write barriers in g1 gc - jfokus...the garbage first collector - pause histogram 32 pause time in...

Write Barriers in Garbage First Garbage Collector

-By Monica Beckwith Code Karam LLC

@mon_beck; [email protected]

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mailto:[email protected]

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About me

• Java performance engineer

• I am working as a consultant and an instructor

• I have worked with Oracle, Sun, AMD …

• I used to work in the capacity of G1 GC performance lead @Oracle.

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Agenda• Heap regions and additional data structures

• RSets

• Barriers

• Concurrent refinement

• G1 GC Stages

• Concurrent marking in G1 GC

• Pre-write barrier

• SATB algorithm

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Regionalized Heap

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Traditional Java Heap

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Contiguous Java Heap

Eden S0 S1

Old Generation

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Garbage First GC - Heap Regions

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Contiguous Java Heap

Free Region

Non-Free Region

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G1 GC Heap Regions

• Young Regions - Regions that house objects in the Eden and Survivor Spaces

• Old Regions - Regions that house objects in the Old generation.

• Humongous Regions - Regions that house Humongous Objects.

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Additional Data Structures

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G1 GC Collection Set & Remembered Sets

• Additional data structure to help with maintenance and collection

• Add a slight footprint overhead (~5%)

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Remembered Sets (RSets)

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• Maintains and tracks incoming references into its region

• old-to-young references

• old-to-old references

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Remembered Sets

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• Remembered sets have varying granularity based on the “popularity” of objects or regions.

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Remembered Sets

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Remembered Sets

• Different granularities:

• sparse per-region-table (PRT)

• fine-grained PRT

• coarse-grained bitmap

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Figure 2.3 Remembered sets with incoming object references

RSet Maintenance: Barriers + Refinement

Threads

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Write Barrier

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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.

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Post-Write BarrierG1 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

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Concurrent Refinement Threads

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• The refinement threads will scan cards in the filled update log buffers to update the RSets for their corresponding regions.

• The refinement threads are always active

• G1 GC deploys them in a tiered manner to keep up with the filled buffers

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• Mutator threads can be enlisted to help with processing of filled buffers.

• Avoid this scenario, since the Java application will be halted until the filled buffers are processed!

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G1 GC Stages

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A Young Collection

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G1 GC Stages - Young Collection

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• When young regions are full and no further allocations can happen

• Need to start a stop-the-world collection

• Age objects in survivor regions

• Promote aged objects into the old regions

Marking Threshold

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Initiating Heap Occupancy Percent

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• Threshold to start a marking cycle to identify candidate old regions for collection during a mixed/incremental collection

• When old generation occupancy crosses this adaptive threshold, a marking cycle can start

Marking

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Stages of Marking

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• Initial-mark

• Root region scan

• Concurrent mark

• Remark/ Final mark

• Cleanup

A Mixed Collection

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G1 GC Stages - Mixed Collection

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• When candidate old regions are available and the potential of recovered space is over the (internal) reclaimable threshold

• This is a stop-the-world collection

• All regions in the young generation are included in this collection

• Candidate old regions are added based on the reclaimable space and other min/max thresholds

• Can have multiple mixed collection pauses based on the total old regions identified and a (internal) count target.

G1 GC Pause Histogram

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The Garbage First Collector - Pause Histogram

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Paus

e tim

e in

milli

seco

nds

0

30

60

90

120

Timestamps3415 3416.3 3417.2 3418.4 3419 3422 3423.4 3432.2 3433.2 3436.8 3437.6 3438.9 3440

Young Collection Initial Mark Remark Cleanup Mixed Collection

Initiating Heap Occupancy Percent

Concurrent Marking in G1 GC

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Recap: Stages of Marking

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• Initial-mark

• Root region scan

• Concurrent mark

• Remark/ Final mark

• Cleanup

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Marking Highlights

• Employs ‘Snapshot At The Beginning (SATB) algorithm

• Incremental and concurrent marking algorithm.

• A pre-write barrier is needed to gather the snapshot

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SATB Algorithm For Concurrent & Incremental

Marking

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A Java Heap (Showing PTAMS) With Previous Bitmap

End

Bottom

Previous Bitmap

PTAMS

Objects Are Already Marked

Top

~ ~ ~ ~

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A Java Heap (Showing PTAMS + NTAMS) With Previous Bitmap

Top

Bottom

Next Bitmap

PTAMS

NTAMS

End~ ~ ~ ~

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Same Java Heap During Concurrent Marking

End

Bottom

PTAMS

NTAMSNew Objects Are

Allocated In This SpaceTop ~ ~ ~ ~

Next Bitmap

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Bottom

PTAMS

NTAMS

End

Same Java Heap During Concurrent Marking

Top

Objects Will Be Considered

Implicitly Live

~ ~ ~ ~

Next Bitmap

Pre-Write Barrier

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Pre-Write Barrier

• To record the previous value of the reference fields of objects that were reachable at the start of marking + were a part of the snapshot.

• Prevents those objects from being overwritten by the mutator thread

• Mutator thread logs the previous value of the pointer in an SATB buffer

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Pre-Write Barrier - Pseudo Code

if (marking_is_active) {

pre_val:= x.f;

if (pre_val:= NULL) {

satb_enqueue(pre_val);

}

}

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End

Bottom

PTAMS

NTAMS

All Live Objects Are Marked

Next Bitmap

Top ~ ~ ~ ~

Same Java Heap At The End Of Remark Pause

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Same Java Heap At The End Of Remark Pause

End

Bottom

PTAMS

NTAMS

Top

All Objects Are Implicitly Live

~ ~ ~ ~

Next Bitmap

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References and Additional Reading

• D. L. Detlefs, C. H. Flood, S. Heller, and T. Printezis. Garbage-First Garbage Collection.

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

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write barriers in g1 gc - jfokus...the garbage first collector - pause histogram 32 pause time in...

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