1 Difference between wait-die and wound-wait deadlock prevention algorithms PDF

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Difference between “wait-die” and “woundwait” deadlock prevention algorithms Wait-Die scheme It is a non-preemptive technique for deadlock prevention. When transaction Tn requests a data item currently held by Tk, Tn is allowed to wait only if it has a timestamp smaller than that of Tk (That is Tn is older than Tk), otherwise Tn is killed ("die"). In this scheme, if a transaction requests to lock a resource (data item), which is already held with a conflicting lock by another transaction, then one of the two possibilities may occur:

1. Timestamp(Tn) < Timestamp(Tk) − that is Tn, which is requesting a conflicting lock, is older than Tk − then Tn is allowed to "wait" until the data-item is available. 2. Timestamp(Tn) > Timestamp(Tk) − that is Tn is younger than T k − then Tn is killed ("dies"). Tn is restarted later with a random delay but with the same timestamp(n). This scheme allows the older transaction to "wait" but kills the younger one ("die").

Example Suppose that transaction T5, T10, T15 have time-stamps 5, 10 and 15 respectively. If T5 requests a data item held by T10 then T5 will "wait". If T15 requests a data item held by T10, then T15 will be killed ("die").

Wound-Wait scheme It is a preemptive technique for deadlock prevention. It is a counterpart to the wait-die scheme. When Transaction Tn requests a data item currently held by Tk, Tn is allowed to wait only if it has a timestamp larger than that of Tk, otherwise Tk is killed (i.e. Tk is wounded by Tn). In this scheme, if a transaction requests to lock a resource (data item), which is already held with conflicting lock by some another transaction, one of the two possibilities may occur:

1. Timestamp(Tn) < Timestamp(Tk), then T n forces Tk to be killed − that is T n "wounds" Tk. Tk is restarted later with a random delay but with the same timestamp(k). 2. Timestamp(Tn) > Timestamp(Tk), then T n is forced to "wait" until the resource is available.

This scheme allows the younger transaction requesting a lock to "wait" if the older transaction already holds a lock, but forces the younger one to be suspended ("wound") if the older transaction requests a lock on an item already held by the younger one.

Example Again, suppose that Transactions T5, T10, T15 have time-stamps 5, 10 and 15 respectively. If T5 requests a data item held by T10, then data item will be preempted from T10 and T10 will be suspended. ("wounded") If T15 requests a data item held by T10, then T15 will "wait".

Summary In both the cases, only the transaction that enters the system at a later timestamp (i.e. the younger transaction) might be killed and restarted.

Summary Parth has given a detailed answer. Here I summarize it in a different way. Assume that Tn requests a lock held by Tk. The following table summarizes the actions taken for wait-die and wound-wait scheme: Tn is younger than Tk Tn is older than Tk

wait-die Tn dies Tn waits

wound-wait Tk waits Tk aborts

Both schemes prefer older transactions with an older timestamp.

https://stackoverflow.com/questions/32794142/what-is-the-difference-between-wait-die-and-wound-waitdeadlock-prevention-a...