OpenJDK Concurrent Collectors
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This document provides an overview and comparison of the algorithms, phases, and commonalities of modern concurrent garbage collectors in HotSpot, including G1, Shenandoah, and Z GC. It begins with laying the groundwork on stop-the-world vs concurrent collection and heap layout. It then introduces the key differences between the three collectors in their marking, barrier, and compaction approaches. The goal of the document is to provide a technical introduction and high-level differences between these concurrent garbage collectors.
OpenJDK Concurrent Collectors
- 1. Java Team OpenJDK: In the New Age of Concurrent Garbage Collectors HotSpot’s Regionalized GCs Monica Beckwith JVM Performance java-performance@Microsoft @mon_beck
- 2. Agenda Part 1 – Groundwork & Commonalities Laying the Groundwork Stop-the-world (STW) vs concurrent collection Heap layout – regions and generations Basic Commonalities Copying collector – from and to spaces Regions – occupied and free Collection set and priority July 10th, 2020
- 3. Agenda Part 2 – Introduction & Differences Introduction to G1, Shenandoah and Z GCs Algorithm Basic Differences GC phases Marking Barriers Compaction July 10th, 2020
- 4. Groundwork : Stop-the world vs concurrent collections
- 5. Stop-the-world aka STW GC Application Threads GC Threads Application Threads Safepoint Requested GC Complete d Application Threads GC Threads Application Threads Safepoint Requested GC Complete d Handshakes Thread local handshakes vs Global Time To Safepoint (TTSP)
- 6. Concurrent GC Application Threads GC Threads
- 7. Groundwork : Heap layout - regions and generations
- 8. Heap Layout Heap Z GC Shenandoah GC Young Generation G1 GC Old Generation
- 9. Commonalities : Copying collector – from and to spaces From To
- 10. HeapFrom Space To Space O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O GC ROOTS THREAD 1 STACK THREAD N STACK STATIC VARIABLES ANY JNI REFERENCES Copying aka Evacuating Collector
- 11. O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O GC ROOTS THREAD 1 STACK THREAD N STACK O O O O O STATIC VARIABLES ANY JNI REFERENCES O OO O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O Copying aka Evacuating Collector
- 12. Copying aka Evacuating Collector O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O
- 13. Commonalities : Regions – occupied and free
- 14. Occupied and Free Regions O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O • List of free regions • In case of generational heap (like G1), the occupied regions could be young, old or humongous
- 15. Commonalities : Collection set and priority
- 16. Collection Priority and Collection Set O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O OO OO OO OO OO OO OO OO OO OO OO OO OO OO OO OO OO OO OO OO OOOO • Priority is to reclaim regions with most garbage • The candidate regions for collection/reclamation/relocation are said to be in a collection set • There are threshold based on how expensive a region can get and maximum regions to collect • Incremental collection aka incremental compaction or partial compaction • Usually needs a threshold that triggers the compaction • Stops after the desired reclamation threshold or free-ness threshold is reached • Doesn’t need to be stop-the-world
- 17. Introduction : G1, Shenandoah & Z - Algorithms
- 18. Algorithm and Other Considerations Garbage Collectors G1 GC Shenandoah GC Z GC Regionalized? Yes Yes Yes Generational? Yes No No Compaction? Yes, STW, Forwarding address in header Yes, Concurrent, Forwarding Pointer Yes, Concurrent, Colored Pointers Target Pause Times? 200ms 10ms 10ms Concurrent Marking Algorithm? SATB SATB Striped
- 19. Differences – G1
- 20. GC Phases of Marking and Compaction G1 GC Gist Initial Mark Mark objects directly reachable by the roots Concurrent Root Region Scanning Since initial mark is piggy-backed on a young collection, the survivor regions need to be scanned Concurrent Marking Snapshot-at-the-beginning (SATB) algorithm Final Marking Drain SATB buffers; traverse unvisited live objects Cleanup Identify and free completely free regions, sort regions based on liveness and expense STW Compaction Move objects in collection set to “to” regions; free regions in collection set •C. Hunt, M. Beckwith, P. Parhar, B. Rutisson. Java Performance Companion.
- 21. Concurrent Marking Logical snapshot of the heap SATB marking guarantees that all garbage objects that are present at the start of the concurrent marking phase will be identified by the snapshot But application mutates its object graph Any new objects are considered live For any reference update, the mutator needs to log the previous value in a log queue This is enabled by a pre-write barrier •C. Hunt, M. Beckwith, P. Parhar, B. Rutisson. Java Performance Companion. •https://www.jfokus.se/jfokus17/preso/Write-Barriers-in-Garbage-First-Garbage-Collector.pdf Snapshot-at-the-beginning (SATB) Algorithm
- 22. Barriers SATB Pre-Write Barrier The pseudo-code of the pre-write barrier for an assignment of the form x.f := y is: if (marking_is_active) { pre_val := x.f; if (pre_val != NULL) { satb_enqueue(pre_val); } } •C. Hunt, M. Beckwith, P. Parhar, B. Rutisson. Java Performance Companion.
- 23. Barriers Post Write Barrier Consider the following assignment: object.field = some_other_object G1 GC will issue a write barrier after the reference is updated, hence the name. G1 GC filters the need for a barrier by way of a simple check as explained below: (&object.field XOR &some_other_object) >> RegionSize If the check evaluates to zero, a barrier is not needed. If the check != zero, G1 GC enqueues the card in the update log buffer https://www.jfokus.se/jfokus17/preso/Write-Barriers-in-Garbage-First-Garbage-Collector.pdf •C. Hunt, M. Beckwith, P. Parhar, B. Rutisson. Java Performance Companion.
- 24. STW Compaction Forwarding Pointer in Header BodyHeader A Java Object Pointer Pointer to an InstanceKlass Mark Word b b GC workers compete to install the forwarding pointer From source: • An InstanceKlass is the VM level representation of a Java class. It contains all information needed for at class at execution runtime. • When marked the bits will be 11
- 25. Differences – Z
- 26. GC Phases of Marking and Compaction Z GC Gist Initial Mark Mark objects directly reachable by the roots Concurrent Marking Striping - GC threads walk the object graph and mark Final Marking Traverse unvisited live objects; weak root cleaning Concurrent Prepare for Compaction Identify collection set; reference processing Start Compaction Handles roots into the collection set Concurrent Compaction Move objects in collection set to “to” regions Concurrent Remap (done with Concurrent Marking of next cycle since walks the object graph) Fixup of all the pointers to now-moved objects http://cr.openjdk.java.net/~pliden/slides/ZGC-Jfokus-2018.pdf
- 27. Striping Heap divided into logical stripes GC threads work on their own stripe Minimizes shared state Load barrier to detect loads of non-marked object pointers Concurrent reference processing Thread local handshakes http://cr.openjdk.java.net/~pliden/slides/ZGC-Jfokus-2018.pdf Heap GC Thread0 GC Thread1 GC Threadn … 0 1 … n 0 1 … n 0 1 … n Stripe 0 Stripe 1 Stripe n Concurrent Marking
- 28. Barriers Read Barrier – For References Update a “bad” reference to a “good” reference Can be self-healing/repairing barrier when updates the source memory location Imposes a set of invariants – “All visible loaded reference values will be safely “marked through” by the collector, if they haven’t been already. All visible loaded reference values point to the current location of the safely accessible contents of the target objects they refer to.” Tene, G.; Iyengar, B. & Wolf, M. (2011), C4: The Continuously Concurrent Compacting Collector, in 'Proceedings of the international symposium on Memory management' , ACM, New York, NY, USA , pp. 79--88 . Loaded Reference Barrier
- 29. Example Object o = obj.fieldA; // Loading an object reference from heap load_barrier(register_for(o), address_of(obj.fieldA)); if (o & bad_bit_mask) { slow_path(register_for(o), address_of(obj.fieldA)); }
- 30. Example mov 0x20(%rax), %rbx // Object o = obj.fieldA; test %rbx, (0x16)%r15 // Bad color? jnz slow_path // Yes -> Enter slow path and mark/relocate/remap, // adjust 0x20(%rax) and %rbx http://cr.openjdk.java.net/~pliden/slides/ZGC-Jfokus-2018.pdf
- 31. Core Concept Colored Pointers http://cr.openjdk.java.net/~pliden/slides/ZGC-Jfokus-2018.pdf Object Address 041 Unused M a r k e d 0 M a r k e d 1 R e m a p p e d F i n a l i z a b l e 4663 Object is known to be marked? Object is known to not be pointing into the relocation set? Object is reachable only through a Finalizer? Metadata stores in the unused bits of the 64-bit pointers Virtual address mapping/tagging Multi-mapping on x86-64 Hardware support on SPARC, aarch64
- 32. Concurrent Compaction Load barrier to detect object pointers into the collection set Can be self-healing Off-heap forwarding tables enable to immediately release and reuse virtual and physical memory http://cr.openjdk.java.net/~pliden/slides/ZGC-Jfokus-2018.pdf Off-Heap Forwarding Tables
- 34. GC Phases of Marking and Compaction https://wiki.openjdk.java.net/display/shenandoah/Main Shenandoah GC Gist Initial Mark Mark objects directly reachable by the roots Concurrent Marking Snapshot-at-the-beginning (SATB) algorithm Final Marking Drain SATB buffers; traverse unvisited live objects; identify collection set Concurrent Cleanup Free completely free regions Concurrent Compaction Move objects in collection set to “to” regions Initial Update Reference Initialize the update reference phase Concurrent Update Reference Scans the heap linearly; update any references to objects that have moved Final Update Reference Update roots to point to to-region copies Concurrent Cleanup Free regions in collection set
- 35. Concurrent Marking •C. Hunt, M. Beckwith, P. Parhar, B. Rutisson. Java Performance Companion. •https://www.jfokus.se/jfokus17/preso/Write-Barriers-in-Garbage-First-Garbage-Collector.pdf Snapshot-at-the-beginning (SATB) Algorithm
- 36. Barriers SATB Pre-Write Barrier - Recap •C. Hunt, M. Beckwith, P. Parhar, B. Rutisson. Java Performance Companion. Needed for all updates Check if “marking-is-active” SATB_enqueue the pre_val
- 37. Barriers Read Barrier – For Concurrent Compaction Here’s an assembly code snippet for reading a field: mov 0x10(%rsi),%rsi ; *getfield value Here’s what the snippet looks like with Shenandoah: mov -0x8(%rsi),%rsi ; read of forwarding pointer at address object - 0x8 mov 0x10(%rsi),%rsi ; *getfield value *Flood, Christine & Kennke, Roman & Dinn, Andrew & Haley, Andrew & Westrelin, Roland. (2016). Shenandoah: An open-source concurrent compacting garbage collector for OpenJDK. 1-9. 10.1145/2972206.2972210.
- 38. Barriers Copying Write Barrier – For Concurrent Compaction Needed for all updates to ensure to-space invariant Check if “evacuation_in_progress” Check if “in_collection_set” and “not_yet_copied” CAS (fwd-ptr(obj), obj, copy) *Flood, Christine & Kennke, Roman & Dinn, Andrew & Haley, Andrew & Westrelin, Roland. (2016). Shenandoah: An open-source concurrent compacting garbage collector for OpenJDK. 1-9. 10.1145/2972206.2972210.
- 39. Barriers Read Barrier – For Concurrent Compaction *Flood, Christine & Kennke, Roman & Dinn, Andrew & Haley, Andrew & Westrelin, Roland. (2016). Shenandoah: An open-source concurrent compacting garbage collector for OpenJDK. 1-9. 10.1145/2972206.2972210.
- 40. Barriers Copying Write Barrier – For Concurrent Compaction *Flood, Christine & Kennke, Roman & Dinn, Andrew & Haley, Andrew & Westrelin, Roland. (2016). Shenandoah: An open-source concurrent compacting garbage collector for OpenJDK. 1-9. 10.1145/2972206.2972210.
- 41. Barriers Loaded Reference Barrier - Recap Tene, G.; Iyengar, B. & Wolf, M. (2011), C4: The Continuously Concurrent Compacting Collector, in 'Proceedings of the international symposium on Memory management' , ACM, New York, NY, USA , pp. 79--88 . https://developers.redhat.com/blog/2019/06/27/shenandoah-gc-in-jdk-13-part-1-load-reference-barriers/ Ensure strong ‘to-space invariant’ Utilize barriers at reference load Check if fast-path-possible; else do-slow-path
- 42. Concurrent Compaction Brooks Style Indirection Pointer BodyHeader A Java Object Indirection Pointer Forwarding pointer is placed before the object Additional work of dereferencing per object
- 43. Concurrent Compaction Brooks Style Indirection Pointer Forwarding pointer is placed before the object Additional work of dereferencing per object
- 44. Concurrent Compaction Forwarding Pointer in Header BodyHeader To Space Copy Java Object Body Forwarding Pointer From Space Java Object X https://developers.redhat.com/blog/2019/06/28/shenandoah-gc-in-jdk-13-part-2-eliminating-the-forward- pointer-word/
- 45. Performance!
- 46. Variability: OpenJDK 8 LTS OpenJDK 11 LTS JDK 11 LTS significantly less variability than JDK 8 LTS for responsiveness 0.00 0.20 0.40 0.60 0.80 1.00 1.20 Run 1 Run 2 Run 3 Run 4 Run 5 Run 6 Run 7 SPECjbb2015 JDK 8 LTS Full System Capacity Responsiveness 0.00 0.20 0.40 0.60 0.80 1.00 1.20 Run 1 Run 2 Run 3 Run 4 Run 5 Run 6 Run 7 SPECjbb2015 JDK 11 LTS Full System Capacity Responsiveness 0 5 10 15 JDK 8 LTS JDK 11 LTS % STD Dev Full System Capacity Responsiveness With G1 GC
- 47. 0.00 0.25 0.50 0.75 1.00 1.25 1.50 JDK 8 LTS JDK 11 LTS JDK 12 JDK 13 Full System Capacity Responsiveness Out-of-box* GC Performance OpenJDK 8 LTS - > OpenJDK 11 LTS "-Xmx150g –Xms150g -Xmn130g" G1 GC became the default GC Higher is Better
- 48. Out-of-box* OpenJDK GC Performance Innovation happens at tip *With Xmx=Xms 0.85 0.90 0.95 1.00 1.05 1.10 Full System Capacity Responsiveness PGC JDK tip vs JDK 11 G1GC JDK tip vs JDK 11 ZGC JDK tip vs JDK 11 Higher is Better
- 49. GCs Head-to-Head Performance 0.00 0.25 0.50 0.75 1.00 1.25 1.50 shenandoah z g1, base+ng parallel, base+xmn parallel, base+ng Full System Capacity Responsiveness Higher is Better
- 51. © Copyright Microsoft Corporation. All rights reserved.
Editor's Notes
- #11: Root set includes: thread local variables, references embedded in generated code, interned Strings, references from classloaders (e.g. static final references), JNI references, JVMTI references. Having larger root set generally means longer pauses with Shenandoah, see below for diagnostic techniques
- #13: Compacting garbage collection algorithms have been shown to have smaller memory footprints and better cache locality than in place algorithms like Concurrent Mark and Sweep (CMS)
- #22: Objects allocated during the concurrent marking phase will be considered live but they are not traced, thus reducing the marking overhead. The technique guarantees that all live objects that were alive at the start of the marking phase are marked and traced and any new allocations made by the concurrent mutator threads during the marking cycle are marked as live and consequently not collected.
- #23: The marking_is_active condition is a simple check of a thread local flag that is set to true at the start of marking, during the initial mark pause. Guarding the rest of the pre-barrier code with this check reduces the overhead of executing the remainder of the barrier code when marking is not active. Since the flag is thread-local and it's value may be loaded multiple times, it is likely that any individual check will hit in cache - further reducing the overhead of the barrier.
- #24: Remembered Sets
- #25: Klass pointer Live objects that need to be evacuated are copied to thread-local GC allocation buffers (GCLABs) allocated in target regions. Worker threads compete to install a forwarding pointer to the newly allocated copy of the old object image. With the help of work stealing [2], a single “winner” thread helps with copying and scanning the object. Work stealing also provides load balancing between the worker threads. Each section here marks a heap word. That would be 64 bits on 64-bit architectures and 32 bits on 32-bit architectures. The first word is the so-called mark word, or header of the object. It is used for a variety of purposes. For example, it can keep the hash-code of an object; it has 3 bits that are used for various locking states; some GCs use it to track object age and marking status; and it can be “overlaid” with a pointer to the “displaced” mark, to an “inflated” lock, or, during GC, the forwarding pointer. The second word is reserved for the klass pointer. This is simply a pointer to the Hotspot-internal data structure that represents the class of the object. Arrays would have an additional word next to store the array length. What follows is the actual payload of the object, that is, fields and array elements.
- #27: Weak root cleaning (string table)
- #28: Finalizable mark: Final reachable: object about to be finalized
- #29: writes and reads always happen into/from the to-space copy = strong to-space invariant.
- #32: Finalizable mark: Final reachable: object about to be finalized
- #33: Self-healing is where Java threads will help out
- #38: SGC no need to update refs, fwding pointer.; When we start register %rsi contains the address of the object, and the field is at offset 0x10.
- #39: To-space invariant – all writes are made into the object in to-space Even primitives and locking of objects – exotic barriers acmp (pointer comparison), CAS, clone
- #42: writes and reads always happen into/from the to-space copy = strong to-space invariant.
- #43: Memory and throughput overhead