time - CS6253: Time and Clocks Introduction Clocks, events...

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Unformatted text preview: CS6253: Time and Clocks Introduction Clocks, events and process states Synchronizing physical clocks Logical time and logical clocks Global States These slides are based on material provided by the authors of the textbook: Coulouris, Dollimore and Kindberg 10.1 Introduction We need to measure time accurately: to know the time an event occurred at a computer to do this we need to synchronize its clock with an authoritative external clock Algorithms for clock synchronization useful for concurrency control based on timestamp ordering authenticity of requests e.g. in Kerberos There is no global clock in a distributed system this chapter discusses clock accuracy and synchronisation Logical time is an alternative It gives ordering of events - also useful for consistency of replicated data Computer clocks and timing events Each computer in a DS has its own internal clock used by local processes to obtain the value of the current time processes on different computers can timestamp their events but clocks on different computers may give different times computer clocks drift from perfect time and their drift rates differ from one another. clock drift rate : the relative amount that a computer clock differs from a perfect clock Even if clocks on all computers in a DS are set to the same time, their clocks will eventually vary quite significantly unless corrections are applied Clocks, events and process states A distributed system is defined as a collection P of N processes p i , i = 1,2, N Each process p i has a state s i consisting of its variables (which it transforms as it executes) Processes communicate only by messages (via a network) Actions of processes : Send, Receive, change own state Event: the occurrence of a single action that a process carries out as it executes e.g. Send, Receive, change state Events at a single process p i , can be placed in a total ordering denoted by the relation i between the events. i.e. e i e if and only if e occurs before e at p i A history of process p i: is a series of events ordered by i history (p i )= h i = <e i , e i 1 , e i 2 , > Clocks We have seen how to order events (happened before) To timestamp events, use the computers clock At real time, t , the OS reads the time on the computers hardware clock H i ( t ) It calculates the time on its software clock C i ( t )= H i ( t ) + e.g. a 64 bit number giving nanoseconds since some base e....
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This note was uploaded on 11/15/2011 for the course ECKOW 123 taught by Professor Riuyt during the Spring '11 term at University of Damascus.

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time - CS6253: Time and Clocks Introduction Clocks, events...

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