Thanks to Steve for his Immix presentation from 2008.

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Immix: A Mark-Region Garbage Collector

Jennifer SartorCS395T Presentation

Mar 2, 2009

Thanks to Steve for his Immix presentation from 2008.

2

Contributions

• Another family of collectors: Mark-Region• Immix has all of: space efficiency, fast

collection, and mutator performance– Bump allocation– Locality– Opportunistic defragmentation, mark+copy in

single pass

Thanks to Steve for his Immix presentation from 2008.

3

GC FundamentalsAlgorithmic Components

Allocation ReclamationIdentification

Bump Allocation

Free List

`

Tracing(implicit)

Reference Counting(explicit)

Sweep-to-Free

Compact

Evacuate

3 1

4

Mark-Compact [Styger 1967]

Bump allocation + trace + compact

GC FundamentalsCanonical Garbage Collectors

`

Sweep-to-Free

Compact

Evacuate

Mark-Sweep [McCarthy 1960]

Free-list + trace + sweep-to-free

Semi-Space [Cheney 1970]

Bump allocation + trace + evacuate

Thanks to Steve for his Immix presentation from 2008.

5

Sweep-To-Regionand Mark-Region

`

Sweep-to-Free

Compact

Evacuate

Reclamation

Sweep-to-Region

Mark-SweepFree-list + trace + sweep-to-free

Mark-CompactBump allocation + trace + compact

Semi-SpaceBump allocation + trace + evacuate

Mark-RegionBump alloc + trace + sweep-to-region

Thanks to Steve for his Immix presentation from 2008.

6

Compared to Existing CollectorsCollector Space

EfficiencyFast

CollectionMutator

PerformanceReclamation

Mark-Sweep

X X Sweep-to-free

Semi-Space

X X Evacuation

Mark-Compact

X X Compaction

Mark-Region

X X X Sweep-to-region

Thanks to Steve for his Immix presentation from 2008.

7

Naïve Mark-Region

• Contiguous allocation into regionsExcellent locality– For simplicity, objects cannot span regions

• Simple mark phase (like mark-sweep)– Mark objects and their containing region

• Unmarked regions can be freed

0

Thanks to Steve for his Immix presentation from 2008.

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Heap Organization

• Blocks– Recyclable– Immix block = 32KB

• Lines– Objects can span lines– Immix line = 128B

• Opportunistic defragmentation– Candidate and target blocks– Single pass to mark and copy

Thanks to Steve for his Immix presentation from 2008.

9

Immix: Lines and Blocks

Small Regions

Large Regions

✗ Fragmentation (can’t fill blocks)

✓ More contiguous allocation ✗ Fragmentation (false marking)

Lines & BlocksN pages approx 1 cache line

✓ Less fragmentation Objects span lines

✓ Fast common case Lines marked with objects

✗ Increased metadata o/h

✗ Constrained object sizes

0

TLB locality, cache locality Block > 4 X max object size

Free FreeRecyclable Recyclable

Thanks to Steve for his Immix presentation from 2008.

10

Allocation Policy(Recycling)

• Recycle partially marked blocks firstMinimize fragmentationMaximize sharing of freed blocks

• Recycle in address order– We explored other options

• Allocate into free blocks lastEffect on locality and fragmentation?

Thanks to Steve for his Immix presentation from 2008.

11

Opportunistic Defragmentation

0

• Identify source and target blocks– (see paper for heuristics)

• Evacuate objects in source blocks– Allocate into target blocks

• Opportunistic– Leave in place if no space, or object pinned

• Opportunistically evacuate fragmented blocks– Lightweight, uses same allocation mechanism– No cost in common case (specialized GC)

• Source = most holes• Other heuristics?

Thanks to Steve for his Immix presentation from 2008.

12

Details

• Parallelizable– Coarse sweeping– Defragmentation

• Demand-driven overflow allocations– Medium objects

• Metadata space overheads– For parallel synch: mark bytes (not bits)– Line and block mark, not just object mark– Defragmentation headroom– Overflow allocation block– Conservative line marking

Thanks to Steve for his Immix presentation from 2008.

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Other Optimizations

Implicit Marking

✓ Most objects small Small objects implicitly mark next line✓ V. Fast common case Large objects mark lines exactly Implicit line mark

Line mark

Overflow Allocation

Multi-line objects may skip many small holes Overflow allocation (used on failure)

✓ Large objects uncommon ✓ V. effective solution✓

Thanks to Steve for his Immix presentation from 2008.

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Garbage Collection

Space

Tim

e

Total Performance

MarkSweepMarkCompactSemiSpaceImmix

Space

Tim

e

Mark-Region: Immix(Bump Allocation + Trace + Sweep-to-Region)

Mutator

Space

Tim

e

Minimum Heap

Spac

e

✓Simple, very

fast collection

✓Space

efficient

✓Good

locality

Actual data, taken from geomean of DaCapo, jvm98, and jbb2000 on 2.4GHz Core 2 Duo

✓Excellent

performance

Thanks to Steve for his Immix presentation from 2008.

15

Minimum Heap

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.61111111111111 3.0377358490566

MarkSweep MarkCompact SemiSpace Immix

Thanks to Steve for his Immix presentation from 2008.

16

Sticky Performance

Geomean of DaCapo, jvm98 and jbb2000 on 2.4GHz Core 2 Duo

1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 61

1.05

1.1

1.15

1.2

1.25

StickyMS StickyIX GenMS (Production)

Heap Size (Normalized)

Tota

l Tim

e (N

orm

alize

d)

Benefits of Sticky?

Thanks to Steve for his Immix presentation from 2008.

17

Discussion

• Compare to Beltway!– Increments & belts vs. lines & blocks– Both: reduce space util, good locality– Differences: incrementality and copying

• Compare to MC2 • Caching/paging?– Do tuned line/block sizes work?

• What is missing in experimental results?• Heap utilization?

Thanks to Steve for his Immix presentation from 2008.

18

ImmixEfficient Mark-Region Garbage Collection

Thanks to Steve for his Immix presentation from 2008.

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Total Performance

Geomean of DaCapo, jvm98 and jbb2000 on 2.4GHz Core 2 Duo

1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 61

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8

1.9

2

MarkSweep MarkCompact SemiSpace Immix

Heap Size (Normalized)

Tota

l Tim

e (N

orm

alize

d)