Chapter 18: Problem 23

Prove that the wait-die and wound-wait protocols avoid deadlock and starvation.

Short Answer

Expert verified

Wait-die and wound-wait are deadlock prevention methods that ensure older transactions are prioritized and younger transactions either wait or are rolled back to prevent cyclic waits. In case of continuous rollbacks, a transaction cannot be starved of resources because it will eventually become the oldest transaction in the system, and as such, is guaranteed to gain the necessary resources.

Step by step solution

Understanding the Wait-Die Protocol

In the wait-die method, if a transaction \(T_i\) requests a resource currently held by \(T_j\), the timestamp of \(T_i\) is checked. If the timestamp of \(T_i\) is older than or equal to \(T_j\) (i.e., \(T_i\) has been waiting for a longer time), \(T_i\) is allowed to wait for \(T_j\) to release the resource. This is because older transactions have higher priority. If \(T_i\) is younger than \(T_j\), \(T_i\) is rolled back or killed. So no younger transaction can wait for an older one, preventing any possibility of cyclic waits and thus, deadlocks.

Understanding Wound-Wait Protocol

The wound-wait strategy is a little different. Here, when a transaction \(T_i\) requests a resource held by \(T_j\), their timestamps are compared. If \(T_i\) is older than \(T_j\), \(T_j\) is rolled back or wounded to permit \(T_i\) to proceed; if \(T_i\) is younger, it's made to wait until the resource is available. This scheme prevents a younger transaction from waiting for an older one, thus preventing deadlocks.

Preventing Starvation

In both the wait-die and wound-wait strategies, an older transaction always has priority over the younger ones. In the case of continuous rollbacks, the rolled-back transactions are restarted with the same timestamp. Therefore, a transaction cannot be starved for resources because after a number of rollbacks, the transaction will become the oldest transaction in the system and will get all the resources it needs. This shows that the protocols prevent both deadlock and starvation.

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Prove that the wait-die and wound-wait protocols avoid deadlock and starvation.

Short Answer

Step by step solution

Understanding the Wait-Die Protocol

Understanding Wound-Wait Protocol

Preventing Starvation

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