Mutual%20Exclusion

1
Mutual Exclusion

• An Optimal Algorithm
• For
• Computer Networks

2
Assumptions Involved

• communication by message passing
• no shared memory
• error free network links
• node to node latency might vary
• nodes are assumed to function properly

3
How is the Algorithm Optimal ?

• a symmetrical, distributed algorithm cannot use
  fewer messages if requests are processed by each
  node concurrently
• 2(N-1) messages for mutual exclusion
• symmetrical ? same of msg. in/out of nodes
• distributed ? running on all nodes in network
• concurrently ? decision making is independent

4
Breaking Down the Node

• 3 processes
• wakes upon the need for mutual exclusion
• receives and processes REQUEST messages
• receives and processes REPLY messages
• constant parameters
• each node has a unique number ID
• number of nodes in network N

5
Breaking Down the Node II

• dynamic local parameters
• int ourSequenceNum
• int highestSequenceNum ? 0
• int outstandingReplyCount
• bool requestingCriticalSection
• int 1-N replyDeferred ? FALSE
• one semaphore
• protect local shared variables of 3 processes

6
Process One -- invoke M.E. --

• requestingCriticalSection TRUE
• ourSequenceNum highestSequenceNum 1
• outstandingReplyCount N 1
• send REQUEST to all other nodes
• wait( outstandingReplyCount 0 )
• ENTER THE CRITICAL SECTION

7
Process 1 -- after CS --

• LEAVE CRITICAL SECTION
• requestingCriticalSection FALSE
• send REPLY to all deferred nodes

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Process 2 -- parses REQUEST --

• update highestSequenceNum if required
• determine who has priority
• compare sequence number tie ? compare ID
• if we have priority
• make note of node ID to reply to later
• if requesting node has priority
• send a REPLY to node ID

9
Process 3 -- parse REPLY --

• decrease outstandingReplyCount

10
REPLY Decision Table
11
Further Details

• satisfies all three CS conditions
• mutual exclusion
• deadlock
• starvation
• unbounded sequence number count
• similar to Lamports Bakery Algorithm
• range never more than (N-1)
• can solve using modulo gt 2N

12
New Node Joining Network

• new node must have list of other nodes
• new node must be inserted into other lists
• contact a sponsor node already participatory
• sponsor node will tell others using mutual
  exclusion
• alternatively broadcast to all and await REPLYs

13
Node Leaving Network

• notify all other nodes of intention
• in itself considered a REPLY to requesting nodes
• wait for REPLY from all nodes
• do not invoke mutual exclusion during process

14
Node Restart and Failure

• restart
• restarting node first performs removal
• next the node performs insertion
• failure
• must use some type of timeout-timer

15
Modifications

• network supports broadcasting
• message traffic reduces to (N)
• implicit REPLY
• approaches (N-1) messages with low CS activity
• combination of the two above
• can be as low as (1) messages

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Readers / Writers Problem

• writers
• follow the original algorithm
• will be given priority over readers
• readers
• always send REPLY
• never defer a write REQUEST

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History and Additional Info

• if nodes act concurrently with NO access to
  timing-derived information, the algorithm in
  minimal time to obtain the mutual exclusion
• the only earlier proposed mutual exclusion
  algorithm for computer networks is from Lamport
  and takes approximately 3(n-1) messages