lect14-lamport-eraser

lect14-lamport-eraser - Notes Midterm exam on November 17...

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CMSC 714 Lecture 14 Lamport Clocks and Eraser Alan Sussman (with thanks to Chris Ackermann) 2 Notes ! Midterm exam on November 17 – sample exam questions posted a couple weeks before ! Research project proposals due tomorrow ! Guest lecturer on Thursday for Performance Metrics CMSC 714 - Alan Sussman 3 Lamport Clocks ! Distributed systems are inherently concurrent, asynchronous, and nondeterministic, so executing programs on multiple machines requires coordination ! Lamport introduce methods to define an ordering of events ! Want to create a partial ordering of events (instructions, message passing, or whatever) ! Define a happens before relation: a ! b – event a happened before event b – event a can causally affect event b CMSC 714 - Alan Sussman 4 Happens Before Relation 1. If a and b are events in the same process, and a comes before b, then a ! b 2. If a is sending of a message by one process and b is the receipt of the same message by another process, then a ! b 3. If a ! b and b ! c then a ! c (transitivity) ! Partial Order: Unordered events are concurrent CMSC 714 - Alan Sussman
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5 Logical Clocks ! Clock Condition: For any events a, b : if a ! b then C ! a " < C ! b " ! Holds if C1 and C2 are satisfied: C1. If a and b are events in Process P i , and a comes before b, then C i ! a " < C i ! b " C2. If a is the sending of a message by process P i and b is the receipt of that message by process P j , then C i ! a " < C i ! b " ! Implementation IR1. Each process P i increments C i between any two successive events IR2a. If event a is the sending of a message m by Process Pi, then the message m contains a timestamp T m = C j ! a " . IR2b. Upon receiving a message m, process P
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This note was uploaded on 01/12/2012 for the course CMSC 714 taught by Professor Staff during the Fall '07 term at Maryland.

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lect14-lamport-eraser - Notes Midterm exam on November 17...

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