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  What to do when Deadlock is detected

What to do when Deadlock is detected:

Each transaction in a deadlock is waiting. The merely way to get things going again is to grant some waiter. However this can only be achieved after a lock is pre-empted from some holder. Because the victim is waiting he will get the “deadlock” response from lock manager rather than the “granted” response.

In breaking the deadlock some set of victims will be pre-empted. We want to minimize the amount of work lost by these pre-emptions. Thus deadlock resolution wants to pick a minimum cost set of victims to break deadlocks.

Transaction management should associate a cost with each transaction. In the lack of policy decisions: the cost of a sufferer is the cost of undoing his work and then redoing it. The length of the transaction log is a basic estimate of this cost. In any case transaction management should provide lock management with an estimate of the cost of each transaction. Lock manager may perhaps implement either of the following two protocols:

•    For every cycle choose the minimum cost victim in that cycle.
•    Choose the least cost cut-set of the deadlock graph.

The difference among these two options is best visualized by the picture:

848_detection of deadlock.jpg

If T1 and T3 have a cost of 2 and T2 has a cost of 3 then a cycle-at-a-time algorithm will choose T1 and T3 as victims whereas a minimal cut set algorithm will choose T2 as a victim.

The cost of resulting a minimal cut set is considerably greater (seems to be NP complete) than the cycle-ata-time scheme. If there are N familiar cycles the cycle-at-a-tine scheme is at most N times worse than the minimal cut set scheme. Consequently it seems that the cycle-at-a-time scheme is better.

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