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COMP30112: Concurrency
Topic 9: Termination Detection
Alan Williams
Room 2.107, email: alanw@cs.man.ac.uk
February 2007
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The Problem
- Task requires a set of n processes to complete the job
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COMP30112: Concurrency Topic 9: Termination Detection Alan Williams - - PowerPoint PPT Presentation
COMP30112: Concurrency Topic 9: Termination Detection Alan Williams - - PowerPoint PPT Presentation
COMP30112: Concurrency Topic 9: Termination Detection Alan Williams Room 2.107, email: alanw@cs.man.ac.uk February 2007 1 / 8 The Problem Task requires a set of n processes to complete the job 2 / 8 The Problem Task requires a set of
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SLIDE 3
The Problem
- Task requires a set of n processes to complete the job
(variations possible!)
2 / 8
SLIDE 4
The Problem
- Task requires a set of n processes to complete the job
(variations possible!)
- processes communicate via synchronous one-to-one
message-passing
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SLIDE 5
The Problem
- Task requires a set of n processes to complete the job
(variations possible!)
- processes communicate via synchronous one-to-one
message-passing (more variations possible!)
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SLIDE 6
The Problem
- Task requires a set of n processes to complete the job
(variations possible!)
- processes communicate via synchronous one-to-one
message-passing (more variations possible!)
- each process is active or passive
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SLIDE 7
The Problem
- Task requires a set of n processes to complete the job
(variations possible!)
- processes communicate via synchronous one-to-one
message-passing (more variations possible!)
- each process is active or passive
- active processes can generate and send messages
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SLIDE 8
The Problem
- Task requires a set of n processes to complete the job
(variations possible!)
- processes communicate via synchronous one-to-one
message-passing (more variations possible!)
- each process is active or passive
- active processes can generate and send messages
- active or passive processes can receive messages
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SLIDE 9
The Problem
- Task requires a set of n processes to complete the job
(variations possible!)
- processes communicate via synchronous one-to-one
message-passing (more variations possible!)
- each process is active or passive
- active processes can generate and send messages
- active or passive processes can receive messages
- an inactive process becomes active on receiving a
message
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SLIDE 10
The Problem
- Task requires a set of n processes to complete the job
(variations possible!)
- processes communicate via synchronous one-to-one
message-passing (more variations possible!)
- each process is active or passive
- active processes can generate and send messages
- active or passive processes can receive messages
- an inactive process becomes active on receiving a
message
- The task is complete when all processes are inactive
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SLIDE 11
The Problem
- Task requires a set of n processes to complete the job
(variations possible!)
- processes communicate via synchronous one-to-one
message-passing (more variations possible!)
- each process is active or passive
- active processes can generate and send messages
- active or passive processes can receive messages
- an inactive process becomes active on receiving a
message
- The task is complete when all processes are inactive
Problem: How to determine when task is finished??
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Termination Detection Algorithm
- Reference: Dijkstra, Feijen and van Gasteren: Derivation
- f a Termination Detection Algorithm for Distributed
- Computation. Information Processing Letters, volume 16,
1983 pages 217–219, North-Holland.
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SLIDE 13
Termination Detection Algorithm
- Reference: Dijkstra, Feijen and van Gasteren: Derivation
- f a Termination Detection Algorithm for Distributed
- Computation. Information Processing Letters, volume 16,
1983 pages 217–219, North-Holland.
- algorithm + derivation only 3 pages
3 / 8
SLIDE 14
Termination Detection Algorithm
- Reference: Dijkstra, Feijen and van Gasteren: Derivation
- f a Termination Detection Algorithm for Distributed
- Computation. Information Processing Letters, volume 16,
1983 pages 217–219, North-Holland.
- algorithm + derivation only 3 pages
- use a token to detect termination by ‘probing’ processes
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Termination Detection Algorithm
- Reference: Dijkstra, Feijen and van Gasteren: Derivation
- f a Termination Detection Algorithm for Distributed
- Computation. Information Processing Letters, volume 16,
1983 pages 217–219, North-Holland.
- algorithm + derivation only 3 pages
- use a token to detect termination by ‘probing’ processes
- connect processes in a ring: process i + 1 passes token to
process i (process 0 passes token to process n − 1)
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SLIDE 16
Termination Detection Algorithm
- Reference: Dijkstra, Feijen and van Gasteren: Derivation
- f a Termination Detection Algorithm for Distributed
- Computation. Information Processing Letters, volume 16,
1983 pages 217–219, North-Holland.
- algorithm + derivation only 3 pages
- use a token to detect termination by ‘probing’ processes
- connect processes in a ring: process i + 1 passes token to
process i (process 0 passes token to process n − 1)
- process 0 initiates the probe and signals completion
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Algorithm Rules
Probe starts and finishes with token at process 0. Processes and token have colour: all start coloured white
- Rule 0:
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Algorithm Rules
Probe starts and finishes with token at process 0. Processes and token have colour: all start coloured white
- Rule 0:
– keep/accept token while process active
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Algorithm Rules
Probe starts and finishes with token at process 0. Processes and token have colour: all start coloured white
- Rule 0:
– keep/accept token while process active – accept/pass token while process passive
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Algorithm Rules
Probe starts and finishes with token at process 0. Processes and token have colour: all start coloured white
- Rule 0:
– keep/accept token while process active – accept/pass token while process passive
- Rule 1: Set process colour to red if it sends a message to
a process with higher index
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Algorithm Rules
Probe starts and finishes with token at process 0. Processes and token have colour: all start coloured white
- Rule 0:
– keep/accept token while process active – accept/pass token while process passive
- Rule 1: Set process colour to red if it sends a message to
a process with higher index
- Rule 2:
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Algorithm Rules
Probe starts and finishes with token at process 0. Processes and token have colour: all start coloured white
- Rule 0:
– keep/accept token while process active – accept/pass token while process passive
- Rule 1: Set process colour to red if it sends a message to
a process with higher index
- Rule 2:
– A red (passive) process passes a red token
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SLIDE 23
Algorithm Rules
Probe starts and finishes with token at process 0. Processes and token have colour: all start coloured white
- Rule 0:
– keep/accept token while process active – accept/pass token while process passive
- Rule 1: Set process colour to red if it sends a message to
a process with higher index
- Rule 2:
– A red (passive) process passes a red token – A white (passive) process passes the same colour token
as input
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SLIDE 24
Algorithm Rules
Probe starts and finishes with token at process 0. Processes and token have colour: all start coloured white
- Rule 0:
– keep/accept token while process active – accept/pass token while process passive
- Rule 1: Set process colour to red if it sends a message to
a process with higher index
- Rule 2:
– A red (passive) process passes a red token – A white (passive) process passes the same colour token
as input
- Rule 3: For Process 0: if token or process are red, initiate
a new probe
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- Rule 4: Initiate a new probe: pass a white token; set
process colour to white
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- Rule 4: Initiate a new probe: pass a white token; set
process colour to white
- Rule 5: On passing token, a process sets colour to white
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- Rule 4: Initiate a new probe: pass a white token; set
process colour to white
- Rule 5: On passing token, a process sets colour to white
Done when Process 0 is passive, white, and receives a white token.
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Termination Example
* = active process
- 1. Initially P2 active, all other
processes passive You Do It: Apply Termination Detection Algorithm
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Termination Example
* = active process
- 1. Initially P2 active, all other
processes passive
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Termination Example
* = active process
- 2. P2 sends message to P1:
both P1, P2 active
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Termination Example
* = active process
- 3. P2 passive; P1 sends
message to P3: both P1, P3 active
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Termination Example
* = active process
- 4. P1 passive; P3 active
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Termination Example
* = active process
- 5. P3 active
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Termination Example
* = active process
- 6. P3 active
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Termination Example
* = active process
- 7. P3 becomes passive; all
processes passive
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