What is Garbage Collection and why should I care? Some theory, real world examples and best practices.

1. (JVM) Garbage Collection & Co.
Brown Bag Session
Jens Hadlich
2015-12-08

2. Garbage Collection
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3. Why should I care?

4. Garbage Collection
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Can have a huge impact
30 seconds
20 seconds
10 seconds
~16 hours

5. Outline
• Some Theory
• Practice & real world examples
• Tools & live demos
• Configuration best practices
• Discussion
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6. Some Theory

7. Garbage Collection
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8. Garbage Collection
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9. Terminology
Garbage collection
… is a form of automatic memory management.
Garbage
… memory occupied by objects that are no longer used.
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10. Pros and Cons
Pros
- Programmer does not need to worry about memory (de-)allocation
(leading to fewer memory leaks)
Cons
- Performance overhead
- Can lead to more complex programming language design
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11. More Terminology
Collector
Heap
Mutator
Roots
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12. Roots
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13. Garbage Collection
Algorithms
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Tracing Reference Counting
Collection Style Batch Incremental
Cost Per Mutation None High
Throughput High Low
Pause Times Long Short
Real Time? No Yes
Collects Cycles? Yes No

14. Tracing algorithms
Mark & Sweep
Copying
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15. Think of a bookshelf
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16. Tracing algorithms
Mark & Sweep
Copying
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17. Mark & Sweep
Mark the books you want to keep
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18. Mark & Sweep
Throw away the books that you don’t want to keep
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19. Mark & Sweep
Problem: Fragmentation
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This size book won’t fit!

20. Tracing algorithms
Mark & Sweep
Copying
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21. Copying
Need 2 shelves
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22. Copying
Move (“copy”) books you want to keep
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23. Copying
Throw away books you don’t want to keep
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24. Further readings
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25. Java

26. HotSpot JVM Architecture
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http://www.oracle.com/webfolder/technetwork/tutorials/obe/java/gc01/index.html

27. Java
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Metaspace
(Java 8 and later)
(Java 7 and earlier)

28. Java
Generational Garbage Collection
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Hypothesis:
Most objects die young

29. Generational Garbage Collection
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30. Tools & Practice

31. Garbage Collection
Tools
(Memory Analyzer)
Java VisualVM
jClarity Censum
Grafana
Icinga
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32. A little bit of GC Tuning

33. GC Tuning
Performance Triangle
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Memory Footprint
Throughput Latency
M
T L

34. GC Tuning
Better Performance Triangle
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Compactness
Throughput Responsiveness
C
T R
C x T x R = a
Optimization: Increase a

35. Responsiveness vs. Throughput
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36. Collectors
Collectors (new generation)
-XX:+UseSerialGC (Copy)
-XX:+UseParNewGC
(used together with old gen CMS)
-XX:+UseG1GC
(same for old gen)
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37. Collectors
Collectors (old generation)
Throughput:
-XX:+UseSerialGC (MarkSweepCompact)
-XX:+UseParallelGC
-XX:+UseParallelOldGC
Low-pause:
-XX:+UseConcMarkSweepGC
-XX:+UseG1GC
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38. Concurrent Mark-Sweep
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39. Fun with JVM Options
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40. Fun with JVM Options
Most important
-server
Crucial for all long-running server applications.
http://stackoverflow.com/questions/198577/real-differences-between-
java-server-and-java-client
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41. Fun with JVM Options
Most important (cont’d)
-Xms (-XX:InitialHeapSize)
-Xmx (-XX:MaxHeapSize)
Setting Xms = Xmx (slightly) saves some overhead.
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42. Fun with JVM Options
Enable GC logs
-Xloggc:logs/gc-<app>.log
-XX:+PrintGCCause
-XX:+PrintGCDetails
-XX:+PrintGCTimeStamps
-XX:+PrintHeapAtGC
-XX:+PrintTenuringDistribution
-XX:+PrintGCApplicationStoppedTime
-XX:+UseGCLogFileRotation
-XX:NumberOfGCLogFiles=2
-XX:GCLogFileSize=128M
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43. Fun with JVM Options
Enable JMX remoting
-Dcom.sun.management.jmxremote
-Dcom.sun.management.jmxremote.port=<port>
-Dcom.sun.management.jmxremote.local.only=false
-Dcom.sun.management.jmxremote.authenticate=false
-Dcom.sun.management.jmxremote.ssl=false
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44. Think about your program
https://flink.apache.org/news/2015/05/11/Juggling-with-Bits-and-Bytes.html
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45. Best practices

46. Best practices
“Good” JVM options to start with
-server
-Xms1g
-Xmx1g
-XX:+AlwaysPreTouch
-XX:+UseConcMarkSweepGC
-XX:+UseParNewGC
-XX:+DisableExplicitGC
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47. Best practices
Notes on G1GC
-XX:+UseG1GC
http://www.oracle.com/technetwork/articles/java/
g1gc-1984535.html
(G1GC shall become the default collector in Java 9)
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48. Best practices
“Good” JVM options to start with (G1GC)
-server
-Xms1g
-Xmx1g
-XX:+AlwaysPreTouch
-XX:+UseG1GC
-XX:+DisableExplicitGC
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49. Best practices
Keep the heap size below 30 GB if possible.
Why? Compressed oops.
https://wiki.openjdk.java.net/display/HotSpot/
CompressedOops
http://docs.oracle.com/javase/7/docs/technotes/guides/vm/
performance-enhancements-7.html
(enabled by default since Java 7 for 64-bit)
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50. Best practices
Look out for specific tuning guides /
recommendations.
https://www.elastic.co/guide/en/elasticsearch/guide/
current/heap-sizing.html
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51. The End?

52. The End?
Other things that can go wrong.

53. Linux OOM Killer
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System (3GB memory)
JVM (Xmx=2g)
/var/log/kern.log
jvm.memory.total.used

54. OOM
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hs_err_pid<pid>.log

55. Questions?

56. Add-on slides

57. Go
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58. Java Profiling
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