Finding your garbage

CMS and G1 garbage collection algorithms walkthrough

Transcript

1. Finding Your Garbage Dmitry Buzdin
2. Summary • Intro to JVM Memory Areas • Intro to GC principles • Introducing CMS and G1 • Configuration possibilities • Measuring GC
3. Memory Management • In languages like C++ you have to free memory • JVM cleans garbage after you • It does not mean that you can do whatever you want
4. Common Memory Problems • Memory Leaks • “Stop the world” pauses • High CPU consumption by GC • Memory fragmentation
5. Memory Leaks https://plumbr.eu/blog/memory-leaks/memory-leaks-fallacies- and-misconceptions OK Memory Leak
6. Why do you need to know GC? • Understand whats going on under the hood • Interpret monitoring metrics • Understand how to adapt your code to JVM • Don’t optimise prematurely!
7. Lets revisit what goes into heap
8. JVM Data Areas
9. What Goes Where?
10. Heap Size
11. Generational GC
12. Heap Structure “Metaspace” since JDK8
13. Object Promotion Object age is recorded and incremented with each GC cycle
14. What gets Collected? • Garbage = unreachable objects • How do you know if object is unreachable?
15. GC Roots • Local variables of currently executed methods • Active threads • Static field of loaded classes
16. Ideal GC Algorithm • Predictable pauses • Low latency • Supports large heaps • Not CPU intensive • Uses memory efficiently
17. HotSpot GC Algorithms • SerialGC -XX:UseSerialGC • ParallelGC -XX:UseParallelGC • CMS -XX:+UseConcMarkSweepGC • G1 JDK 9 default
18. CMS Algorithm • Mostly Concurrent mark and sweep • Designed to avoid long pauses -XX:+UseConcMarkSweepGC
19. CMS Heap Structure
20. Before GC
21. Young Generation Collection
22. After Young GC
23. Old Gen Collection Occupancy rate is reached -XX:CMSInitiatingOccupancyFraction=75
24. CMS Collection Steps • Initial mark (STW) - collect all GC roots • Concurrent mark - mark all live objects in old gen • Concurrent preclean - mark dirty live objects • Concurrent abortable preclean - continuation of above • Final remark (STW) - finalize marking of live objects • Concurrent sweep - reclaiming space concurrently • Concurrent reset - reset internal data structures
25. Concurrent Marking
26. After Sweeping
27. CMS in JDK9 • CMS is deprecated and to be removed in JDK 10? • Risk is that it outperforms G1 in some cases http://openjdk.java.net/jeps/291
28. G1 Algoritm • Garbage First • Lots of small regions instead of large ones • Predictable “stop-the-world” pauses • Large heap sizes with low latency
29. G1 Heap Structure Fixed Size Memory Regions (1mb - 32mb) ± 2000
30. G1 Step by Step
31. Young GC Also called Evacuation Phase All young regions plus some old ones Stop the World GC
32. End of Young GC
33. Initial Marking Phase G1 GC marks the roots during the initial-mark phase. Happens with Evacuation Pause
34. Concurrent Marking Phase G1 GC looks for reachable objects across the entire heap. This phase happens concurrently.
35. Remark Phase Empty regions are removed Stop the world pause Completes the marking process
36. Copying/Cleanup Phase Stop the world + concurrent
37. After Copying/Cleanup Phase Space has been reclaimed Memory regions compacted
38. Configuring G1 • -XX:MaxGCPauseMillis (realistic soft limitation) • -XX:ParallelGCThreads • -XX:ConcGCThreads • -XX:InitiatingHeapOccupancyPercent • and more
39. G1 Recap • Based on similar concepts as CMS • Additional dimensions with RSets and CSets • Support for humongous objects
40. There is no best GC algorithm • Tradeoff between • Throughput • Latency • Heap size
41. GC Monitoring • JMX and JConsole • Visual VM • GC Logs • GC Heap Dumps
42. JMX JConsole
43. VisualVM
44. GC Logging http://openjdk.java.net/jeps/158
46. Heap Dump -XX:-HeapDumpOnOutOfMemoryError
47. How to find the best one for your project • Experiment • Try different settings • Observe metrics • Understand the effect • Try again…
48. Java GC Future • Further focus on G1 and improvements • Shenandoah https://rkennke.wordpress.com/
49. References • http://www.oracle.com/webfolder/technetwork/ tutorials/obe/java/gc01/index.html • http://www.oracle.com/technetwork/tutorials/ tutorials-1876574.html • https://plumbr.eu/blog/memory-leaks/memory- leaks-fallacies-and-misconceptions • http://www.slideshare.net/buzdin/java7-garbage- collector-g1