淺談 Java GC 原理、調教和 新發展
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淺談 Java GC 原理、調教和 新發展 - Java Developer Day 2014 @ Taiwan
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淺談 Java GC 原理、調教和 新發展
- 1. 淺談 Java GC 原理、調教和 新發展 Leon Chen 陳逸嘉 Java Architect 文曄科技
- 2. 陳逸嘉 (Leon Chen) • Master of NTHU CS • More than 15 years in Java/JEE, as programmer, architect, developer leader and consultant, in finance/telecom domain. • 工研院 • 得捷 • 新光人壽 • Ericsson • Oracle • 文曄科技 (WT Microelectronics) 2
- 3. 著作 • TW Patent 182927, 一種文章切割方法, 2003, 陳逸嘉, 林一中 • TW Patent 206819, 自動服務組合方法及系統, 2004, 陳逸嘉, 許維德, 洪 鵬翔 • US Patent 7,617,174, Method and system for automaRc service composiRon, 2009, Leon Chen, Wei-‐Tek Hsu, Peng-‐Hsiang Hung 3
- 4. Garbage CollecRon 4 我真的需要花時間去 Tune GC 嗎?
- 5. Performance Impact of GC 5 Java SE 6 HotSpot[tm] Virtual Machine Garbage CollecRon Tuning h[p://www.oracle.com/technetwork/java/javase/gc-‐tuning-‐6-‐140523.html 0.6 0.2 8 32
- 6. 有資格被回收的物件? 6
- 15. JAVA HEAP STRUCTURE AND GC 15
- 16. Hotspot JVM Heap Layout 16 Young Old (Tenured) Permanent Eden From To Survivor Spaces NaRve -‐Xms -‐Xmx -‐XX:PermSize -‐XX:MaxPermSize NO
- 17. GC Fundamentals Garbage DistribuRon – Objects die young 17
- 18. 18 GC Fundamentals Garbage DistribuRon – Objects die young
- 19. Minor GC (Young GeneraRon GC) 19 Young Old Eden From To Survivor Spaces
- 20. Minor GC (Young GeneraRon GC) 20 Young Old Eden From To Survivor Spaces ! ! !! ! ! !
- 21. Minor GC (Young GeneraRon GC) 21 Young Old Eden From To Survivor Spaces 1 1 1
- 22. Minor GC (Young GeneraRon GC) 22 Young Old Eden To From Survivor Spaces 1 1 1
- 23. Minor GC (Young GeneraRon GC) 23 ! Young Old Eden To From Survivor Spaces ! ! ! ! ! ! ! 1 ! 1 1 1
- 24. Minor GC (Young GeneraRon GC) 24 Young Old Eden To From Survivor Spaces 1 2 1 2
- 25. Minor GC (Young GeneraRon GC) 25 Young Old Eden From To Survivor Spaces 1 2 1 2
- 26. Minor GC (Young GeneraRon GC) 26 Young Old Eden From To Survivor Spaces 16 1 16 2 ! ! ! ! ! ! ! ! 16 16 Promo
- 27. Minor GC (Young GeneraRon GC) 27 Young Old Eden To From Survivor Spaces 17 17 1 2 1 3
- 28. Major GC (Old GeneraRon GC, Full GC) 28 Young Old Eden To From Survivor Spaces
- 29. Major GC (Old GeneraRon GC, Full GC) 29 Young Old Eden To From Survivor Spaces ! ! !! !
- 30. Major GC (Old GeneraRon GC, Full GC) 30 Young Old Eden To From Survivor Spaces Compact
- 31. Tuning Guideline • 讓 heap 的 size 足以容納程式的一般運作 • Major GC 通常會比 Minor GC 花更多的時間 • 盡可能減少 Major GC 的次數 31
- 33. 33 花太多時間做 GC → Performance變慢 Performance變慢→花太多時間做 GC 不一定!!!
- 34. Performance Tuning Methodology 34 Advanced JVM Tuning; Java One 2011; David Keenan, Monica Beckwith
- 35. GC Logging in ProducRon • Don’t be afraid to enable GC logging in producRon – Very helpful when diagnosing producRon issues • Extremely low / non-‐existent overhead – Maybe some large files in your file system. – We are surprised that customers are sRll afraid to enable it 35
- 36. 36 If Someone doesn’t enable GC logging in producEon I shoot them!
- 37. Most Important GC Logging Parameters • You need at least: – -‐XX:+PrintGCDetails – -‐XX:+PrintGCTimeStamps • Add -‐XX:+PrintGCDateStamps if you must know the Rme • Also useful – -‐Xloggc:file – Rotate GC Log, Aqer Java 1.6_34 (or 1.7_2): -‐XX:+UseGCLogFileRotaRon -‐XX:NumberOfGCLogFiles=5 -‐XX:GCLogFileSize=10M 37
- 38. GCViewer – Offline analysis of GC logs h[ps://github.com/chewiebug/GCViewer 38
- 39. GCViewer – Memory Leak Pa[ern 39
- 42. Hotspot Tuning -‐ Sizing Heap • Young GeneraRon size determines • Frequency of minor GC • Number of objects reclaimed in minor GC • Old GeneraRon Size • Should hold applicaRon's steady-‐state live data size • Try to minimize frequency of major GC’s • JVM footprint should not exceed physical memory • Max of 80-‐90% RAM (leave room for OS) • Thumb Rule: Try to maximize objects reclaimed in young gen. Minimize Full GC frequency 42
- 43. Calculate Live Data Size (LDS) • 經過 full GC 後, 所剩下的 heap size • Make sure you get full GCs during steady-‐state • Induce a couple if necessary – JConsole / VisualVM – jmap -‐histo:live pid – System.gc • Remove it before deployment. :-‐) 43
- 44. Calculate Live Data Size • GC log example: 44
- 45. Calculate Live Data Size by GCViewer 45
- 46. IniRal Heap ConfiguraRon • Rule of thumb – Set -‐Xms and -‐Xmx to 3x to 4x LDS – Set both -‐XX:PermSize and -‐XX:MaxPermSize to around 1.2x to 1.5x the max perm gen size – Set the generaRon sizes accordingly • Young gen should be around 1x to 1.5x LDS • Old gen should be around 2x to 3x LDS • e.g., young gen should be around 1/3-‐1/4 of the heap size 46
- 47. Hotspot JVM Heap Layout 47 Young –Xmn768m Old (Tenured) Permanent NaRve -‐Xms2g –Xmx2g -‐XX:PermSize -‐XX:MaxPermSize For LDS of 512m : -‐Xmn768m -‐Xms2g -‐Xmx2g
- 49. High Throughput vs. Low Latency 49 High Throughput Low Latency App Thread GC Thread 專心 分心 較低 較高 較長 較短 Stop the world Total Cost
- 50. High Throughput vs. Low Latency • For most applicaRons, GC overhead is small – 2% – 5% • High Throughput GCs – Move most work to GC pauses – ApplicaRon threads do as li[le as possible – Least overall GC overhead • Low Latency (Pause) GCs – Move work out of GC pauses – ApplicaRon threads do more work • Bookkeeping for GC more expensive – More overall GC overhead 50
- 51. ApplicaRon Requirement • Different applicaRons have different requirements – Higher Throughput: • Batch processing • Web applicaRon: pauses during garbage collecRon may be tolerable, or simply obscured by network latencies – Lower Latencies: • InteracRve graphics applicaRon 51
- 52. HotSpot Garbage Collectors in Java SE 6 52 G1 -XX:+UseSerialGC -XX:+UseConcMarkSweepGC -XX:+UseParallelOldGC -XX:+UseParallelGC -‐XX:+UseParNewGC
- 53. HotSpot Garbage Collectors in Java SE 6 53 G1 -XX:+UseSerialGC -XX:+UseConcMarkSweepGC -XX:+UseParallelOldGC -XX:+UseParallelGC -‐XX:+UseParNewGC
- 54. Serial Collector 54 App Thread GC Thread stop-‐the-‐world pause Serial Collector
- 55. HotSpot Garbage Collectors in Java SE 6 55 G1 -XX:+UseSerialGC -XX:+UseConcMarkSweepGC -XX:+UseParallelOldGC -XX:+UseParallelGC -‐XX:+UseParNewGC
- 56. Serial VS Parallel Collector 56 App Thread GC Thread stop-‐the-‐world pause Serial Collector Parallel Collector
- 57. HotSpot Garbage Collectors in Java SE 6 57 G1 -XX:+UseSerialGC -XX:+UseConcMarkSweepGC -XX:+UseParallelOldGC -XX:+UseParallelGC -‐XX:+UseParNewGC
- 58. CMS Collector 58 App Thread GC Thread iniRal mark concurrent marking remark concurrent sweeping
- 59. CMS Collector with ParNewGC 59 App Thread GC Thread
- 60. GC SuggesRons 60 Low Latency Collectors Throughput Collectors GeneraRon Young Old 2+ CPUs -XX:+UseParNewGC 2+ CPUs -XX:+UseParallelGC -XX: +UseConcMarkSweepGC -XX:+UseParallelOldGC New G1 Collector as of WLS10.3.1/JDK 6 Update 14
- 62. Problems of Current GC • Memory is cheaper and cheaper • 64 bits JVM • We can use very large heap! Young Old How to handle large heap size?
- 63. G1 Collector • The Garbage-‐First Collector – Since JDK 6u14, officially supported as of JDK 7u4 – -‐XX:+UseG1GC – Future CMS Replacement – Server “Style” low latency collector • Parallel • Concurrent – GeneraRonal – Good Throughput – CompacRng – Improved ease-‐of-‐use – Predictable (Soq Real-‐Time) 63
- 64. G1 – Heap Layout 64 S S Old Eden S Survivor Free Region Type
- 65. CMS vs G1 Collectors 65 • Heap split into regions • Young generaRon (A set of regions) • Old generaRon (A set of regions)
- 66. G1 Collector: Parallelism Concurrency 66 App Thread GC Thread
- 67. GC Comparison 67
- 68. GC Comparison 68
- 69. GC Comparison 69
- 71. 什麼時候該使用G1? • Large heaps – Typically ~6GB or larger heaps • ApplicaRons that needs low pauses 71
- 72. 什麼時候不該使用 G1? • G1需要更多的compuRng resources和 memory 來運行 • No large heaps – Typically smaller than 6GB • ApplicaRons that don’t need low pauses 72
- 73. SUMMARIZE
- 74. Summarize • Professional developer 必須非常了解 variables, objects 的 life cycle, objects 何時有 資格被會收 • GC will hurt system performance • Must monitor GC acRviRes • Must enable GC logging
- 75. Tuning Procedures • Observer LDS to decide iniRal size se~ng • Use parallel GC first • If the GC pause Rme can’t be accepted, then trying to use CMS • If heap is over 6G, and look for lower down pause Rme, you can consider G1 • Monitoring -‐ Analyzing -‐ Tuning -‐ Monitoring -‐ … 75
- 76. REFERENCES 76
- 77. References • Step-‐by-‐Step: Garbage CollecRon Tuning in the Java HotSpot™ Virtual Machine – JavaOne 2010 • The Garbage CollecRon Mythbusters – JavaOne 2010 • GC Tuning for the HotSpot JVM – JavaOne 2009 • uPortal Performance OpRmizaRon. Faizan Ahmed. Architect and Engineering Group. Enterprise Systems Services. RUTGERS. faizan@rutgers.edu. • h[p://java.sun.com/javase/technologies/hotspot/gc/ gc_tuning_6.html
- 78. • Java Performance – Charlie Hunt, Binu John • HotSpot GC Tuning Guide for Java SE 6 h[p://www.oracle.com/technetwork/java/javase/gc-‐ tuning-‐6-‐140523.html • Memory Management in the Java HotSpot VM h[p://www.oracle.com/technetwork/java/javase/ memorymanagement-‐whitepaper-‐150215.pdf • Java 6 Performance Whitepaper h]p://java.sun.com/performance/reference/whitepapers/6_performance.html 78
- 79. QA 79
- 80. Parallel Concurrent (CMS) G1