-Xloggc:$CATALINA_BASE/logs/gc.log -XX:+PrintGCDetails -XX:+PrintGCDateStamps. Leaving a GC log does not
particularly affect the performance of Java applications. You are recommended to leave a GC log as much as possible.
GC algorithm
-XX:+UseParNewGC -XX:+CMSParallelRemarkEnabled -XX:+UseConcMarkSweepGC-
XX:CMSInitiatingOccupancyFraction=75.This is only a generally recommendable configuration. Other choices could be better
depending on the characteristics of the application.
Creating a heap
dump when an
OOM error occurs
-XX:+HeapDumpOnOutOfMemoryError -XX:HeapDumpPath=$CATALINA_BASE/logs
Actions after an
OOM occurs
-XX:OnOutOfMemoryError=$CATALINA_HOME/bin/stop.sh or -
XX:OnOutOfMemoryError=$CATALINA_HOME/bin/restart.sh. After leaving a heap dump, take a proper operation according
to a management policy.

12. Ref and suggestion links to understand
more this topic

•http://www.cubrid.org/blog/dev-platform/understanding-java-garbage-collection/ - To understand garbage
collection
•http://www.cubrid.org/blog/dev-platform/how-to-monitor-java-garbage-collection/ - To Monitor Garbage
Collection
•https://blog.codecentric.de/en/2011/08/create-and-understand-java-heapdumps-act-4/
•http://www.cubrid.org/blog/dev-platform/the-principles-of-java-application-performance-tuning/
•http://javabook.compuware.com/content/memory/how-garbage-collection-works.aspx

13. MemoryAndThreadExample.zip
Find attached zip project, I have added MemoryLeak and Deadlock example.
That we will analysed using jvisualVM

14. Suggestion
and
Question / Answer

15. Thank You!!!