Banker's Algorithm
In this analogy
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Customers
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≡
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processes
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Units
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≡
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resources, say, tape drive
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Banker
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≡
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Operating System
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Customers
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Used
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Max
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A
B
C
D
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0
0
0
0
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6
5
4
7
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Available Units = 10
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We observe four customers each of whom has been granted a number of credit nits. The banker reserved only 10 units somewhat than 22 units to service them. At definite moment the situation becomes
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Customers
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Used
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Max
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A
B
C
D
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1
1
2
4
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6
5
4
7
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Available Units = 2
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Safe State- The key to a state being secure is that there is at least one way for all users to finish. In other analogy the state of is safe because with 2 units left, the banker is able to delay any request except C's, therefore letting C finish and release all four resources. With four units in hand the banker is able to let either D or B have the necessary units and so on.
Unsafe State- Consider what would take place if a request from B for one more unit were granted
We would have following situation
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Customers
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Used
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Max
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A
B
C
D
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1
2
2
4
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6
5
4
7
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Available Units = 1
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Fig. 3
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This is an unsafe state.
If all the customers namely A, B, C, and D request for their maximum loans, then banker couldn't satisfy any of them and we would have a deadlock.
It is significant to note that an unsafe state does not imply the existence or even the eventual existence a deadlock. What an unsafe state does involve is simply that some unfortunate sequence of events might lead to a deadlock. The Banker's algorithm is therefore to consider each request as it occurs and see if granting it leads to a safe state. If it does, the request is granted or else, it postponed until later.