1 / 17

Immix: A Mark-Region Garbage Collector

Immix: A Mark-Region Garbage Collector. Curtis Dunham CS 395T Presentation Feb 2, 2011. I believe this presentation to be ~ 95% Steve’s PLDI talk and ~4% Jennifer Sartor’s 395T presentation and < 1% mine . Comparison to Prior Work; Contributions. Status Quo Before This Work.

kolina
Download Presentation

Immix: A Mark-Region Garbage Collector

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Immix: A Mark-Region Garbage Collector Curtis Dunham CS 395T Presentation Feb 2, 2011 I believe this presentation to be ~95% Steve’s PLDI talk and ~4% Jennifer Sartor’s 395T presentation and < 1% mine  Thanks to Steve Blackburn and Jennifer Sartor for their 2008 and 2009 Immix presentations, respectively.

  2. Comparison to Prior Work; Contributions Status Quo Before This Work Post-Immix: The New World of GC Mutator Performance Bump Pointer Locality Mark-Compact Copying Immix: Mark-Region w/ Opportunistic Defragmentation Does both (every object either marked or copied) In One Pass Non-Semispace Space efficiency Fast Collection Mark-Sweep

  3. GC FundamentalsAlgorithmic Components Identification Allocation Reclamation ` Sweep-to-Free Tracing (implicit) Free List Compact Reference Counting (explicit) Bump Allocation Evacuate 3 1

  4. GC FundamentalsCanonical Garbage Collectors Mark-Sweep [McCarthy 1960] Free-list + trace + sweep-to-free ` Sweep-to-Free Compact Mark-Compact [Styger 1967] Bump allocation + trace + compact Evacuate Semi-Space [Cheney 1970] Bump allocation + trace + evacuate Thanks to Steve for his Immix presentation from 2008.

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

  6. Naïve Mark-Region • 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

  7. Heap Organization • Blocks – analogous to Regions • Recyclable • Immix block = 32KB • Lines • Objects can span lines • Immix line = 128B • Opportunistic defragmentation • Candidate and target blocks • Single pass to mark and copy Reusable for (more) allocation 256 per Block Move from mostly-empty to mostly-full

  8. Immix: Lines and Blocks ✓More contiguous allocation Large Regions ✗ Increased metadata o/h ✗ Constrained object sizes ✗ Fragmentation (false marking) “In a mark-region collector, region size embodies the collector’s space-time tradeoff.” Small Regions Lines & Blocks N pages approx 1 cache line Free Recyclable Free Recyclable ▫ TLB locality, cache locality ▫ Block > 4 X max object size ▫ Objects span lines ▫ Lines marked with objects ✗ Fragmentation (can’t fill blocks) ✓Less fragmentation ✓Fast common case

  9. Allocation Policy(Recycling) • Recycle partially marked blocks first • Minimize fragmentation • Maximize sharing of freed blocks • Recycle in address order • We explored other options • Allocate into free blocks last Effect on locality and fragmentation?

  10. Opportunistic Defragmentation • 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?

  11. 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

  12. Other Optimizations ✓Most objects small ▫ Small objects implicitly mark next line Line mark ✓V. Fast common case Implicit line mark ▫ Large objects mark lines exactly Overflow Allocation Implicit Marking ✓ ▫ Multi-line objects may skip many small holes ▫ Overflow allocation (used on failure) ✓Large objects uncommon ✓V. effective solution

  13. Mark-Region: Immix(Bump Allocation + Trace + Sweep-to-Region) Good locality Space efficient ✓ ✓ Excellent performance ✓ Simple, very fast collection ✓ Actual data, taken from geomean of DaCapo, jvm98, and jbb2000 on 2.4GHz Core 2 Duo

  14. Total Performance Geomean of DaCapo, jvm98 and jbb2000 on 2.4GHz Core 2 Duo

  15. Discussion • Necessity of two-level hierarchy? • Caching/Paging? • Efficacy of tuned line/block sizes:e.g. actual TLB miss reduction? • Implicit Marking • advantages overcome possible fragmentation? • Methodology and Results

  16. Minimum Heap

  17. Sticky Performance Benefits of Sticky? Geomean of DaCapo, jvm98 and jbb2000 on 2.4GHz Core 2 Duo

More Related