Consider a system with 14 dedicated devices of the same type. All jobs currently

Question

Consider a system with 14 dedicated devices of the same type. All jobs currently running on this system require a maximum of five devices to complete their execution but they each run for long periods of time with just three devices and request the remaining two only at the very end of the run. Assume that the job stream is endless and that your operating system’s device allocation policy is a very conservative one: No job will be started unless all the required drives have been allocated to it for the entire duration of its run. Using banker's algorithm,

a.   What is the maximum number of jobs that can be in progress at once? Explain your answer.
b.   What are the minimum and maximum number of devices that may be idle as a result of this policy? Under what circumstances would an additional job be started?

Explanation / Answer

The maximum number of active jobs is 3, because the Banker's Algorithm will allow the safe state of: 2 jobs running using 4 drives each, and a third job running in the device allocation state of 5 tape drives, which allows it to complete.

The minimum idle drive count is 0, occurring when 2 running jobs are using 4 drives each and the third job is using 5 drives: 13 - (2 * 4) - 5 = 0

The maximum idle drive count is 1, occurring when 3 running jobs are using the minimal drive allocation of 4 drives each: 13 - (3 * 4) = 1

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