(JVM) Garbage Collection - Brown Bag Session
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The document discusses the importance and impact of garbage collection in Java, explaining its role in automatic memory management. It covers various garbage collection algorithms, tools for analysis and tuning, and best practices for configuration. Real-world examples and a discussion on JVM options highlight how proper garbage collection management can enhance performance.
(JVM) Garbage Collection - Brown Bag Session
- 1. (JVM) Garbage Collection & Co. Brown Bag Session Jens Hadlich 2015-12-08
- 3. Why should I care?
- 4. Garbage Collection 4 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 5
- 6. Some Theory
- 9. Terminology Garbage collection … is a form of automatic memory management. Garbage … memory occupied by objects that are no longer used. 9
- 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 10
- 12. Roots 12
- 13. Garbage Collection Algorithms 13 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 14
- 15. Think of a bookshelf 15
- 16. Tracing algorithms Mark & Sweep Copying 16
- 17. Mark & Sweep Mark the books you want to keep 17
- 18. Mark & Sweep Throw away the books that you don’t want to keep 18
- 19. Mark & Sweep Problem: Fragmentation 19 This size book won’t fit!
- 20. Tracing algorithms Mark & Sweep Copying 20
- 22. Copying Move (“copy”) books you want to keep 22
- 23. Copying Throw away books you don’t want to keep 23
- 25. Java
- 27. Java 27 Metaspace (Java 8 and later) (Java 7 and earlier)
- 28. Java Generational Garbage Collection 28 Hypothesis: Most objects die young
- 30. Tools & Practice
- 31. Garbage Collection Tools (Memory Analyzer) Java VisualVM jClarity Censum Grafana Icinga 31
- 32. A little bit of GC Tuning
- 33. GC Tuning Performance Triangle 33 Memory Footprint Throughput Latency M T L
- 34. GC Tuning Better Performance Triangle 34 Compactness Throughput Responsiveness C T R C x T x R = a Optimization: Increase a
- 36. Collectors Collectors (new generation) -XX:+UseSerialGC (Copy) -XX:+UseParNewGC (used together with old gen CMS) -XX:+UseG1GC (same for old gen) 36
- 37. Collectors Collectors (old generation) Throughput: -XX:+UseSerialGC (MarkSweepCompact) -XX:+UseParallelGC -XX:+UseParallelOldGC Low-pause: -XX:+UseConcMarkSweepGC -XX:+UseG1GC 37
- 39. Fun with JVM Options 39
- 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 40
- 41. Fun with JVM Options Most important (cont’d) -Xms (-XX:InitialHeapSize) -Xmx (-XX:MaxHeapSize) Setting Xms = Xmx (slightly) saves some overhead. 41
- 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 42
- 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 43
- 44. Think about your program https://flink.apache.org/news/2015/05/11/Juggling-with-Bits-and-Bytes.html 44
- 45. Best practices
- 46. Best practices “Good” JVM options to start with -server -Xms1g -Xmx1g -XX:+AlwaysPreTouch -XX:+UseConcMarkSweepGC -XX:+UseParNewGC -XX:+DisableExplicitGC 46
- 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) 47
- 48. Best practices “Good” JVM options to start with (G1GC) -server -Xms1g -Xmx1g -XX:+AlwaysPreTouch -XX:+UseG1GC -XX:+DisableExplicitGC 48
- 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) 49
- 50. Best practices Look out for specific tuning guides / recommendations. https://www.elastic.co/guide/en/elasticsearch/guide/ current/heap-sizing.html 50
- 51. The End?
- 52. The End? Other things that can go wrong.
- 53. Linux OOM Killer 53 System (3GB memory) JVM (Xmx=2g) /var/log/kern.log jvm.memory.total.used
- 55. Questions?
- 56. Add-on slides
- 57. Go 57