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Garbage First Collectors

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by Péter Varga on 11 August 2014

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Transcript of Garbage First Collectors

Garbage First Garbage Collector Algorithm
Basics
Generational Collectors
Serial/Parallel/Concurrent
Garbage First Garbage Collector
Java Garbage Collectors
Weak Generational Hypothesis
Generational
Detects Not Used Objects
Reclaims memory
Compacts HEAP
Basics
Serial?
Garbage First
String a = "A";
String b = "B";
String c = "C";
String d = "D";
String e = "E";
A
B
C
D
E
STACK
a
d
e
b
c
HEAP
A
B
C
D
E
STACK
a
d
e
b
c
HEAP
Young Generation
Old Generation
Eden
Majority of new objects allocated here

After GC, the surviving objects are moved to one of the Survivor Spaces
Survivor
Space
After GC in the Eden space, objects are piled up into the Survival space

Once a Survival space is full, surviving objects are moved to the other Survival space. Then the Survival space which was full will be treated as empty.

Objects that survived GC-s in the Young Generation a number of times are moved to the old generation.
Survivor
Space
Different algorithms
SerialGC (-XX:+UseSerialGC)
Paralell GC (-XX:UseParalellGC)
Paralell Old GC (-XX:UseParallelOldGC)
CMS GC (-XX:UseConcMarkSweepGC)
Garbage First GC (-XX:UseG1GC)
Generational Garbage Collection
Before GC
During GC
After GC
HEAP Regions and Chunks
Remembered Set for Region 0
Remembered Set for Region 1
Remembered Set for Region 2
After Marking Phase
During Concurrent Marking...
Marking Bitmaps in Action
Garbage Collectors
Most objects soon become unreachable.

References from old objects to young objects only exists in small numbers.
Garbage Collector
Remembered Set
Initial Mark
Concurrent
Maintenance
Using write barriers
Pauses the world
Creates a logical snapshot of the HEAP

Copying TAMS pointers
Final
Marking
Pause
Evacuation
Pauses the world
Copying survivor objects
Can evacuate to different regions depending on age
Mark
Pauses the world
Using write barrier
Heuristics
Determining Collections Sets
Based on Costs Prediction
Guarantees Pause Time Goal
A
D
E
STACK
a
d
e
b
c
HEAP
Parallel?
Concurrent?
Top At Mark Start (TAMS)
Saved at the beginning of Marking phase
After this point all objects treated as LIVE
The goal of marking phase is to mark live objects before this TAMS point
Evacuation explained
Pure Garbage First
Soft Realtime GC
Predictable Pauses
Write Barrier explained
Thank you for watching!
b = null;
c = null;
(C) 2012
peter.varga@drotposta.hu
DPC Consulting
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