Wound-Wait Scheme: A demonstration applet

CS238 Operating Systems

Ching-yi Au

Minnie Ko

Introduction:

A Deadlock occurs when there are a set of processes that are each holding some resources. Although each one are requesting some resources, none of them can obtain what it needs to progress.

There are four basic situations that if held simultaneously, can cause a deadlock to arise:

1. Mutual Exclusion
2. Hold and wait
3. No preemption
4. Circular Wait

The Wound – Wait scheme:

In order to prevent a deadlock, use of resources may be constrained. The Wound-Wiat scheme is based on a preemptive technique; counterpart to the waitdie system.

If P_i requests a resource currently held by P_j , P_i is allowed to wait only if it has a larger timestamp than does P_j (P_i is younger than P_j ). Otherwise, P_j is rolled back (P_j is wounded by P_i ).

Example: Suppose that processes P₁, P₂, and P₃ have timestamps 5, 10, and 15, respectively .

If P₁ requests a resource held by P₂ , then the resource will be preempted from P₂ and P₂ will be rolled back.

If P₃ requests a resource held by P₂, then P₃ will wait.

An introduction describing demonstration.

Diagram describing the event sequence of three transactions: T1, T2 and T3.