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Explain Distributed Approach for providing Mutual Exclusion
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Mutual Exclusion in Distributed System:

  • Mutual Exclusion ensures that no other process will use shared resources at same time.

    1) Centralized Algorithm

    2) Distributed Algorithm

    3) Token Ring Algorithm.

  • One process is elected as coordinator.

  • Whenever process wants to enter a critical region , it sends request msg to coordinator asking for permission.

  • If no other process is currently in that critical region, the coordinator sends back a reply granting permission.

  • When reply arrives, the requesting process enters the critical region.

  • If the coordinator knows that a different process is already in critical regions, so it cannot be granted permission.

Centralized Algorithm:

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Advantages:

  • Guarantees mutual exclusion.

  • Fair Approach (Request Granted In FCFS).

  • No Starvation.

  • Easy to Implement.

  • Only 3 Msgs per use of Critical Section (request, grant, release).

Drawbacks:

  • Single point of failure.

  • Dead co-ordinate & permission denied cannot distinguish.

  • In large systems, single coordinators can create performance bottleneck.

Distributed Algorithm:

  • Timestamps are used for distributed mutual exclusion.

  • Kieart & Agarwala’s Algorithm:

  • When process wants to enter critical region, it builds message containing name of critical

region its process number and current time

  • It sends msg to all including itself.

  • If receiver if not in critical region and doesn’t want to enter it sends back Ok msg to sender.

  • If the receiver is already in critical region, it doesn’t reply, instead it queues request.

  • If the receiver wants to enter critical region but has not yet done, so it compares the timestamp in the incoming msg the lowest one wins.

  • If its own msg has lower timestamp, the receiver queues the incoming request and sends nothing.

Drawbacks:

  • If any process fails (Crashes), then it does not respond to request.

  • It can be misinterpreted as denial of permission thus may cause blocking of all processes.

RPCART & AGARWALA’S SOLUTION :

  • Same algorithm with reply always given by receiver either by granting or denying permission.

  • Other problem is either group communication must be used, or each process must maintain group information (who enters, who left, etc)

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  • Other problems is bottleneck

  • Solution can be given with permission from simple majority of processes rather than from all with conditions that a process granted permission to one that a process granted permission to one process cant grant other permission until first has released.

  • Other problems

      i) Slower
    
      ii) Complicated
    
      iii) More expensive.
    

TOKEN RING ALGORITHM

  • Mutual Exclusion – Token Ring Algorithm

    • In software, a logical ring is constructed in which each process is assigned a position in the ring.

    • The ring positions may be allocated in numerical order of network address or some other means.

    • It does not matter what the ordering is all that matters is that each process knows who is next in line after itself.

Working of Token Ring Algorithm :

  • When the ring is initialized, process 0 is given a token.

  • The token circulates around the ring.

  • It passes from process K to process Kt(module the ring size) in point to point messages.

  • When a process acquires the token from its neighbor, it checks to see if it is attempting to enter a critical region.

  • If so, the process enters the region, does all the work it needs to, and leaves the region.

  • After it h excited, it passes the token along the ring.

  • It is not permitted to enter the second critical region with using the same token.

  • If a process is handed the token by its neighbor & is not interested in entering a critical region, it just passes it along.

  • As a consequence, when no processes want to enter any critical regions the token just circulates at high speed around the ring.

  • Only one process has the token at any instant, so only one process can actually be in the critical region.

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                 Fig. An Unordered group of process in network

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     Figure : A Logical ring Constructed in S/W
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