synch_2 - Distributed Synchronization Clock Synchronization...

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Distributed Synchronization
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Clock Synchronization When each machine has its own clock, an event that occurred after another event may nevertheless be assigned an earlier time.
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Physical Clocks Clock Synchronization Maximum resolution desired for global time keeping determines the maximum difference which can be tolerated between “synchronized” clocks The time keeping of a clock, its tick rate should satisfy: The worst possible divergence δ between two clocks is thus: So the maximum time Δ t between clock δ ρ δ δ ρ + < < - 1 1 t C t = ρ δ 2 ρ δ 2 = t
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Physical Clocks Clock Synchronization Christian’s Algorithm Periodically poll the machine with access to the reference time source Estimate round-trip delay with a time stamp Estimate interrupt processing time figure 3-6, page 129 Tanenbaum Take a series of measurements to estimate the time it takes for a timestamp to make it from the reference machine to the synchronization target This allows the synchronization to converge within δ with a certain degree of confidence Probabilistic algorithm and guarantee
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Physical Clocks Clock Synchronization Wide availability of hardware and software to keep clocks synchronized within a few milliseconds across the Internet is a recent development Network Time Protocol (NTP) discussed in papers by David Mill(s) GPS receiver in the local network synchronizes other machines What if all have GPS receivers Increasing deployment of distributed system algorithms depending on synchronized clocks Supply and demand constantly in flux
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Physical Clocks (1) Computation of the mean solar day.
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Physical Clocks (2) TAI seconds are of constant length, unlike solar seconds. Leap seconds are introduced when necessary to keep in phase with the sun.
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Clock Synchronization Algorithms The relation between clock time and UTC when clocks tick at different rates.
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Cristian's Algorithm Getting the current time from a time server.
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The Berkeley Algorithm a) The time daemon asks all the other machines for their clock values b) The machines answer c) The time daemon tells everyone how to adjust their clock
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Lamport Timestamps a) Three processes, each with its own clock. The clocks run at different rates. b) Lamport's algorithm corrects the clocks.
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Example: Totally-Ordered Multicasting Updating a replicated database and leaving it in an inconsistent state.
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Global State (1) a) A consistent cut b) An inconsistent cut
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Global State (2) a) Organization of a process and channels for a distributed snapshot
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Global State (3) b) Process Q receives a marker for the first time and records its local state c) Q records all incoming message d) Q receives a marker for its incoming channel and finishes recording the state of the incoming channel
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The Bully Algorithm (1) The bully election algorithm Process 4 holds an election Process 5 and 6 respond, telling 4 to stop Now 5 and 6 each hold an election
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Mutual Exclusion Distributed components still need to coordinate their
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