Operating Systems. Designed and Presented by Dr. Ayman Elshenawy Elsefy

Transcription

1 Operating Systems Designed and Presented by Dr. Ayman Elshenawy Elsefy Dept. of Systems & Computer Eng.. AL-AZHAR University Website : eaymanelshenawy.wordpress.com eaymanelshenawy@yahoo.com Reference Operating System Concepts, ABRAHAM SILBERSCHATZ

2 Chapter 7: Deadlocks The Deadlock Problem System Model Deadlock Characterization Methods for Handling Deadlocks Deadlock Prevention Deadlock Avoidance Deadlock Detection Recovery from Deadlock

3 The Deadlock Problem Deadlock: A process requests resources; If the resources are not available at that time, the process enters a waiting state. Sometimes, a waiting process can not change its state, because the resources it has requested are held by other waiting processes. Example of deadlocks: 1. A system with CD1 and CD2 drives allocated to P1 and P2. P1 requested CD2 and P2 requested CD1. Each is waiting for the event CD is released, which can be caused only by one of the other waiting processes(same resource type deadlock). 2. A system with one printer and one DVD. Suppose that process Pi is holding the DVD and process Pj is holding the printer. If Pi requests the printer and Pj requests the DVD drive, a deadlock occurs.

4 System Model A system have a finite number of resources R1,R2,R3, Rm. These resource was divided into several resource types/classes (CPU cycles, files, and I/O devices) consisting of some number of identical instances (CPU1 & CPU2). These resources can requested by a number of processes P 1,P 2,., P n. If a process requests an instance of a resource type, any instance of the type should satisfy the request. Else they are not identical. A process may utilize a resource in only the following sequence: Request: The process requests the resource. If it cannot be granted immediately, the process must wait until it can acquire the resource. Use: The process can operate on the resource. Release: The process releases the resource. The request and release of resources may be system calls request(),release() device - open(),close() file, allocate() free() memory system calls.

5 System Model Resource allocation table: Is a system table that records whether each resource is free or allocated by which process. If a process requests a resource type that is currently allocated to another process, it can be added to a queue of processes waiting for this resource. Each process of a set of processes is in a deadlocked state is waiting for an event that can be caused only by another process in the set. This event is resource acquisition and release.

6 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 up if a deadlock occurs Starvation is possible Note Most OS do not prevent or deal with deadlocks

7 Deadlock Characterization Deadlock can arise if four conditions hold simultaneously. Mutual exclusion: Only one process at a time can use a resource, If another process requests that resource, the requesting process must be delayed until the resource has been released. 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 optionally by the process holding it completing its task. Circular wait: There exists a set {P0, P1,, Pn} 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 Pn is waiting for a resource that is held by P0.

8 Resource Allocation Graph A directed graph used to describe deadlocks, and consisted of : A set of vertices V (Active processes in the system P 1, P 2,..., P n ). All resource types in the system Resource R = (R 1, R 2,..., R m ). A set of edges E: Request Edge (P i R J ): Process Pi has requested an instance of resource type R j and is currently waiting for that resource. points to only the rectangle R j assignment edge (R j P i ): An instance of resource type R j has been allocated to process Pi. One dot in the rectangular.

9 Resource Allocation Graph Work Sequence: Each process P i as a circle Each resource type R j as a rectangle (each instance as a dot within the rectangle). When process Pi requests an instance of resource type Rj, A request edge is inserted in the resource-allocation graph. The request edge is instantaneously transformed to an assignment edge When this request can be fulfilled. When the process no longer needs the resource, it releases the resource.

10 Example of a Resource Allocation Graph

11 Resource Allocation Graph With A Deadlock iven the resource-allocation graph: If the graph contains no cycles, no deadlock. If the graph contains a cycle, then a deadlock may exist. If each resource type has several instances, then a cycle does not necessarily imply that a deadlock has occurred. Each process involved in the cycle is deadlocked. Graph with deadlock Graph with a cycle but no deadlock

12 Resource Allocation Graph

13 Methods for Handling Deadlocks Ensure that the system will never enter a deadlock state, the system can use: A. Deadlock prevention: A set of methods to ensure that at least one of the necessary conditions cannot hold. B. Deadlock avoidance: Requires that the OS be given additional information in advance concerning which resources a process will request and use during its lifetime. the OS can decide for each request whether or not the process should wait. C. Deadlock Detection and recovery: Allow the system to enter a deadlock state and then recover, by provide an algorithm that examines the state of the system to determine whether a deadlock has occurred and an algorithm to recover from the deadlock. D. Ignore the problem and suppose that deadlocks never occur in the system; used by most OS, including UNIX and windows, the application developer to write programs that handle deadlocks

14 Deadlock Prevention By ensuring that at least one of these conditions cannot hold, we can prevent the occurrence of a deadlock. Mutual Exclusion Not required for sharable resources. must hold for non-sharable resources Read-only files are a good example of a sharable resource. If several processes attempt to open a read-only file at the same time, they can be granted simultaneous access to the file. Hold and Wait Whenever a process requests a resource, it does not hold any other resources. Protocol 1: each process to request and be allocated all its resources before it begins execution (system calls requesting resources for a process precede all other system calls. Protocol 2: allows a process to request resources only when it has none. A process may request some resources and use them. Before it can request any additional resources, it must release all the resources that it use. Example: A process copies data from a DVD drive to a file on disk, sorts the file, and then prints the results to a printer. Low resource utilization and starvation possible

15 Deadlock Prevention 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 waiting impose a total ordering of all resource types and to require that each process requests resources in an increasing order of enumeration. let R = {R1, R2,..., Rm} be the set of resource types. We assign to each resource type a unique integer number, which allows us to compare two resources and to determine whether one precedes another in our ordering. For example, if the set of resource types R includes tape drives, disk drives, and printers, then the function F might be defined as follows: F(tape drive) = 1 F(disk drive) = 5 F(printer) = 12

16 Deadlock Avoidance Requires that the system has some additional a priori information available Each process declare the maximum number of resources of each type that it may need The deadlock-avoidance algorithm dynamically examines the resourceallocation state to ensure that there can never be a circular-wait condition Resource-allocation state is defined by: The number of available resources (Available ). The number of allocated resources ( Max ). The maximum demands of the processes (Needs )

17 Safe State When a process requests an available resource, system must decide if this allocation leaves the system in a safe state System is in safe state That is: If P 0 requested resource are not immediately available. P 0 can wait until all P 1 have finished, When P 1 is finished, P 0 can obtain requested resources, execute, return allocated resources, and terminate When P i terminates, P i +1 can obtain its needed resources, and so on Basic Facts: If a system is in safe state no deadlocks If a system is in unsafe state possibility of deadlock Avoidance ensure that a system will never enter an unsafe state. Single instance of a resource type Use a resource-allocation graph Multiple instances of a resource type Use the banker s algorithm

18 Resource-Allocation Graph Scheme Used If we have a resource-allocation with only one instance/ resource type. A new type of edge called 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 Request edge converted to an assignment edge when the resource is allocated to the process When a resource is released by a process, assignment edge reconverts to a claim edge Resources must be claimed a priori in the system before starting execution of the process all its claim edges must appear on the resource allocation graph.

19 Multiple instances per each resource. Banker s Algorithm 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[i]: Vector of length m. If available [j] = k, there are k instances of resource type R j available Max[I,j]: n x m matrix. If Max [i,j] = k, then process P i may request at most k instances of type R j. Allocation[I,j]: n x m matrix. If Allocation[i,j] = k then P i is currently allocated k instances of R j Need[I,j] n x m matrix. If Need[i,j] = k, then P i may need k more instances of R j to complete its task Need [i,j] = Max[i,j] Allocation [i,j]

20 Safety Algorithm

21 Resource-Request Algorithm for Process Pi

22 Example of Bankers Algorithm Consider a system with five processes P0 through P4 and three resource types A (10 instances), B (5 instances), and C (7 instances). Suppose that, at time T0, the following snapshot of the system: Available = System resources Allocated by all process Max is max required for execute process Need = Max(P) Allocated (P) IF P1 requests additional (A(1),B(0),C(2)). To decide whether this request can be immediately granted, we first check that Request t1 Available (1,0,2) (3,3,2).

23 Deadlock Detection If a system does not employ either a deadlockprevention or a deadlock avoidance algorithm, then a deadlock situation may occur. In this environment, the system may provide: Two algorithms are used An algorithm must examines the state of the system to determine whether a deadlock has occurred An algorithm to recover from the deadlock

24 Single Instance of Each Resource Type Make a small modification on the Resource Allocation Graph ( Wait-For Graph) Maintain wait-for graph Waiting for process Pj An edge from Pi to Pj in a wait-for graph implies that process Pi is to release a resource that Pi needs. An edge Pi Pj exists in a wait-for graph if and only if the corresponding resource allocation graph contains two edges Pi Rq and Rq Pj for some resource Nodes are processes P i P j if P i is waiting for P j Periodically invoke an algorithm that searches for a cycle in the graph. If there is a cycle, there exists a deadlock An algorithm to detect a cycle in a graph is O(n 2 ), where n is the number of vertices in the graph. Resource-Allocation Graph Corresponding wait-for graph

25 Several Instances of a Resource Type Available: A vector of length m indicates the number of available resources of each type. Allocation: An n x m matrix defines the number of resources of each type currently allocated to each process. Request: An n x m matrix indicates the current request of each process. If Request [i][j] = k, process P i is requesting k more instances of resource type. R j.

26 Several Instances of a Resource Type

27 Several Instances of a Resource Type

28 Example of Detection Algorithm Five processes P 0 through P 4 ; three resource types A (7 instances), B (2 instances), and C (6 instances) The snapshot of the system at time T0 is: P2 request(0,0,1) We claim that the system is not in a deadlocked state. Indeed, if we execute our algorithm, we will find that the sequence <P0, P2, P3, P1, P4> results in Finish[i] == true for all i. Deadlock have occurred because the requested is less than the available

29 Recovery from Deadlock 2. Process Termination Abort all deadlocked processes Abort one process at a time until the deadlock cycle is eliminated In which order should we choose to abort? Priority of the process How long process has computed, and how much longer to completion Resources the process has used and Resources process needs to complete How many processes will need to be terminated Is process interactive or batch? 3. Resource Preemption Selecting a victim minimize cost Rollback return to some safe state, restart process for that state Starvation same process may always be picked as victim, include number of rollback in cost factor

30 End of Chapter 7