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Broadcast a mestamped request to all 2 upon receiving

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Unformatted text preview: (clock) with ini\al value of zero –  The counter is incremented by and assigned to each event, as its \mestamp. –  A send(message) event carries its \mestamp –  For a receive(message) event the counter is updated by Max(receiver.counter, message.\mestamp) + 1 Distributed Mutex: Coordina\on •  Five algorithms discussed •  None of the algorithms is perfect… Distributed Mutex: Coordina\on •  Safety - At most one process may execute in CS at any \me. –  No deadlock too. •  Liveness – Every request for a CS is eventually granted –  This is called progress. •  Fairness – Requests are granted in FIFO order and bounded wait Centralized Lock Server •  A central coordinator –  Grants permission to enter CS & –  keeps a queue of requests to enter the CS. •  Ensures only one thread at a \me can access the CS. Centralized Lock Server • To enter cri\cal sec\on: – send REQUEST to central server –wait for permission from server • To leave: – send RELEASE to central server • Server: – Has an internal queue of all REQUESTs it’s received but to which it hasn’t yet sent OK – Delays sending OK back to process un\l process is at head of queue Centralized Lock Server •  •  •  •  The lock server implementa\on in YFS Safety, liveness and order are guaranteed It takes 3 messages per entry/exit opera\on. Client delay: one round trip \me –  request + grant •  The coordinator becomes performance bo•leneck and single point of failure –  This is bad Token Ring Approach •  Processes are organized in a logical ring –  p(i) has a communica\on channel to p(i+1)mod N •  Opera\ons: –  Only the process holding the token can enter the CS. –  To exit the CS, the process sends the token onto its neighbor. If a process does not require to enter the CS when it receives the token, it forwards the token to the next neighbor. Token Ring Approach •  Safety & liveness are guaranteed, but ordering is not. •  Client delay –  0 to N message transmissions. A shared priority queue •  Each process i locally maintains Q(i) , part of a shared priority queue •  To run cri\cal sec\on, must have replies from all other processes AND be at the front of Q(i) •  When you...
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