5- time_and_synchronization

5- time_and_synchronization - CSE 486/586 Distributed...

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CSE 486/586, Spring 2012 CSE 486/586 Distributed Systems Time and Synchronization Steve Ko Computer Sciences and Engineering University at Buffalo
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CSE 486/586, Spring 2012 Last Time Failure detectors Properties – , together unachievable in asynchronous systems but achievable in synchronous systems Most apps require 100% completeness, but can tolerate inaccuracy 2 failure detector algorithms Heartbeating and Ping Distributed FD through heartbeating Centralized, ring, all-to-all Metrics : bandwidth, detection time, scale, accuracy Other types of failures 2
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CSE 486/586, Spring 2012 The Next Few Lectures Several aspects of it 3 I want to shake my tail. No, I don’t want to. OK No way!
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CSE 486/586, Spring 2012 Today’s Question 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? Expect to see lots of equations! 4
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CSE 486/586, Spring 2012 Some definitions: 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. Real-life examples Ever had “make: warning: Clock skew detected. Your build may be incomplete.”? It’s reported that in the worst case, there’s 1 sec/day drift in modern HW. Almost all physical clocks experience this. 5
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CSE 486/586, Spring 2012 Synchronizing Physical Clocks Ci(t): the reading of the software clock at process i when the real time is t . External synchronization
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5- time_and_synchronization - CSE 486/586 Distributed...

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