Bankers Algorithm Example

Banker’s Algorithm for deadlock avoidance The algorithm avoids deadlock by denying or postponing the reque st if it determines that accepting the request could put the system in an unsafe state (one where deadlock could occur). When a new process enters a system, it must declare the maximum number of instances of each resource type that may not exceed the total number of resources in the system. Also, when a process gets all its requested resources it must return them in a finite amount of time.

Resources or the !anker"s algorithm to work, it needs to know three things#

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$ow much of each resource each process could possibly request $ow much of each resource each process is currently holding

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$ow much of each resource the system currently has a%ailable

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&esources may be allocated to a process only if it satisfies the following conditions# '. request  max, else set error condition as process has crossed maximum claim made by it. . request  a%ailable, else process waits until resources are a%ailable. The !anker"s Algorithm deri%es its name from the fact that this algorithm could be used in a banking system to ensure that the bank does not run out of resources, because the bank would ne%er allocate its money in such a way that it can no longer satisfy the needs of all its customers. !y using the !anker"s algorithm, the bank ensures that when customers request money the bank ne%er lea%es a safe state. *f the customer"s request does not cause the bank to lea%e a safe state, the cash will be allocated, otherwise the customer must wait until some other customer deposits enough. !asic data structures to be maintained to implement the !anker"s Algorithm# +et n be the number of processes in the system and m be the number of resource types. Then we need the following data structures# •

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A%ailable# A %ector of length m indicates the number of a%ailable resources of each type. *f A%ailable- / k, there are k instances of resource type &- a%ailable. 0ax# An n1m matrix defines the maximum demand of each process. *f 0axi,- / k, then 2i may request at most k instances of resource type & -. Allocation# An n1m matrix define s the numbe r of resources of each type currently allocated to each process. *f Allocationi,- / k, then process 2 iis currently allocated k instance of resource type & -. 3eed# An n1m matrix indicates the remaining resource need of each process. *f 3eedi,- / k, then 2imay need k more instances of resource type &- to complete task.

3ote# 3eed / 0ax 4 Allocation. Example

Assuming that the system distinguishes between four types of resources, (A, !, 5 and 6), the following is an example of how those resources could be distributed. Available system resources are: A B C D 3 1 1 2 Processes (currently allocated resources): A B C D P1 1 2 2 1 P2 1 0 3 3 P3 1 1 1 0 Processes (maximum resources): A B C D P1 3 3 2 2 P2 1 2 3 4 P3 1 1  0

Need=Max-Allocation Processes Need! A B C P1 2 1 0 P2 0 2 0 P3 0 0 4

D 1 1 0

"afe and #nsafe "tates A state (as in the abo%e example) is considered safe if it is possible for all processes to finish executing (terminate). Any state where no such set exists is an unsafe state. Example

We can show that the state gi%en in the pre%ious example is a safe state by showing that it is possible for each process to acquire its maximum resources and then terminate. '. 2' acquires  A, ' ! and ' 6 more resources, achie%ing its maximum o The system now still has ' A, no !, ' 5 and ' 6 resource a%ailable . 2' terminates, returning 7 A, 7 !,  5 and  6 resources to the system o

The system now has 8 A, 7 !, 7 5 and 7 6 resources a%ailable

7. 2 acquires  ! and ' 6 extra resources, then terminates, returning all its resources o

The system now has 9 A, 7 !, : 5 and : 6 resources

8. 27 acquires 8 5 resources and terminates o

The system now has all resources# : A, 8 !, ; 5 and : 6

9. !ecause all processes were able to terminate, this state is safe

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Assume process 7 requests  units of resource 5.

'. There is not enough of resource 5 a%ailable to grant the request . The request is denied

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Assume process 7 requests ' unit of resource 5.

'. There are enough resources to grant the request . Assume the request is granted o

The new state of the system would be#

Available system resources A B C D !ree 3 1 0 2 Processes (currently allocated resources): A B C D P1 1 2 2 1 P2 1 0 3 3 P3 1 1 2 0 Processes (Need)

A B C P1 2 1 0 P2 0 2 0 P3 0 0 3

D 1 1 0

'. 6etermine if this new state is safe '. 2' can acquire  A, ' ! and ' 6 resources and terminate . Then, 2 can acquire  ! and ' 6 resources and terminate 7. inally, 27 can acquire 7 5 resources and terminate 8. Therefore, this new state is safe .