24_Distributed_Systems - CSC 4103 - Operating Systems Fall...

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1 CSC 4103 - Operating Systems Fall 2009 Tevfik Ko ! ar Louisiana State University December 1 st , 2009 Lecture - XXIV Distributed Systems Distributed Coordination • Ordering events and achieving synchronization in centralized systems is easier. We can use common clock and memory • What about distributed systems? No common clock or memory happened-before relationship provides partial ordering How to provide total ordering? Event Ordering Happened-before relation (denoted by ! ) If A and B are events in the same process (assuming sequential processes), and A was executed before B , then A ! B If A is the event of sending a message by one process and B is the event of receiving that message by another process, then A ! B If A ! B and B ! C then A ! C If two events A and B are not related by the ! relation, then these events are executed concurrently . Relative Time for Three Concurrent Processes Which events are concurrent and which ones are ordered? Exercise Which of the following event orderings are true? (a) p0 --> p3 : (b) p1 --> q3 : (c) q0 --> p3 : (d) r0 --> p4 : (e) p0 --> r4 : Which of the following statements are true? (a) p2 and q2 are concurrent processes. (b) q1 and r1 are concurrent processes. (c) p0 and q3 are concurrent processes. (d) r0 and p0 are concurrent processes. (e) r0 and p4 are concurrent processes. 5 Implementation of ! Associate a timestamp with each system event Require that for every pair of events A and B, if A ! B, then the timestamp of A is less than the timestamp of B Within each process Pi, define a logical clock The logical clock can be implemented as a simple counter that is incremented between any two successive events executed within a process • Logical clock is monotonically increasing A process advances its logical clock when it receives a message whose timestamp is greater than the current value of its logical clock Assume A sends a message to B, LC 1 (A)=200, LC 2 (B)=195 If the timestamps of two events A and B are the same, then the events are concurrent We may use the process identity numbers to break ties and to create a total ordering
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Distributed Mutual Exclusion (DME) • Assumptions The system consists of
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This note was uploaded on 10/01/2011 for the course CSC 4103 taught by Professor Ullmer,b during the Fall '08 term at LSU.

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24_Distributed_Systems - CSC 4103 - Operating Systems Fall...

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