Ulterior Reference Counting: Fast Garbage Collection without a Long Wait

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Ulterior Reference CountingFast Garbage
Collection without a Long Wait

• Author Stephen M Blackburn
• Kathryn S McKinley
• Presenter Jun Tao

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Outline

• Introduction
• Background
• Ulterior Reference Counting
• Methodology
• Results
• Conclusion

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Introduction

• A long-standing and unachieved goal for GC
  (garbage collector)
• Short pause times
• Excellent throughput
• Throughput and responsiveness of generational
  collector (BG-MS) and Reference counting

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Introduction

• A new generational collector Ulterior Reference
  Counting (URC)
• Copying nursery
• Reference counting mature space
• Use a deferred reference counting
• Divides the heap into logical partitions of RC
  and non-RC objects
• Reference count only infrequently mutated fields
• Defer the fields of highly mutated objects and
  enumerate them
• Trace the deferred pointers
• Use a write barrier to remember them

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Introduction

• Object lifetime and mutation demographics combine
  well to fit that requirement
• Young objects mutate frequently and die at a high
  rate
• Favor generational collection, with copying
  algorithm using fast contiguous allocations for
  the young objects
• Old objects mutate infrequently and die at a
  slower rate
• Favor a space efficient free-list allocator and a
  reference counting algorithm

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Introduction

• BG-RC a hybrid generational collector
• BG bounded copying nursery
• RC Reference counts the mature space
• Ignores nursery pointer mutations
• For surviving nursery objects
• Enumerates live pointers during tracing
• Copies them to RC space
• Computes reference counts for the mature space
• Reclaims objects with no reference

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Background

• Generational Collection
• Use tracing to identify dead objects indirectly
• Copying collector
• Copies all live objects into another space
• Works well when few objects survive
• Use bump-pointer allocation
• Mark-sweep collector
• Marks live objects
• Frees all unmarked objects
• Uses free-list allocation
• Needs no copy reserve
• Poor memory utilization without compaction

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Background

• Generational Collection
• How to organize nursery collection
• Flexible-sized nursery
• Fixed-size nursery
• Bounded nursery

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Background

• Reference Counting
• Advantage
• The work of garbage detection is spread out over
  every mutation
• Disadvantage
• Additional algorithms must reclaim cycles
• Tracking every pointer mutation is expensive

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Background

• Reference Counting (continued)
• Mechanisms
• Deferral
• Examines certain heavily mutated pointers
  periodically
• Deferral phase in which RCs are not correct
• RC phase in which they are
• Buffering
• Do not perform RC increments and decrements
  immediately
• Buffer and process them in RC phase
• Coalescing
• The periodicity of deferred RC implies that only
  the initial values and final values of pointer
  fields are relevant
• Uses the differences to generate increments and
  decrements

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Background

• Reference Counting (continued)
• RC Formal Definitions
• Mutation event RCM(p)
• i.e., RC(p before)--, RC(p after)
• May be buffered or performed immediately
• Retain event RCR(p)
• Deferral
• No mutation generates RCM(p)
• Need a RCR(p) to preserve objects

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Ulterior Reference Counting

• Generalizing Deferral

A RC integrate event RCI(p) changes a deferred
pointer to not-deferred is needed.
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Ulterior Reference Counting

• Deferral policies
• Determines for each pointer whether or not to
  perform mutation events
• Three approaches to enumerates deferral set
• Trace Deferred Fields
• Trace all live deferred fields just prior to
  every RC phase
• Record Mutated Deferred Fields
• Record Mutated Deferred Objects

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Ulterior Reference Counting

• A Generational RC Hybrid Collector (BG-RC)
• For young objects
• Bump-pointer allocation
• Copying collection
• For old objects
• Free-list allocation
• Reference counting collection

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Ulterior Reference Counting

• A Generational RC Hybrid Collector (BG-RC)
  (continued)

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Ulterior Reference Counting

• A Generational RC Hybrid Collector (BG-RC)
  (continued)
• Write Barrier
• Remembers pointers into the nursery from the
  non-nursery spaces
• An object remembering coalescing barrier

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Ulterior Reference Counting

• Write Barrier (continued)

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Ulterior Reference Counting

• Controlling Pause Times
• Pause time Nursery collection time reference
  counting time
• Appropriate nursery size
• Large
• Small
• Frequent collections
• Diminishes the effect of coalescing in RC
• Gives nursery objects less time to die
• Bound the accumulation of RC work between
  collections
• Limit the growth of meta data
• Modified object buffer
• Decrement buffer

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Ulterior Reference Counting

• Cycle Detection
• Bacon and Rajans trial deletion algorithm
• Colors the objects from all decrements which do
  not go to zero purple and puts them on a list
• If a purple still non-zero at the end of a RC
  phase, computes the object and objects reachable
  from it gray and decrements their RCs
• All gray objects with RC0 are cyclic garbage
• The authors choice
• Perform cycle detection with increasing
  probability when the available heap space falls
  toward a user-defined limit

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Methodology

• Jikes RVM and JMTk
• Collectors
• RC coalescing, deferred
• BG-RC
• MS
• BG-MS bounded copying nursery MS mature space

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Results
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Results
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Results
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Results

• Sensitivity to Heap size
• Generational collectors perform better on both GC
  time and mutator time
• Benefit from bump-pointers
• As the heap size shrinks, BG-RC degrades more
  rapidly than BG-MS
• Pause time guidelines prevent it from reclaiming
  cyclic garbage promptly
• Mutator time
• Performance MS gt BG-MS gt BG-RC

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Results
BG-RC Sensitivity to Variations in Collection
Triggers (defaults are 4MB nursery, 60ms time
cap, and 512KB cycle trigger)
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Conclusion

• Matches allocation and collection policies to the
  behaviors of older and younger objects
• Copying collector on nursery
• Reference Counting collector on mature space
• Achieves good performance on both throughput and
  responsiveness