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Fast Garbage Collection without a Long Wait

Ulterior Reference Counting:. Fast Garbage Collection without a Long Wait. Steve Blackburn – Kathryn McKinley Presented by: Na Meng. mutator. mutator. poor responsiveness. maximum pause. GC. CPU Utilization (  time  ). Throughput vs. Responsiveness. Goal

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Fast Garbage Collection without a Long Wait

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  1. Ulterior Reference Counting: Fast Garbage Collection without a Long Wait Steve Blackburn – Kathryn McKinley Presented by: Na Meng

  2. mutator mutator poor responsiveness maximum pause GC CPU Utilization (time) Throughput vs. Responsiveness • Goal • Better responsiveness (shorter pause time) • Better throughput (running time) Thanks to Dimitris for his presentation in 2009.

  3. Copying • GC time proportional to survivors • Ideal for collecting high mortality young objects • Reference Counting • GC time proportional to mutations • Ideal for collecting objects with low mutation • Young objects • High mortality • High mutation • Old objects • Low mortality • Low mutation Ulterior Reference Counting

  4. Basic Idea Young nursery space RC-mature space 1 2 0 1 • Thanks to Curtis for his presentation recently.

  5. Ulterior Reference Counting Parameter Control Pause time Write Barrier Old-new pointer Integrate Non-RC to RC RC buffering Expensive RC tracking Bounded nursery Expensive cycle detection Cycle detection Cyclic structures

  6. 3 3 3 2 0 1 1 2 Live Data 2 1 2 0 1 1 1 1 1 0 1 1 0 2 2 2 2 3 1 1 1 3 1 1 1 1 1 0 1 0 0 0 0 3 1 3 1 1 1 1 2 3 3 0 1 0 2 3 0 0 1 0 2 Cycle Detection Root Buffer Cycle Buffer 1. Process Decrements 4. Collect White 2. Mark Gray 5. Calculate external in-degree 3. Scan 6. If 0, GC/decrement neighbors 7. If non-0, restore Modifiedfrom David Bacon’s slide in 2001.

  7. Bounded Nursery • Situation: mature space is almost full 1 2 1 2 • Is this understanding correct? • Will maximum pause time increase?

  8. RC Buffering • Situation: tracking RC is expensive Stacks Registers Boot Image & Immortal Inc Buffer Dec Buffer a c a e f b c d 0 1 1 1 2 1 1 1 a c

  9. e g Integration • When doing nursery collection Stacks Registers g e d f Boot Image & Immortal Stacks Registers d c c f g e 0 0 0 0 0 0 Boot Image & Immortal Inc Buffer f d b b 1 1 • Decreased in the next collection to make the increment temporary. Why?

  10. Write Barrier • Prepare for nursery collection Stacks Registers e d Boot Image & Immortal b srcObj b srcSlot &b &b e d tgtobj Modified Object Set Dec Buffer c c b 1 2 • What will happen if b points back to c again?

  11. Pause Time Control • Phases for Ulterior Garbage Collector • Nursery Collection • RC buffering • RC collection • Cycle collection Bounded pause time Negligible Unbounded in worst case Unbounded in worst case • To control RC collection time • Limit size of modified object buffer, dec buffer • Whynot limit inc buffer size ? • To control cycle collection time • Limit time spent on this work

  12. Discussion • Cycle detection • On every RC collection, create a candidate set of potential cycle roots from all the decrements which do not go to zero • Cycle detection starts from the candidate set • Do we need a limit to the length of candidate set? • Do such decrements guarantee finding all cycles if time allows?

  13. Thank You ! www.themegallery.com

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