OpenJDK: In the New Age of Concurrent Garbage Collectors · 2019-11-05 · Algorithm and Other...

Java Team

OpenJDK: In the New Age of Concurrent Garbage CollectorsHotSpot’s Regionalized GCs

Monica BeckwithJVM Performance java-performance@Microsoft

@mon_beck

AgendaPart 1 – Groundwork & Commonalities

Laying the GroundworkStop-the-world (STW) vs concurrent collection

Heap layout – regions and generations

Basic CommonalitiesCopying collector – from and to spaces

Regions – occupied and free

Collection set and priority

Oct 8th, 2019

AgendaPart 2 – Introduction & Differences

Introduction to G1, Shenandoah and Z GCsAlgorithm

Basic DifferencesGC phases

Marking

Barriers

Compaction

Oct 8th, 2019

Groundwork : Stop-the world vs concurrent collections

Stop-the-world aka STW GC

Application Threads

GC Threads

Application Threads

Safepoint Requested

GC Completed

Application Threads GC Threads Application Threads

Safepoint Requested

GC Completed

Handshakes

Thread local handshakes vs Global

Time To Safepoint (TTSP)

Concurrent GC

Application Threads

GC Threads

Groundwork : Heap layout -regions and generations

Heap Layout

Shenandoah GC

Young GenerationG1 GCOld Generation

Commonalities : Copying collector – from and to spaces

From To

HeapFrom Space To SpaceO 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

STATICVARIABLES

ANY JNI REFERENCES

Copying Collector aka Compacting Collector aka Evacuation

O O O O O O O O O OO O O O O O O O O OO O O O O O O O O O

GC ROOTS

THREAD 1 STACK

THREAD N STACK

O OO O

STATICVARIABLES

ANY JNI REFERENCES

O O O O O O O O O OO O O O O O O O O OO O O O O O O O O O

Copying Collector aka Compacting Collector aka Evacuation

O O O O O O O OO O O O O O OO O O O O O O

O O OO O O

Commonalities : Regions – occupied and free

Occupied and Free Regions

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

Commonalities : Collection set and priority

Collection Priority and Collection Set

O O O O O O O O O O

O O O O

• 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

Introduction : G1, Shenandoah & Z - Algorithms

Algorithm and Other Considerations

Garbage Collectors G1 GC Shenandoah GC Z GCRegionalized? Yes Yes YesGenerational? Yes No NoCompaction? Yes, STW, Forwarding

address in headerYes, Concurrent, Forwarding Pointer

Yes, Concurrent, Colored Pointers

Target Pause Times? 200ms 10ms 10msConcurrent Marking Algorithm? SATB SATB Striped

Differences – G1

GC Phases of Marking and Compaction

G1 GC GistInitial Mark Mark objects directly reachable by the rootsConcurrent Root Region Scanning Since initial mark is piggy-backed on a young collection, the survivor

regions need to be scannedConcurrent Marking Snapshot-at-the-beginning (SATB) algorithmFinal Marking Drain SATB buffers; traverse unvisited live objectsCleanup Identify and free completely free regions, sort regions based on liveness

and expenseSTW Compaction Move objects in collection set to “to” regions; free regions in collection

•C. Hunt, M. Beckwith, P. Parhar, B. Rutisson. Java Performance Companion.

Concurrent Marking

Logical snapshot of the heapSATB 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 liveFor 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

BarriersSATB 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); }

BarriersPost 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

STW CompactionForwarding Pointer in Header

BodyHeader

A Java Object

Pointer

Pointer to an InstanceKlassMark 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

Differences – Z

Z GC GistInitial Mark Mark objects directly reachable by the rootsConcurrent Marking Striping - GC threads walk the object graph and markFinal Marking Traverse unvisited live objects; weak root cleaningConcurrent Prepare for Compaction Identify collection set; reference processingStart Compaction Handles roots into the collection setConcurrent Compaction Move objects in collection set to “to” regionsConcurrent 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

Striping

Heap divided into logical stripesGC threads work on their own stripeMinimizes shared state

Load barrier to detect loads of non-marked object pointersConcurrent reference processingThread local handshakes

GC Thread0

GC Thread1

GC Threadn

0 1 … n 0 1 … n 0 1 … n

Stripe0 Stripe1 Stripen

Colored PointersConcurrent Marking

BarriersRead Barrier – For References

Update a “bad” reference to a “good” referenceCan be self-healing/repairing barrier when updates the source memory locationImposes 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

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)); }

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

Core ConceptColored Pointers

Object Address041

Unused

Marked0

Marked1

Remapped

Finalizable

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/taggingMulti-mapping on x86-64Hardware support on SPARC, aarch64

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

Off-Heap Forwarding Tables

Differences – Shenandoah

https://wiki.openjdk.java.net/display/shenandoah/Main

Shenandoah GC GistInitial Mark Mark objects directly reachable by the rootsConcurrent Marking Snapshot-at-the-beginning (SATB) algorithmFinal Marking Drain SATB buffers; traverse unvisited live objects; identify

collection setConcurrent Cleanup Free completely free regionsConcurrent Compaction Move objects in collection set to “to” regionsInitial Update Reference Initialize the update reference phaseConcurrent Update Reference Scans the heap linearly; update any references to objects

that have movedFinal Update Reference Update roots to point to to-region copiesConcurrent Cleanup Free regions in collection set

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

BarriersSATB Pre-Write Barrier - Recap

Needed for all updatesCheck if “marking-is-active”SATB_enqueue the pre_val

BarriersRead 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.

BarriersCopying Write Barrier – For Concurrent Compaction

Needed for all updates to ensure to-space invariantCheck if “evacuation_in_progress”Check if “in_collection_set” and “not_yet_copied”CAS (fwd-ptr(obj), obj, copy)

Superseded

BarriersRead Barrier – For Concurrent Compaction

Superseded

BarriersCopying Write Barrier – For Concurrent Compaction

BarriersLoaded 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 loadCheck if fast-path-possible; else do-slow-path

Concurrent CompactionBrooks Style Indirection Pointer

BodyHeader

A Java Object

Indirection Pointer

Forwarding pointer is placed before the objectAdditional work of dereferencing per object

Superseded

Concurrent CompactionBrooks Style Indirection Pointer

Forwarding pointer is placed before the objectAdditional work of dereferencing per object

Concurrent CompactionForwarding Pointer in Header

BodyHeader

To Space Copy Java Object

BodyForwarding Pointer

From Space Java Object

https://developers.redhat.com/blog/2019/06/28/shenandoah-gc-in-jdk-13-part-2-eliminating-the-forward-pointer-word/

Performance!

Test SetupAzure VM

Azure VM + Linux:

Intel(R) Xeon(R) CPU E5-2673 v3 @ 2.40GHz

16GB; 4 cores

https://adoptopenjdk.net/releases.html#x64_linux

Out-of-box* GC PerformanceThroughput and Responsiveness

G1 (200ms) G1 (50ms) Shenandoah Z

Throughput Responsiveness

*With Xmx=Xms

OpenJDK: In the New Age of Concurrent Garbage Collectors · 2019-11-05 · Algorithm and Other...

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