Introduction to Garbage Collection

GC Fundamentals Algorithmic Components AllocationReclamation 2 Identification Bump Allocation Free List ` Tracing (implicit) Reference Counting (explicit) Sweep-to-Free Compact Evacuate 31

Mark-Compact [Styger 1967] Bump allocation + trace + compact GC Fundamentals Canonical Garbage Collectors 3 ` Sweep-to-Free Compact Evacuate Mark-Sweep [McCarthy 1960] Free-list + trace + sweep-to-free Semi-Space [Cheney 1970] Bump allocation + trace + evacuate

GC Fundamentals The Time–Space Tradeoff 4

Mark-Sweep (Free List Allocation + Trace + Sweep-to-Free) 5 ✓ ✓ Space efficient ✓ ✓ Simple, very fast collection Poor locality

Mark-Compact (Bump Allocation + Trace + Compact) 6 ✓ ✓ Good locality Good locality ✓ ✓ Space efficient Expensive multi-pass collection

Semi-Space (Bump Allocation + Trace + Evacuate) 7 ✓ ✓ Good locality Space inefficient

Sweep-To-Region and Mark-Region 8 ` Sweep-to-Free Compact Evacuate Reclamation Sweep-to-Region Mark-Sweep Free-list + trace + sweep-to-free Mark-Compact Bump allocation + trace + compact Semi-Space Bump allocation + trace + evacuate Mark-Region Bump alloc + trace + sweep-to-region

Mark-Region (Bump Allocation + Trace + Sweep-to-Region) 9 ✓ ✓ Simple, very fast collection ✓ ✓ Space efficient ✓ ✓ Good locality ✓ ✓ Excellent performance Excellent performance

Naïve Mark-Region 10 Contiguous allocation into regions Excellent 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 0

Generational [Ungar 1984] Most objects die young 11 Nursery space Mature space

Immix [Blackburn and McKinley 2008] 0 0 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 12 block line recyclable lines object markline mark