Discuss mutual exclusion algorithms such as centralized, decentralized, token-based, and distributed algorithms.

1. Mutual Exclusion in
Distributed Memory Systems
CS4532 Concurrent Programming
Dilum Bandara
Dilum.Bandara@uom.lk
Some slides adapted from U Uthaiyashankar (WSO2) and Rajkumar Buyya’s

2. Exercise
 Come up with a solution to enforce following
synchronization between 2 threads x & y
 Thread x writes to a file f. Then only thread y should
read from f
 Now suppose x & y are 2 processes on 2
different nodes (assume a shared file system)
 Propose a suitable solution to achieve
synchronization
2

3. Mutual Exclusion Problems
 Centralized algorithms for mutual exclusion
 Semaphores, Monitors, Condition Variables
 Mutual exclusion in distributed memory systems
 No shared memory
 Usually, no centralized instance similar to OS kernel
that would coordinate access
 Based on asynchronous, usually failure-prone,
network infrastructure
 Need a solution for distributed mutual exclusion,
based solely on message passing
3

4. Mutual Exclusion Requirements
 Safeness property
 Liveness property
 Ordering
 A fairness issue
 Efficient
4
Source: http://pk.org/417/notes/10-mutex.html

5. Question from 2013 S7 Paper
 There are monkeys living on 2 very high rocks. The
northern monkeys live on the northern rock that provides
water but no food. Conversely, the southern monkeys
live on the southern rock that provides food but no water.
However, both the northern and southern monkeys have
to eat and to drink!
 There is a small rope between the 2 rocks. The rope can
carry up to MaxOnRope monkeys. Concurrent crossing
in both directions is not possible.
1. What type of a solution would you recommend for this
problem? Briefly explain. [3 marks]
2. Provide a pueudocode to solve the problem. [5 marks]
5

6. Mutual Exclusion Algorithms
6
Mutual Exclusion
algorithms
Centralized
Locks Semaphores Monitors
Distributed
Contention
Based
Voting schemes
Timestamp
priority based
Controlled
(Token based)
Central
server
Ring
Structure
Tree
Structure

7. Mutual Exclusion Algorithms (Cont.)
1. Centralized algorithm
2. Decentralized algorithm
3. Token-based algorithm
4. Distributed algorithm
7

8. 1. Centralized Algorithm
 Central server that grants permission to enter
critical section
8
Source: http://lycog.com/distributed-systems/centralized-algorithm-mutual-exclusion-in-distributed-systems/

9. Central Server Algorithm
 Mutual exclusion requirements
 Safeness property – satisfied
 Liveness property – satisfied
 Ordering – not satisfied
 Due to network delay
 Other concerns
 2 messages per request + 1 message to release
 Resource can’t be accessed until at least for a round
trip time
 Delay between 1 process leaving critical section &
another entering
 Single point of failure 9

10. 2. Decentralized Algorithm
10

11. 2. Decentralized Algorithm (Cont.)
 Use many replicas of resources & many
coordinators
 To enter a critical section, a process sends a
request message to coordinators & awaits reply
 Need majority vote from coordinators
 If denied, back-off & retry later
 Can lead to low utilization, particularly under high
contention
 Avoids problem of single point of failure
11

12. 3. Token-Based Algorithm
 Pass a token around
 Whoever having token has exclusive access to
shared resource
 Examples, token ring
12
Source: http://lycog.com/distributed-systems/token-ring-mutual-exclusion-in-distributed-systems/

13. Ring-Based Algorithm
 Mutual exclusion requirements
 Safeness property – satisfied
 Liveness property – satisfied
 Ordering – not satisfied
 Follow ring order, not order requested by processes
 Other concerns
 Token passing
 Worst case waiting time O(N)
 N – no of processes in the ring
 Delay between 1 process leaving critical section &
another entering is O(N)
13

14. 4. Distributed Algorithm
 By Lamport in 1978, Improved by Ricart & Agrawala in 1981
14
Source: http://lycog.com/distributed-systems/distributed-algorithm-mutual-exclusion-distributed-systems/

15. Distributed Algorithm
 Mutual exclusion requirements
 Safeness property – satisfied
 Liveness property – satisfied
 Ordering – satisfied
 Events are in total order
 Other concerns
 High overhead
 Gaining entry requires 2(N-1) messages per request
 N – 1 to multicast request followed by N – 1 replies
 Resource can’t be accessed until at least for a round
trip time
 Delay between 1 process leaving critical section &
another entering is 1 message 15

16. Distributed Algorithm (Cont.)
 N points of failures
 Before granting access each process needs to reply
 If 1 process failed?
 Probability of failure p vs. Np
 N bottlenecks
 Each process need to respond to messages from all
other processes
16

17. Mutual Exclusion Algorithm
Comparison
 Comparison assumes that messages are passed
sequentially (sent 1 after another) over a network
17

18. Contention vs. Token Based
Contention
 Competition among
processes
 May need to retry
repeatedly
 No token
 Voting improves fault
tolerance
 What if no majority?
 More complicated to
implement
Token
 Avoid deadlocks
 Avoid starvation by
efficient organization of
processes
 Lost token?
 How to create a new one?
 How to create only 1
token?
18