Deadlocks pptx

Wait-die  Transactions given a timestamp when they arrive …. ts(Ti)  Ti can only wait for Tj if ts(Ti)< ts(Tj) else die T1 (ts =10) T2 (ts =20) T3 (ts =25) Wait for A Wait for C? Wait fo B T1 (ts =22) T2 (ts =20) T3 (ts =25) wait(A) requests A: wait for T 2 or T 3 or both? (in my html notes, I assume both) Note: ts between 20 and 25. Wait-die-1 T1 (ts =22) T2 (ts =20) T3 (ts =25) wait(A) wait(A) One option: T 1 waits just for T 3 , transaction holding lock. wait(A) But when T 2 gets lock, T 1 will have to die! (also lots of WFG revision) Wait-die-1 T1 (ts =22) T2 (ts =20) T3 (ts =25) wait(A) wait(A) wait(A) Another option: T 1 waits for both T 2 , T 3 E.g., (saves having to revise WFG) T 1 allowed to wait iff there is at least one younger trans wait-involved with A. But again, when T2 gets lock, T 1 must die! Wait-die-2 T1 (ts =22) T2 (ts =20) T3 (ts =25) wait(A) Yet another option: T 1 preempts T 2 (T 2 is just waiting idly anyway), so T 1 only waits for T 3 ; T 2 then waits for T 3 But, T 2 may starve? And lots of WFG work for Deadlock Mgr (shifting edges) wait-A Wait-die-3 Wound-wait  Transactions given a timestamp when they arrive … ts(Ti)  Ti wounds Tj if ts(Ti)< ts(Tj) else Ti waits “Wound”: Tj rolls back (if it cannot finish in small interval of time) and gives lock to Ti T1 (ts =25) T2 (ts =20) T3 (ts =10) Wait A Wait B Wait C Wound-wait T1 (ts =15) T2 (ts =20) T3 (ts =10) wait(A) requests A: wait for T 2 or T 3 ? Note: ts between 10 and 20. Wound-wait-2 T1 (ts =15) T2 (ts =20) T3 (ts =10) wait(A) wait(A) One option: T 1 waits just for T 3 , transaction holding lock. But when T 2 gets lock, T 1 waits for T 2 and wounds T 2 . Wound-wait-2 Wait A T1 (ts =15) T2 (ts =20) T3 (ts =10) wait(A) wait(A) wait(A) Another option: T 1 waits for both T 2 , T 3 ⇒ T 2 wounded right away! Wound-wait-3