L3.sp11 - Distributed Systems Distributed Systems CS 425...

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Lecture 5-1 Lecture 5-1 Distributed Systems CS 425 / CSE 424 / ECE 428 Distributed Systems CS 425 / CSE 424 / ECE 428 Time and Synchronization Reading: Sections 11.1-11.4 010, I. Gupta, K. Nahrtstedt, S. Mitra, N. Vaidya, M. T. Harandi, J. Hou
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Lecture 5-2 Lecture 5-2 Why synchronization? Why synchronization? You want to catch the 10 Gold West bus at the Illini Union stop at 6.05 pm, but your watch is off by 15 minutes What if your watch is Late by 15 minutes? What if your watch is Fast by 15 minutes? Synchronization is required for Correctness Fairness
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Lecture 5-3 Lecture 5-3 Why synchronization? Why synchronization? Servers in the cloud need to timestamp events Server A and server B in the cloud have different clock values You buy an airline ticket online via the cloud It’s the last airline ticket available on that flight Server A timestamps your purchase at 9h:15m:32.45s What if someone else also bought the last ticket (via server B) at 9h:20m:22.76s? What if Server A was > 10 minutes ahead of server B? Behind? How would you know what the difference was at those times?
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Lecture 5-4 Lecture 5-4 An Asynchronous Distributed System (DS) consists of a number of processes. Each process has a state (values of variables). Each process takes actions to change its state, which may be an instruction or a communication action ( send , receive ). An event is the occurrence of an action. Each process has a local clock – events within a process can be assigned timestamps , and thus ordered linearly. But – in a DS, we also need to know the time order of events across different processes. Clocks across processes are not synchronized in an asynchronous DS (unlike in a multiprocessor/parallel system, where they are). So… Process clocks can be different Need algorithms for either (a) time synchronization, or (b) for telling which event happened before which Basics – Processes and Events Basics – Processes and Events
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Lecture 5-5 Lecture 5-5 In a DS, each process has its own clock. Clock Skew versus Drift Clock Skew = Relative Difference in clock values of two processes Clock Drift = Relative Difference in clock frequencies (rates) of two processes A non-zero clock drift will cause skew to continuously increase.
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This note was uploaded on 02/08/2012 for the course ECE 428 taught by Professor Hu during the Spring '08 term at University of Illinois, Urbana Champaign.

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L3.sp11 - Distributed Systems Distributed Systems CS 425...

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