Modern Java GC Algorithms Explained
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In JDK 11/12 there have appeared two very efficient GC collectors: ZGC and ShenandoahGC. The main reason why they have appeared is a short GC pause, no matter how the root set size is and so as to the developer nothing has to tune. I'll show you previous options and compare them mostly with G1 GC, which is the default GC collector since JDK9.
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Modern Java GC Algorithms Explained
- 1. Krystian Zybała @k_zybala (Modern) New? GC algorithms in Java
- 2. # whoami • Java Principal Engineer / Java Performance Engineer • Specialization : JVM, Apache Kafka, Cassandra • Hobby : JVM Performance, Reactive Systems • Workshops, Consultations Twitter: @k_zybala Blog: https://kzybala.pl Mail: kontakt@kzybala.pl
- 4. Let's hold our hands and wait for the next GC cycle
- 5. What is GC?
- 6. GC is your friend 🤗
- 8. Previous GC’s algorithms • SerialGC • ParallelGC • CMS - not available in current JDKs builds • G1GC
- 11. The most of young objects are much more likely to die than old objects
- 12. Eden Survivor 1 Survivor 2 Tenured New object Old generation Young generation
- 13. G1
- 14. G1 • Low-pause collector - First papers date back to 2004 • Supported since JDK 7u4 (April 2012) • Replacement for CMS • Low pauses valued more than max throughput • Designed to be really ease to tune • Tuning based on max Stop-The-World pause ( -XX:MaxGCPauseMillis=X) Default: 250ms
- 15. G1 • Default GC since Java 9 • Use regions instead generations • Dynamic size of region • Parallel Full GC in Java10(jdk-10-ea+34)
- 16. G1 Concurrent Marking Concurrent liveness summary Init Marking Remark Clean up GC Thread Application Thread Copy or Space reclamation
- 18. Ergonomic
- 19. bool UseG1GC = false {product} {default} bool UseSerialGC = true {product} {ergonomic} http://hg.openjdk.java.net/jdk/jdk10/ fi le/b09e56145e11/src/hotspot/share/runtime/os.cpp#l1606
- 21. Why we need something new?
- 23. GC Optimized for Best for Pause Multi- threaded Number of cores SerialGC Memory Footprint Single core, small heaps ( micro-service ? ) Yes No 1 ParallelGC Throughput Batch job Yes Yes 2+ G1GC Throughput / Latency Balance request-response db integractions Yes Yes 2+* ZGC Low latency request-response db integractions Yes Yes 2+* ShenandoahGC Low latency request-response db integractions Yes Yes 2+*
- 24. How does it work?
- 25. How does it work? • Requires meta data storing in object • Requires JIT support - Load GC barrier or Store GC barrier
- 26. Mutator problems Headers x = 1 y = 2 z = 1 Object A Object B
- 27. Mutator problems Headers x = 1 y = 2 z = 1 Headers x = 1 y = 2 z = 1 from-space to-space Object A Object B Object A Object B Object C
- 28. Mutator problems Headers x = 1 y = 4 z = 1 Headers x = 3 y = 2 z = 1 from-space to-space Application thread 10 Application thread 3 Object A Object B Object A Object B
- 29. Shenandoah
- 30. Shenandoah • Available in AdoptOpenJDK builds • NOT available in Oracle’s OpenJDK builds • Present in RedHat OpenJDK 8,11+ • Low pause GC • Concurrent Compaction, Single Generation
- 31. Shenandoah • Available for x86 32-bit and 64-bit • ARM ports available • Linux, MacOS, Windows • Region Based
- 32. Shenandoah 1.0 * Maintains a weak to-space invariant * Load barrier * Store barrier
- 34. Shenandoah Mark Class Field1 Field2 Mark Class Field1 Field2 Mark Class Field1 Field2 Mark Class Field1 Field2 Fwd Ptr Fwd Ptr 1.0 Object A Object B Object C Object A Object B Object C
- 35. Shenandoah Problems * More memory need ( due to the forward pointer) worst case 50% common case 5-10% * Complicated GC barriers ( Store and Load)
- 36. Shenandoah 2.0 * Maintains a strong to-space invariant * Load barrier (LRB - Load Reference Barrier)
- 38. Shenandoah 2.0 Class Field1 Field2 Mark Class Field1 Field2 Fwd Ptr Mark Class Field1 Field2 From Space To Space Object A Object B Object C
- 39. Shenandoah Concurrent Mark Concurrent Evacuation Init Mark Final Mark Init U-R Concurrent Update References GC Thread Application Thread Final U-R
- 41. Shenandoah Marking + Evacuating + Update References
- 43. Shenandoah Mark Class Field1 Field2 Evacuation from-space to-space Mark Class Field1 Field2 Application Thread copy Object A Object B Object B’
- 44. Shenandoah Mark Class Field1 Field2 Evacuation Mark Class Field1 Field2 from-space to-space Mark Class Field1 Field2 GC Thread copy Application Thread copy Object A Object B Object B’’ Object B’
- 45. Shenandoah Evacuation Mark Class Field1 Field2 from-space to-space Mark Class Field1 Field2 Class Field1 Field2 Fwd Ptr GC Thread copy Application Thread copy Object A Object B Object B’’ Object B’
- 46. Shenandoah Evacuation from-space to-space Mark Class Field1 Field2 Class Field1 Field2 Fwd Ptr Object A Object B Object B’
- 47. Shenandoah Update references from-space to-space Mark Class Field1 Field2 Class Field1 Field2 Fwd Ptr Object A Object B
- 48. Tuning
- 49. -Xms -Xmx
- 50. Shenandoah Tuning • -XX:+AlwaysPreTouch • -XX:+UseLargePages • -XX:+UseNUMA (not supported yet) • -XX:-UseBiasedLocking • -XX:+DisableExplicitGC
- 51. ZGC
- 52. ZGC • Sub-millisecond max pause times (less than 10ms) (less than 1ms) • Pauses are O(1) • Pause times do not increase with the heap, live-set or root-set size • Handle heaps ranging from a 8MB to 16TB in size • ZGC was initially introduced as an experimental feature in JDK 11, and was declared Production Ready in JDK 15.
- 53. ZGC • Only for Linux x86 64-bit • compressed class pointers • no support compress OOPs • Region based • ZPages • Small ( 2MiB - object size up to 256KiB) • Medium ( 32MiB - object size up to 4MiB) • Large ( 4+ MiB - object size > 4MiB)
- 54. ZGC • Concurrent • Region-based • Compacting • NUMA-aware • Using colored pointers • Using load barriers
- 55. ZGC Uses colored pointers. Color indicates GC metadata Object address - 44 bits 16 bit 4 color bits 64 bits pointer address
- 56. ZGC Multi-Mapping ∂ ∂ 001<addr> Virtual memory Physical memory 010<addr> 100<addr>
- 57. ZGC ZGC uses a load GC barrier Object someObject = person.name; <load_barrier> // String n = person.name; mov 0x10(%rax), %rbx // bad color test %rbx, (0x16)%r15 // If yes, enter slow path jnz slow_path
- 58. ZGC Concurrent Mark Reference processing Unload classes Free memory pages Prepare relocation Mark start Mark End Relocate Concurrent Relocate Concurrent Remap GC Thread Application Thread
- 59. ZGC Concurrent Mark Concurrent Reference processing Unload classes Free memory pages Prepare relocation Concurrent Relocate Remap Concurrent Mark …
- 60. ZGC
- 61. ZGC Relocation Forwarding table A -> A1 B -> B1 C -> C1
- 62. ZGC Relocation Forwarding table A -> A1 B -> B1 C -> C1 D -> D1 Application thread copy
- 63. ZGC Relocation Forwarding table A -> A1 B -> B1 C -> C1 D -> D1 Application thread copy GC thread copy
- 64. ZGC Relocation Forwarding table A -> A1 B -> B1 C -> C1 D -> D1 E -> E1 Application thread copy GC thread copy
- 65. ZGC Remapping
- 66. ZGC Future changed * Generational * Sub-millisecond max pause time * Graal JIT support
- 67. Tuning
- 68. -XMX<Size>
- 69. ZGC Tuning • -XX:ZAllocationSpikeTolerance=factor • -XX:ZCollectionInterval=seconds • -XX:ZFragmentationLimit=percent • -XX:ZUncommitDelay
- 71. Java 19
- 72. Project Loom
- 74. JEP 376 ZGC Concurrent Thread-Stack Processing
- 75. Concurrent Thread-Stack Processing ▪Remove thread-stack processing from ZGC safepoints. ▪Make stack processing lazy, cooperative, concurrent, and incremental. ▪Remove all other per-thread root processing from ZGC safepoints. ▪Provide a mechanism by which other HotSpot subsystems can lazily process stacks.
- 76. Conclusions
- 77. Conclusions * Still JDK8 ? - Shenandoah * JDK 11+ ? - ZGC or Shenandoah * Nothing is for free - you need more resources for achieve low pause
- 78. Is G1 dead?
- 79. JEP-423 Region Pinning for G1
- 80. JEP-423 • No stalling of threads due to JNI critical regions. • No additional latency to start a garbage collection due to JNI critical regions. • No regressions in GC pause times when no JNI critical regions are active. • Minimal regressions in GC pause times when JNI critical regions are active.
- 81. Over 30 major performance improvements
- 83. QA
- 84. Thank you!