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Clock Synchronization

Clock Synchronization. When each machine has its own clock, an event that occurred after another event may nevertheless be assigned an earlier time. Physical Clocks (1). Computation of the mean solar day. Physical Clocks (2).

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Clock Synchronization

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  1. Clock Synchronization When each machine has its own clock, an event that occurred after another event may nevertheless be assigned an earlier time.

  2. Physical Clocks (1) Computation of the mean solar day.

  3. 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.

  4. Clock Synchronization Algorithms The relation between clock time and UTC when clocks tick at different rates.

  5. Cristian's Algorithm Getting the current time from a time server.

  6. 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

  7. Global State (1) • A consistent cut • An inconsistent cut

  8. Global State (2) • Organization of a process and channels for a distributed snapshot

  9. Global State (3) • Process Q receives a marker for the first time and records its local state • Q records all incoming message • Q receives a marker for its incoming channel and finishes recording the state of the incoming channel

  10. 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

  11. Global State (3) • Process 6 tells 5 to stop • Process 6 wins and tells everyone

  12. A Ring Algorithm Election algorithm using a ring.

  13. Mutual Exclusion: A Centralized Algorithm • Process 1 asks the coordinator for permission to enter a critical region. Permission is granted • Process 2 then asks permission to enter the same critical region. The coordinator does not reply. • When process 1 exits the critical region, it tells the coordinator, when then replies to 2

  14. A Distributed Algorithm • Two processes want to enter the same critical region at the same moment. • Process 0 has the lowest timestamp, so it wins. • When process 0 is done, it sends an OK also, so 2 can now enter the critical region.

  15. A Toke Ring Algorithm • An unordered group of processes on a network. • A logical ring constructed in software.

  16. Comparison A comparison of three mutual exclusion algorithms.

  17. The Transaction Model (1) Updating a master tape is fault tolerant.

  18. The Transaction Model (2) Examples of primitives for transactions.

  19. The Transaction Model (3) • Transaction to reserve three flights commits • Transaction aborts when third flight is unavailable

  20. Distributed Transactions • A nested transaction • A distributed transaction

  21. Private Workspace • The file index and disk blocks for a three-block file • The situation after a transaction has modified block 0 and appended block 3 • After committing

  22. Writeahead Log a) A transaction b) – d) The log before each statement is executed

  23. Concurrency Control (1) General organization of managers for handling transactions.

  24. Concurrency Control (2) General organization of managers for handling distributed transactions.

  25. Serializability (d) a) – c) Three transactions T1, T2, and T3 d) Possible schedules

  26. Two-Phase Locking (1) Two-phase locking.

  27. Two-Phase Locking (2) Strict two-phase locking.

  28. Pessimistic Timestamp Ordering Concurrency control using timestamps.

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