Fast Conservative Garbage Collection

Rifat ShahriyarStephen M. BlackburnAustralian National University

Kathryn S. McKinleyMicrosoft Research

GC is Ubiquitous

• GC implementations Exact Conservative

• High performance systems use exact GC• Conservative GC is popular

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conservativeexact

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BDW

Our g

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Performance

16% slowdown a show-stopper in many situations

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Root Conservative GC

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Ambiguous References

• May be pointers– Retain referents, transitively– Excess retention

• May be values– Cannot change values – Pin referents

• May corrupt heap– Validate before updating per-object metadata

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Why Conservative GC

Advantages✔ Disentangles GC from compiler✔ Avoids challenging engineering of stack maps✔ Enables more compiler optimizations

Disadvantages✘ Must handle ambiguous references✘ Performance

Our goal is high performance conservative GC for managed languages

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High Performance Exact Garbage Collectors

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Immix [Blackburn & McKinley 2008]

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• Contiguous allocation into regions– 256B lines and 32KB blocks– Objects span lines but not blocks

• Simple mark phase– Mark objects and containing regions

• Free unmarked regions ✔Recycled allocation and defragmentation

block

line

recyclable linesobject mark line mark

linelineline

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RC Immix [Shahriyar et al. 2013]

• Combines RC and Immix✔Object local collection✔Great mutator locality

• Exploit Immix’s opportunistic copy– New objects can be copied by first GC– Old objects can be copied by backup GC✔Copying with RC to eliminate fragmentation

0 01 3 1 2

11 3 2 1 2200

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Conservative Garbage Collectors

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Non-Moving Free list

• Designed for fully conservative settings • Problems

✘Poor performance✘Overly conservative design for many settings

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Mostly-Copying Semi Space

• a.k.a. Bartlett-style, with many variants• Problems

✘Semi-space suffers from huge collection cost✘Poor performance

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All exact 217

Unique exact 98 0.45x

All cons. 461 2.12x

Unique cons. 157 1.6x

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44 KB 0.02%

Actual pinned 14 KB 0.05%

MCC pinned 462 KB 2.1%

MCC false pinned 282 KB

Immix pinned 36 KB 0.2%

Excess retention

Pinning

Roots

Cost of Conservatismaveraged over 20 Java benchmarks

The direct cost of conservatism is very low

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Heap Organization is Key

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Outline

✔Real source of performance overhead is not conservatism but heap organization

• Design and implementation of– Conservative Immix collectors– Conservative reference counting

• Evaluation

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Design

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Filtering

• Object map filters ambiguous references– Bitmap records locations of all live objects – Allocator sets bit encoding object start address– Collector clears bit for dead objects

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Cost of Filtering

Heap organization trumps object map overhead (2.7%)

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Pinning

• Immix line pinning, opportunistic copying– Pin referents of ambiguous references– Logically pinned lines cannot be reused– Move other objects as space permits

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Results

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Methodology

• 20 benchmarks– DaCapo, SPECjvm98 and pjbb2005

• 20 invocations for each benchmark• Jikes RVM and MMTk– All garbage collectors are parallel

• Intel Core i7 4770, 8GB• Ubuntu 12.04.3 LTS

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Cost of Conservatismnormalized to exact

Low performance penalty for conservative collectors

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CC MSBDW RC RC

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1% 2.3% 2.7% 2.6% 2.7%

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Total Time

RC Immixcons matches Gen Immix and RC Immix

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Total Time v Heap Size

1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 60.98

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Heap Size / Minimum Heap

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cons

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Wider Applicability

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Conservatism and PinningAmenability of existing systems to conservative RC Immix

• Quantify ambiguous references in target applications– Conservative RC Immix tolerates 8x pinning

increase with only 3.4% overhead

• Modify heap organization to use lines and blocks

• Implement full-heap tracing Immix collector (5% improvement)– Depends on code quality

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Summary

• Conservative GC– Dominated by BDW and MCC– Significant overheads– Heap org. key to performance

• New designs– Low overhead object map– Immix line based pinning

• Conservative RC Immix– Matches fastest production

Questions?Available at: https://jira.codehaus.org/browse/RVM-1085