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Temple University – CIS Dept. CIS616– Principles of Data Management

Temple University – CIS Dept. CIS616– Principles of Data Management . V. Megalooikonomou Concurrency Control – Deadlocks (based on slides by C. Faloutsos at CMU and on notes by Silberchatz,Korth, and Sudarshan). Isolation - concurrency control. serializability <-> correctness

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Temple University – CIS Dept. CIS616– Principles of Data Management

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  1. Temple University – CIS Dept.CIS616– Principles of Data Management V. Megalooikonomou Concurrency Control – Deadlocks (based on slides by C. Faloutsos at CMU and on notes by Silberchatz,Korth, and Sudarshan)

  2. Isolation - concurrency control • serializability <-> correctness • precedence graph • automatically correct interleavings: • locks + protocol (2PL, 2PLC) • but: deadlocks!

  3. Deadlocks • detection • handling • (prevention)

  4. Deadlock detection T1 T2 lock manager L(A) Yes L(B) Yes ... L(B) No <waits> L(A) No time DEADLOCK

  5. Algorithm for deadlock detection? • wait-for graph: • nodes  transactions • arcs  Tsource waits for Tsink • if cycle, then deadlock! • Must invoke a deadlock-detection algorithm periodically to look for cycles

  6. Example T1 T2 lock manager L(A) Yes L(B) Yes ... L(B) No <waits> L(A) No time for ‘B’ T1 T2 ‘A’ ‘B’ for ‘A’

  7. T1 T2 T4 T3 Another example • is there a deadlock? • if yes, which xacts are involved?

  8. T1 T2 T4 T3 Another example • now, is there a deadlock? • if yes, which xacts are involved?

  9. Deadlock detection • how often should we run the algorithm? • how many transactions are typically involved?

  10. T1 T2 T4 T3 Deadlock handling • Q: what to do?

  11. T1 T2 T4 T3 Deadlock handling • Q0: what to do? • A: select a ‘victim’ & ‘rollback’ • Q1: which/how to choose?

  12. T1 T2 T4 T3 Deadlock handling • Q1: which/how to choose? • A1.1: by age • A1.2: by progress • A1.3: by # items locked already... • A1.4: by # xacts to rollback • Q2: How far to rollback?

  13. T1 T2 T4 T3 Deadlock handling • Q2: How far to rollback? • A2.1: completely • A2.2: minimally • Q3: Starvation??

  14. T1 T2 T4 T3 Deadlock handling • Q3: Starvation?? • A3.1: include #rollbacks in victim selection criterion

  15. SQL statement • usually, conc. control is transparent to the user, but • LOCK <table-name> [EXCLUSIVE|SHARED]

  16. Concurrency control - conclusions • serializability <-> correctness • automatically correct interleavings: • locks + protocol (2PL, 2PLC, ...) • deadlock detection + handling

  17. Conclusions 2PL schedules serializable schedules serializable schedules 2PLC serial sch’s

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