Chapter Five: Introduction to Syncho.pptduction to Syncho.ppt
- 1. Distributed Systems Chapter 5 Synchronization
- 2. Synchnonization in Distributed Systems
- 3. Time in DS • Time is an interesting and Important issue in DS – Ex. At what time of day a particular event occurred at a particular computer … Consistency (use of timestamp for serialization), e- commerce, authentication etc. • Algorithms that depend upon clock synchronization have been developed for several problems. • Due to loose synchrony, the notion of physical time is problematic in DS – There is no absolute physical “global time” in DS
- 4. Clocks – How time is really measured? » Earlier: Solar day, solar second, mean solar second • Solar day: time between two consequtive transits of the sun • Solar second: 1/86400 of a solar day • Mean solar day: average length of a solar day • Problem: solar day gets longer because of slowdown of earth rotation due to friction (300 million years ago there were 400 days per year)
- 5. Physical Clocks • International Atomic Time (TAI): number of ticks of Cesium 133 atom since 1/1/58 (atomic second) • Atom clock: one second defined as (since 1967) 9,192,631,770 transitions of the atom Cesium 133
- 6. Physical Clocks • Because of slowdown of earth, leap seconds have to be introduced • Correction of TAI is called Universal Coordinated Time (UTC): 30 leap seconds introduced so far • Network Time Protocol (NTP) can synchronize globally with an accuracy of up to 50 msec
- 7. Physical Clocks • TAI seconds are of constant length, unlike solar seconds. Leap seconds are introduced when necessary to keep in phase with the sun.
- 8. Clocks – Computers contain physical clock (crystal oscillator) » Physical time t, hardware time Hi(t), software time Ci(t) » The clock output can be read by SW and scaled into a suitable time unit and the value can be used to timestamp any event Ci(t) = Hi(t) + – Clock skew » The instantaneous difference between the readings of any two clocks – Clock drift: Crystal-based clocks count time at different rates, and so diverge.
- 9. Why synchronization? • You want to catch the 5 pm bus at the Harar stop, 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
- 10. Why synchronization? • Airline reservation system • Server A receives a client request to purchase last ticket on flight ABC 123. • Server A timestamps purchase using local clock 9h:15m:32.45s, and logs it. Replies ok to client. • That was the last seat. Server A sends message to Server B saying “flight full.” • B enters “Flight ABC 123 full” + local clock value (which reads 9h:10m:10.11s) into its log. • Server C queries A’s and B’s logs. Is confused that a client purchased a ticket after the flight became full. – May execute incorrect or unfair actions.
- 11. Clock synchronization • UTC signals are synchronized and broadcast regularly from land-based radio stations and satellites covering many parts of the world – E.g. in the US the radio station WWV broadcasts time signals on several short-wave frequencies – Satellite sources include Geo-stationary Operational Environmental Satellites (GOES) and the GPS
- 12. Clock synchronization – Radio waves travel at near the speed of light. The propagation delay can be accounted for if the exact speed and the distance from the source are known – Unfortunately, the propagation speed varies with atmospheric conditions – leading to inaccuracy – Accuracy of a received signal is a function of both the accuracy of the source and its distance from the source through the atmosphere
- 13. Clock Synchronization Algorithms • The relation between clock time and UTC when clocks tick at different rates. Problem: show that, in order to guarantee that no two clocks differ by more than , clocks must be resynchronized at least every /2 seconds.
- 14. Clock Synchronization Algorithms • The constant is specified by the manufacturer and is known as the maximum drift rate. • If two clocks are drifting from the Universal Coordinated Time (UTC) in opposite direction, at a time t after they are synchronized, they maybe as much as 2* *t apart. • If the operating system designer want to guarantee that no two clocks ever differ by more than , clocks must be synchronized at least every /2 seconds.
- 15. Clock synchronization –Remember the definition of synchronous distributed system? » Known bounds for message delay, clock drift rate and execution time. • Clock synchronization is easy in this case » In practice most DS are asynchronous. • Cristian’s Algorithm • The Berkeley Algorithm
- 16. Clock synchronization in a synchronous system • Consider internal synch between two procs in a synch DS • P sends time t on its local clock to Q in a msg m • In principle, Q could set its clock to the time t + Ttrans, where Ttrans is the time taken to transmit m between them • The two processes would then agree (internal synch)
- 17. Clock synchronization in a synchronous system • Unfortunately, Ttrans is subject to variation and is unknown – All processes are competing for resources with P and Q and other messages are competing with m for the network – But there is always a minimum transmission time min that would be obtained if no other processes executed and no other network traffic existed – min can be measured or conservatively estimated
- 18. Clock synchronization in a synchronous system • In synch system, by definition, there is also an upper bound max on the time taken to transmit any message • Let the uncertainty in the msg transmission time be u, so that u = (max – min) – If Q sets its clock to be (t + min), then clock skew may be as much as u (since the message may in fact have taken time max to arrive). – If Q sets it to (t + max), the skew may again be as large as u. – If, however, Q sets it clock to (t + (max + min)/2), then the skew is at most u/2. – In general, for a synch system, the optimum bound that can be achieved on clock skew when synchronizing N clocks is u(1-1/N) • For an asynch system Ttrans = min + x, where x >=0
- 19. Cristian’s Algorithm (Accuracy) • Assumption – Request & reply via same network – The value of minimum transmission time min is known or conservatively estimated.
- 20. The Berkeley Algorithm (internal synchronization) • A coordinator (time server): master – Periodically the master polls the time of each client (slave) whose clocks are to be synchronized. – Based on the answer (by observing the RTT as in Cristian’s algorithm), it computes the average (including its own clock value) and broadcasts the new time. • This method is suitable for a system in which no machine has a WWV receiver getting the UTC. • The time daemon’s time must be set manually by the operator periodically.
- 21. The Berkeley Algorithm • The balance of probabilities is that the average cancels out the individual clock’s tendencies to run fast or slow • The accuracy depends upon a nominal maximum RTT between the master and the slaves • The master eliminates any occasional readings associated with larger times than this maximum
- 22. The Berkeley Algorithm – Instead of sending the updated current time back to the comps – which will introduce further uncertainty due to message transmission time – the master send the amount by which each individual slave’s clock requires adjustment (+ or - ) – The algorithm eliminates readings from faulty clocks (since these could have significant adverse effects if an ordinary average was taken) – a subset of clock is chosen that do not differ by more than a specified amount and then the average is taken.
- 23. The Berkeley Algorithm • The time daemon asks all the other machines for their clock values • The machines answer • The time daemon tells everyone how to adjust their clock
- 24. Averaging Algorithms • Both Cristian’s and Berkeley’s methods are highly centralized, with the usual disadvantages - single point of failure, congestion around the server, … etc. • One class of decentralized clock synchronization algorithms works by dividing time into fixed-length re- synchronization intervals. • The ith interval starts at T0 + iR and runs until T0 + (i+1)R, where T0 is an agreed upon moment in the past, and R is a system parameter.
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