Deadlock Avoidance (Operating Systems) PDF

Module 7: Deadlocks System Model Deadlock Characterization Methods for Handling Deadlocks Deadlock Prevention Deadlock Avoidance  The Deadlock Problem A set of blocked processes each holding a resource and waiting to acquire a resource held by another pr...

Module 7: Deadlocks System Model Deadlock Characterization Methods for Handling Deadlocks Deadlock Prevention Deadlock Avoidance  The Deadlock Problem A set of blocked processes each holding a resource and waiting to acquire a resource held by another process in the set. Example – System has 2 tape drives. – P1 and P2 each hold one tape drive and each needs another one. Example – semaphores A and B, initialized to 1 P0 P1 wait (A); wait(B) wait (B); wait(A)  Bridge Crossing Example Traffic only in one direction. Each section of a bridge can be viewed as a resource. If a deadlock occurs, it can be resolved if one car backs up (preempt resources and rollback). Several cars may have to be backed upif a deadlock occurs. Starvation is possible.  System Model Resource types R1, R2,..., Rm CPU cycles, memory space, I/O devices Each resource type Ri has Wi instances. Each process utilizes a resource as follows: – request – use – release  Deadlock Characterization Deadlock can arise if four conditions hold simultaneously. Mutual exclusion: only one process at a time can use a resource. Hold and wait: a process holding at least one resource is waiting to acquire additional resources held by other processes. No preemption: a resource can be released only voluntarily by the process holding it, after that process has completed its task. Circular wait: there exists a set {P0, P1, …, P0} of waiting processes such that P0 is waiting for a resource that is held by P1, P1 is waiting for a resource that is held by P2, …, Pn–1 is waiting for a resource that is held by Pn, and P0 is waiting for a resource that is held by P0.  Resource-Allocation Graph A set of vertices V and a set of edges E. V is partitioned into two types: – P = {P1, P2, …, Pn}, the set consisting of all the processes in the system. – R = {R1, R2, …, Rm}, the set consisting of all resource types in the system. request edge – directed edge P1 → Rj assignment edge – directed edge Rj → Pi  Resource-Allocation Graph (Cont.) Process Resource Type with 4 instances Pi requests instance of Rj Pi Rj Pi is holding an instance of Rj Pi Rj Example of a Resource Allocation Graph Resource Allocation Graph With A Deadlock Resource Allocation Graph With A Cycle But No Deadlock  Basic Facts If graph contains no cycles  no deadlock. If graph contains a cycle  – if only one instance per resource type, then deadlock. – if several instances per resource type, possibility of deadlock.  Methods for Handling Deadlocks Ensure that the system will never enter a deadlock state. Allow the system to enter a deadlock state and then recover. Ignore the problem and pretend that deadlocks never occur in the system; used by most operating systems, including UNIX.  Deadlock Prevention Restrain the ways request can be made. Mutual Exclusion – not required for sharable resources; must hold for nonsharable resources. Hold and Wait – must guarantee that whenever a process requests a resource, it does not hold any other resources. – Require process to request and be allocated all its resources before it begins execution, or allow process to request resources only when the process has none.  Deadlock Prevention (Cont.) No Preemption – – If a process that is holding some resources requests another resource that cannot be immediately allocated to it, then all resources currently being held are released. – Preempted resources are added to the list of resources for which the process is waiting. – Process will be restarted only when it can regain its old resources, as well as the new ones that it is requesting. Circular Wait – impose a total ordering of all resource types, and require that each process requests resources in an increasing order of enumeration.  Deadlock Avoidance Requires that the system has some additional a priori information available. Simplest and most useful model requires that each process declare the maximum number of resources of each type that it may need. The deadlock-avoidance algorithm dynamically examines the resource-allocation state to ensure that there can never be a circular-wait condition. Resource-allocation state is defined by the number of available and allocated resources, and the maximum demands of the processes.  Resource-Allocation Graph Algorithm Claim edge Pi → Rj indicated that process Pj may request resource Rj; represented by a dashed line. Claim edge converts to request edge when a process requests a resource. When a resource is released by a process, assignment edge reconverts to a claim edge. Resources must be claimed a priori in the system. Resource-Allocation Graph For Deadlock Avoidance Unsafe State In A Resource-Allocation Graph  Banker’s Algorithm Multiple instances. Each process must a priori claim maximum use. When a process requests a resource it may have to wait. When a process gets all its resources it must return them in a finite amount of time.  Data Structures for the Banker’s Algorithm Let n = number of processes, and m = number of resources types. Available: Vector of length m. If available [j] = k, there are k instances of resource type Rj available. Max: n x m matrix. If Max [i,j] = k, then process Pi may request at most k instances of resource type Rj. Allocation: n x m matrix. If Allocation[i,j] = k then Pi is currently allocated k instances of Rj. Need: n x m matrix. If Need[i,j] = k, then Pi may need k more instances of Rj to complete its task. Need [i,j] = Max[i,j] – Allocation [i,j].  Example of Banker’s Algorithm 5 processes P0 through P4; 3 resource types A (10 instances), B (5instances, and C (7 instances). What is the safe state after p1 request (1 1 2 ) is improved Snapshot at time T0: Allocation Max ABC ABC P0 010 753 P1 3 1 2 322 P2 302 902 P3 211 222 P4 002 433  Example (Cont.) The content of the matrix. Need is defined to be Max – Allocation. Need ABC P0 743 P1 122 P2 600 P3 011 P4 431 The system is in a safe state since the sequence < P1, P3, P4, P2, P0> satisfies safety criteria.