Overview of Distributed Garbage Collection Algorithms

1. Distributed Garbage Collection Algorithms stefan brunthaler

2. Overview • Brief Introduction • Reference Counting • Weighted Reference Counting • Indirect Reference Counting • Mark & Sweep • Marktree • Tracing with timestamps • Stop & Copy

3. Introduction – Definitions, 1 What is a distributed system? “A system in which hardware or software components located at networked computers communicate and coordinate their actions only by message passing.” --Colouris, 2001

4. Introduction – Definitions, 2 What is a distributed system? “A distributed system is a collection of independent computers that appear to the users of the system as a single computer.” --Tanenbaum, 1994

5. Introduction – Nomenclature A distributed system: A B C

6. Introduction – Nomenclature local objects: A B C

7. Introduction – Nomenclature global objects: A B C

8. Introduction – Nomenclature export record: A B C

9. Introduction – Nomenclature import record: A B C

10. Introduction – Nomenclature intrasite references: A B C

11. Introduction – Nomenclature intersite references: A B C

12. Introduction – Networking problems: latency A B C

13. Introduction – Networking problems: non-causality A B C

14. Reference Counting • Reference Counting • Weighted Reference Counting • Indirect Reference Counting

15. Weighted Reference Counting • proposed independently by [Watson and Watson, 1987], and [Bevan, 1987]

16. Weighted Reference Counting Where are we? A B C

17. Weighted Reference Counting Memory cell layout count weight ref

18. 128 96 128 64 64 32 32 Weighted Reference Counting Algorithm

19. 1 1 128 1 128 64 64 Weighted Reference Counting Algorithm

20. Weighted Reference Counting Domino effect [Rudalics, 1990] A B C 1

21. Weighted Reference Counting • pros: • no race conditions (latency) • cons: • susceptible to unreliable messaging • duplicated messages, • message failures.

22. Indirect Reference Counting • proposed by [Ichisugi and Yonezawa, 1990], [Rudalics, 1990], and [Piquer, 1991]

23. Indirect Reference Counting Where are we? A B C

24. Indirect Reference Counting Memory cell layout copies parent ref

25. Indirect Reference Counting Algorithm A B C

26. 1 0 0 Indirect Reference Counting inverted diffusion tree 2 1 2 1 0 0

27. Mark & Sweep • Mark & Sweep • Marktree • Tracing with timestamps

28. Marktree • after [Hudak and Keller, 1982], • based on on-the-fly garbage collector as proposed by [Dijkstra et al., 1978], • instead of recursion uses mark tasks, • back propagation using uptree tasks, • tricolor scheme.

29. Marktree

30. Tracing with timestamps • based on Hughes, 1985

31. Tracing with timestamps Algorithm m&s m&s m&s A B C

32. Stop & Copy • based on Rudalics, 1986 rootspace fromspace tospace

33. Danke für Eure Aufmerksamkeit • Fragen?