Distributed Garbage Collection Algorithms

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Distributed Garbage Collection Algorithms
stefan brunthaler
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Overview

• Brief Introduction
• Reference Counting
• Weighted Reference Counting
• Indirect Reference Counting
• Mark Sweep
• Marktree
• Tracing with timestamps
• Stop Copy

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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. --Colour
is, 2001
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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
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Introduction Nomenclature
A distributed system
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Introduction Nomenclature
local objects
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Introduction Nomenclature
global objects
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Introduction Nomenclature
export record
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Introduction Nomenclature
import record
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Introduction Nomenclature
intrasite references
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Introduction Nomenclature
intersite references
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Introduction Networking
problems latency
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Introduction Networking
problems non-causality
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Reference Counting

• Reference Counting
• Weighted Reference Counting
• Indirect Reference Counting

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Weighted Reference Counting

• proposed independently by Watson and Watson,
  1987, and Bevan, 1987

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Weighted Reference Counting
Where are we?
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B
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Weighted Reference Counting
Memory cell layout
count
weight
ref
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Weighted Reference Counting
Algorithm
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Weighted Reference Counting
Algorithm
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Weighted Reference Counting
Domino effect Rudalics, 1990
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1
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Weighted Reference Counting

• pros
• no race conditions (latency)
• cons
• susceptible to unreliable messaging
• duplicated messages,
• message failures.

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Indirect Reference Counting

• proposed by Ichisugi and Yonezawa, 1990,
  Rudalics, 1990, and Piquer, 1991

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Indirect Reference Counting
Where are we?
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B
C
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Indirect Reference Counting
Memory cell layout
copies
parent
ref
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Indirect Reference Counting
Algorithm
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B
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Indirect Reference Counting
inverted diffusion tree
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1
2
1
0
0
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Mark Sweep

• Mark Sweep
• Marktree
• Tracing with timestamps

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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.

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Marktree
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Tracing with timestamps

• based on Hughes, 1985

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Tracing with timestamps
Algorithm
ms
ms
ms
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B
C
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Stop Copy

• based on Rudalics, 1986

rootspace
fromspace
tospace
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Danke für Eure Aufmerksamkeit
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