Introduction to JVM memory structure and reasons for different OutOfMemoryError.

1. Understanding JVM
Aparna Chaudhary
Friday, October 4, 13

2. VM
Memory
What’s inside my VM?
Friday, October 4, 13

3. VM
Memory
Guest OS Memory
What’s inside my VM?
Friday, October 4, 13

4. JVM Memory
VM
Memory
Guest OS Memory
What’s inside my VM?
Friday, October 4, 13

5. JVM Memory
VM
Memory
ize
ermS
XX:MaxP
-
PermGen
Guest OS Memory
What’s inside my VM?
Friday, October 4, 13

6. JVM Memory
VM
Memory
read
S Per Th
-XS
ize
ermS
XX:MaxP
-
Stack
PermGen
Guest OS Memory
What’s inside my VM?
Friday, October 4, 13

7. HEAP
JVM Memory
VM
Memory
read
S Per Th
-XS
ize
ermS
XX:MaxP
-
Stack
PermGen
Guest OS Memory
What’s inside my VM?
Friday, October 4, 13

8. HEAP
Initial
-Xms
JVM Memory
VM
Memory
read
S Per Th
-XS
ize
ermS
XX:MaxP
-
Stack
PermGen
Guest OS Memory
What’s inside my VM?
Friday, October 4, 13

9. -Xmx
HEAP
Initial
-Xms
JVM Memory
VM
Memory
read
S Per Th
-XS
ize
ermS
XX:MaxP
-
Stack
PermGen
Guest OS Memory
What’s inside my VM?
Friday, October 4, 13

10. OutOfMemoryError
Exception in thread "main": java.lang.OutOfMemoryError:
Java heap space
Exception in thread "main": java.lang.OutOfMemoryError:
PermGen space
Exception in thread "main": java.lang.OutOfMemoryError:
GC overhead limit exceeded
Exception in thread "main": java.lang.OutOfMemoryError:
Unable to create new native thread
and more...
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11. OutOfMemoryError
Exception in thread "main":
java.lang.OutOfMemoryError: PermGen Space
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12. Perm Gen Space
Used for Storing the JVM’s internal
representation of Java Classes
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13. Object
described by
references
Class
references
loaded by
Classl
oader
Life of a class
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14. Object
described by
references
Class
references
loaded by
references
Classl
oader
Life of a class
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15. references
Object
references
Object
Class
Class
references
references
Classloader1
Classloader2
Life of a class
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16. references
Object
references
Object
Class
Class
references
references
Class
Classloader1
Class
Class Class
Class
Class
Class
Class
Classloader2
Class
Class
Class
Class
Class
Class
Class Class
Class
Class
Class
Class
Class
Class
Class
Class
Class
Class
Class
Class
Life of a class
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17. references
Object
references
Object
Class
Class
references
references
Class
Classloader1
Class
Class Class
Class
Class
Class
Class
Classloader2
Class
Class
Class
Class
Class
Class
Class Class
Class
Class
Class
Class
Class
Class
Class
Class
Class
Class
Class
Class
Life of a class
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18. OOM - PermGen
This occurs when JVM wants to load new class
definitions; but there is not enough space in
PermGen space
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19. OOM - PermGen
Symptom
At server startup
Cause
Bigger PermGen footprint
Resolution
Increase PermGen capacity -XX:MaxPermSize
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20. OOM - PermGen
Symptom
PermGen space increases gradually
Cause
Possible classloader leak
Resolution
Investigate any third party API you are using
for any potential class loader leak defect
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21. OOM - PermGen
Symptom
OOM after deploy-redeploy cycles
Cause
Possible classloader leak in the App Server
Resolution
Someone should fix it for you. Hmm..where is
EAP?
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22. OutOfMemoryError
Exception in thread "main":
java.lang.OutOfMemoryError: GC overhead
limit exceeded
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23. OOM - GC overhead
limit exceeded
Policy to allow the VM to detect potential
OutOfMemoryError conditions earlier and before
it runs out of Java Heap space.
Can be turned off -XX:-UseGCOverheadLimit
But don’t!
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24. OOM - GC overhead
limit exceeded
When is this error triggered?
✓Too
✓Too
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many Full GC iterations
much time spent in GC

25. OutOfMemoryError
Exception in thread "main":
java.lang.OutOfMemoryError: Unable to
create new native thread
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26. OOM - Unable to create
new native thread
JVM is asking a new thread from the OS and
the underlying OS cannot allocate a new
thread anymore.
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27. Why?
-Xmx
HEAP
Initial
-Xms
JVM Memory
d
Stack
ize
VM
Memory
PermGen
Threa
-XSS Per
ermS
-XX:MaxP
Guest OS Memory
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not eno
ugh
stack
space

28. OOM - Unable to create
new native thread
✓ Check ulimit - process limit
✓ Consider using thread-pools
✓ Reduce heap space or perform
Friday, October 4, 13
vertical scaling

29. OutOfMemoryError
Exception in thread "main":
java.lang.OutOfMemoryError: Java heap
space
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30. JVM Heap Structure
Survivor
Space
Eden
S0
S1
Young
Generation
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Tenured
Old
Generation

31. Object Allocation
Eden
From
To
Survivor Spaces
Young Gen = Eden + Survivor Spaces
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32. Object Allocation
are allocated in Eden
Just allocated
Eden
From
To
Survivor Spaces
Young Gen = Eden + Survivor Spaces
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33. Filling up Eden Space
Eden
From
To
Survivor Spaces
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34. Filling up Eden Space
are allocated in Eden
Just allocated
Eden
From
To
Survivor Spaces
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35. Minor GC - Copy referenced
objects
Eden
From
To
Survivor Spaces
Unreferenced
Referenced
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36. Minor GC - Copy referenced
objects
Eden
From
To
Survivor Spaces
Unreferenced
Referenced
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37. Minor GC - Copy referenced
objects
Eden
From
To
Survivor Spaces
Unreferenced
Referenced
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38. Minor GC - Copy referenced
objects
Eden
To
1
1
1
1
Survivor Spaces
From
Unreferenced
Referenced
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39. Next Minor GC - Object Aging
Eden
From
1
1
1
1
Survivor Spaces
To
Unreferenced
Referenced
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40. Next Minor GC - Object Aging
Eden
From
1
1
1
1
Survivor Spaces
To
Unreferenced
Referenced
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41. Next Minor GC - Object Aging
Eden
From
1
1
Survivor Spaces
To
Unreferenced
Referenced
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42. Next Minor GC - Object Aging
Eden
From
1
2
Survivor Spaces
To
Unreferenced
Referenced
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43. Next Minor GC - Object Aging
Eden
From
2 2
Survivor Spaces
To
Unreferenced
Referenced
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44. Next Minor GC - Object Aging
Eden
From
2 2 1
Survivor Spaces
To
Unreferenced
Referenced
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45. Next Minor GC - Object Aging
Eden
From
2 2 1
Survivor Spaces
1
To
Unreferenced
Referenced
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46. Next Minor GC - Additional
Aging
Eden
To
2 2 1
Survivor Spaces
1
From
Unreferenced
Referenced
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47. Next Minor GC - Additional
Aging
Eden
To
2
1
Survivor Spaces
From
Unreferenced
Referenced
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48. Next Minor GC - Additional
Aging
Eden
To
1
2 3
Survivor Spaces
From
Unreferenced
Referenced
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49. Next Minor GC - Promotion
Eden
From
1
14
1
2
3
To
Survivor Spaces
Tenured
Unreferenced
Referenced
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50. Next Minor GC - Promotion
Eden
From
1
1
2
3
To
Survivor Spaces
Tenured
14
Unreferenced
Referenced
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51. Major GC
Eden
From
1
1
1
2
3
To
Survivor Spaces
Tenured
Unreferenced
Referenced
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52. Major GC
Eden
From
1
1
1
2
3
To
Survivor Spaces
Tenured
Unreferenced
Referenced
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53. Warning!
This knowledge does not make us JVM
experts!
Always focus first on software before
hacking JVM.
Friday, October 4, 13

54. JVM Diagnosis
jmap
jhat
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55. Class Histogram
jmap -F -histo `pgrep java`
Make it a practice to proactively use this
command after a load test and endurance test.
Friday, October 4, 13

56. Heap Dump
jmap [ option ] <pid>
jmap -F -dump:live,format=b,file=/
var/opt/jboss-as/tmp/io/
heapdump.hprof `pgrep java`
http://docs.oracle.com/javase/6/docs/technotes/tools/share/jmap.html
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57. What Next?
Demo
VisualVM
Eclipse Memory Analyzer Tool
Other tools and commands
GC Algorithms (Optional)
Friday, October 4, 13