Distributed Mutual Exclusion

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Distributed Mutual Exclusion

• Synchronization in Distributed Systems
• Synchronization in distributed systems are often
  more difficult compared to synchronization
  in uniprocessor and multiprocessor systems.

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Synchronization in Distributed Systems

• Synchronization in time achieved by Clock
  Synchronization algorithms
• Synchronization between resources - Mutual
  Exclusion algorithms
• Synchronization in activities - Distributed
  Transaction paradigms

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Distributed Mutual Exclusion

• Mutual Exclusion
• Mutual exclusion (often abbreviated to mutex)
  algorithms are
• used in concurrent programming to avoid the
  simultaneous use
• of a common resource, such as a global variable,
  by pieces
• of computer code called critical sections.

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Distributed Mutual Exclusion

• Critical Sections
• In concurrent programming a critical section
  is a piece of code that accesses a shared
  resource (data structure or device) that must not
  be concurrently accessed by more than one thread
  of execution. A critical section will usually
  terminate in fixed time, and a thread, task or
  process will only have to wait a fixed time to
  enter it. Some synchronization mechanism is
  required at the entry and exit of the critical
  section to ensure exclusive use, for example a
  semaphore.

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Distributed Mutual Exclusion

• Achieving Mutual exclusion
• Hardware solutions - Disabling interrupts on
  entry into the critical section
• System variables - By using semaphores
  implemented as Locks
• Software solutions - Mutual Exclusion Algorithms

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Distributed Mutual Exclusion

• Mutual Exclusion Algorithms
• Centralized algorithms
• Distributed algorithms
• Token ring algorithm

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Mutual Exclusion centralized Algorithm

1. Process 1 asks the coordinator for permission to
   enter a critical region. Permission is granted
2. Process 2 then asks permission to enter the same
   critical region. The coordinator does not reply.
3. When process 1 exits the critical region, it
   tells the coordinator, when then replies to 2

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Centralized Algorithm Advantages Vs
Disadvantages

• Advantages
• Fair algorithm, grants in the order of requests
• The scheme is easy to implement
• Scheme can be used for general resource
  allocation
• Shortcomings
• Single point of failure. No fault tolerance
• Confusion between No-reply and permission denied
• Performance bottleneck of single co-ordinator in
  a large system

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Mutual Exclusion Distributed Algorithm

1. Two processes want to enter the same critical
   region at the same moment.
2. Process 0 has the lowest timestamp, so it wins.
3. When process 0 is done, it sends an OK also, so 2
   can now enter the critical region.

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Distributed Algorithm Advantages Vs
Disadvantages

• Advantages
• Single point of failure is overcome by keeping
  the system running even if one processes fail
• No bottleneck on one process
• Shortcomings
• 2(n-1) messages or more than that
• Waiting for n-1 processes to accept is overkill
• Improvements
• Group membership for easy communication

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A Token Ring Algorithm

1. An unordered group of processes on a network.
2. A logical ring constructed in software.

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Token Ring Algorithm Advantages Vs
Disadvantages

• Advantages
• No co-ordinator and does not depend on other
  processes
• On Crash token is just passed on to the neighbour
• Shortcomings
• Only a physical Q, no logical Q
• Loss of token Regeneration of tokens
• Difficulty in finding the token crash The
  processor might be alive

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Comparison

• A comparison of three mutual exclusion algorithms.

Algorithm Messages per entry/exit Delay before entry (in message times) Problems
Centralized 3 2 Coordinator crash
Distributed 2 ( n 1 ) 2 ( n 1 ) Overkill in waiting
Token ring 1 to ? 0 to n 1 Lost token on process crash
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References

• Distributed systems Principles and Paradigms
• Andrew S Tanenbaum
• A Survey of Mutual-Exclusion Algorithms for
  Multiprocessor Operating Systems
• http//www.teamten.com/lawrence/242.paper/242.pap
  er.html