13(a) Explain the Banker’s Algorithm for Deadlock Avoidance With an Illustration. _ Bituh

2/24/14

13(a) Explain the Banker’s algorithm for deadlock avoidance with an illustration. | bituh.com

bituh.com be ready for your exam in last minute.. Search

About

13(a) Explain the Banker’s algorithm for deadlock avoidance with an illustration. 0 The Banker’s algorithm is the best known of the avoidance strategies. The strategy is modelled after the leading policies employed in banking system. The resource-allocation graph algorithm is suitable to a resource allocation system with single instances of each resource type. It is not suitable for multiple instance of each resource type. Banker’s algorithm is suitable to a resource allocation system with multiple instances of each resource type. The banker’s algorithm makes decisions on granting a resource based on whether or not granting the request will put the system into an unsafe state. Several data structures must be maintained to implement the banker’s algorithm. Let n be the number processes in the system and m be the number of resource types. We need the following data structures: (1) Available (2) Max (3) Allocation (4) Need I) Available: A vector of length m indicates the number of available resources of each type. If available [j] = k, there are k instances of resource type R available. 2) Max: An n x m matrix defines the maximum demand of each process. If Max [i, j] = k, then process P. may request at most k instances of resource type Rr 3) Allocation: An n x m matrix defines the number of resources of each type currently allocated to each process. If allocation [i, j] = k, then P. is currently allocated k instances of resource type R 4) Need: An n x m matrix indicates the remaining resource need of each process. If Need |i, j] = k, then process P. may need k more instances of resource type R, to complete its task. Need [i, j] – Allocation [i, j]. Safety algorithm: Safety algorithm is used to find the state of the system: That is, system may be safe state or unsafe state. Method for this is as follows: 1) Let work and finish be vector of length m and n respectively. Initialize work = Available and Finish [i] = False for i = 1, 2, 3, 4, … n. 2) Find an i such that both a) Finish [i] = False b) Need fj] < work If no such i exist, go to step 4. www.bituh.com/2012/08/22/13a-explain-the-bankers-algorithm-for-deadlock-avoidance-with-an-illustration/

1/6

2/24/14

13(a) Explain the Banker’s algorithm for deadlock avoidance with an illustration. | bituh.com

3) Work : = Work + Allocation i Finish [i] = true to step 2 4) If Finish [i] = true for all i, then the system is in a safe state. Resource-request algorithm: Let request, be the request vector for process P. If Request, fj] = k, then process P. wants k instances of resource type R.. When a request for resources is made by process P, the following actions are taken. 1) If Request. < Need(, then go to step 2. Otherwise, raise an eitor condition since the process has exceeded its maximum claim. 2) If Request < Available, then go to step 3. Otherwise, P. must wait since the resources are not available. 3)

Available : = Available – Request.; Allocation : = Allocation + Request.; Need; : = Needt – Request.;

If the resulting resource allocation stale is safe, the transaction is completed and process P. is allocated its resources. If the new state is unsafe, then P. must wait to the Request, and the old resource allocation state is restored. Examples on Banker’s algorithm: 1) System consists of five processes (P,, P2, P3, P4, Ps ) and three resources (R,, R„ R3). Resource type R, has 10 instances, resource type R, has 5 instances and R} has 7 instances. The following snapshot of the system has been taken

ProcessAllocation R,

Max

r2

Ri

P,0

l

0

i

5

P22

0

0

3

2

P3P43

0

2

9

0

2

I

1

2

2

Ps0

0

2

4

3

3 2

www.bituh.com/2012/08/22/13a-explain-the-bankers-algorithm-for-deadlock-avoidance-with-an-illustration/

2/6

2/24/14

13(a) Explain the Banker’s algorithm for deadlock avoidance with an illustration. | bituh.com

2 2 2 3 Available

R, R.

R,

R,

Content of the matrix need is calculated as Need = Max – A1 Process

Need R,

R2

x,

P,

7

4

3

P2

1

2

2

P3

6

0

0

P,

0

1

1

P5

4

3

1

Currently the system is in safe state. Safe sequence: Safe sequence is calculated as follows: 1) Need of each process is compared with available. If need. < available;, then the resources are allocated to that process and process will release the resource. 2) If need is greater than available, next process need is taken for comparison. 3) In the above example, need of process P. is (7, 4, 3) and available is (3, 3, 2). need > available —> False So system will move for next process.

location.

4) Need for process P2 is (1, 2, 2) and available (3, 3, 2), so need < available (work) (1, 2, 2) < (3, 3, 2) = True then Finish [i] = True www.bituh.com/2012/08/22/13a-explain-the-bankers-algorithm-for-deadlock-avoidance-with-an-illustration/

3/6

2/24/14

13(a) Explain the Banker’s algorithm for deadlock avoidance with an illustration. | bituh.com

Request of P, is granted and processes P2 is release the resource to the system. Work : = Work + Allocation Work : = (3, 3, 2) + (2, 0, 0) : = (5, 3, 2) This procedure is continued for all processes. 5) Next process P3 need (6, 0,0) is compared with new available (5, 3,2). Need > Available = False (6 0 0) > (5 3 2) 6) Process P4 need (0, 1, 1) is compared with available (5, 3, 2). Need < available (0 1 1) < (5 3 2) = True Available = Available + Allocation = (5 3 2) + (2 1 1) = (7 4 3) (New available) 7) Then process P5 need (4 3 1) is compared with available (7 4 3) Need < Available (4 3 1) < (7 4 3) = True Available = Available + Allocation = (7 4 3) + (0 0 2) = (7 4 5) (New available) 8) Process P, need (7 4 3) and available (7 4 5). If this request is granted then the system may be in the deadlock state. After granting the request, available resource is (0 0 2) so the system is in unsafe state. 9) Process P3 need is (6 0 0) and available (7 4 5) .’. Need < Available (6 0 0) < (7 4 5) = True Available = Available + Allocation = (7 4 5) + (3 0 2) = (10 4 7) = (New available) 10) Last the remaining process PI need (7 4 3) and available is (10 4 7) Need < Available (7 4 3) < (10 4 7) = True

Available = Available + Allocation = (10 4 7) + (0 I 0) = (10 5 7) Safe sequence is < P1P4P5P0 P,> Weakness of Banker’s algorithm: 1. It requires that there be a fixed number of resources to allocate. www.bituh.com/2012/08/22/13a-explain-the-bankers-algorithm-for-deadlock-avoidance-with-an-illustration/

4/6

2/24/14

13(a) Explain the Banker’s algorithm for deadlock avoidance with an illustration. | bituh.com

2. 3. 4. 5.

The algorithm requires that users sLate their maximum needs (request) in advance. Number of users must remain fixed. The algorithm requires that the bankers grant all requests within a finite time. Algorithm requires that process returns ail resource within a finite time.

Categories: BTECH [IT], NOV-DEC-2006, Operating Systems This post was written by shalini, posted on August 22, 2012 Wednesday at 7:46 am

Leave a Reply Your email address will not be published. Name Email Website

Comment You may use these HTML tags and attributes: Post Comment

Search

Categories open all | close all BE [Civil] (313) BE [CS] (1500) BE [ECE] (522) BE [EEE] (15) BTECH [IT] (1088) Others (58) www.bituh.com/2012/08/22/13a-explain-the-bankers-algorithm-for-deadlock-avoidance-with-an-illustration/

5/6

2/24/14

13(a) Explain the Banker’s algorithm for deadlock avoidance with an illustration. | bituh.com

Disclaimer:All data and information provided on this site is for informational purposes only. http://bituh.com makes no representations as to accuracy, completeness, currentness, suitability, or validity of any information on this site and will not be liable for any errors, omissions, or delays in this information or any losses, injuries, or damages arising from its display or use. Copyright © bituh.com. All rights reserved.

www.bituh.com/2012/08/22/13a-explain-the-bankers-algorithm-for-deadlock-avoidance-with-an-illustration/

6/6