A multiprocessor with eight processors has 20 attached tape drives. There are a

Question

A multiprocessor with eight processors has 20 attached tape drives. There are a large

number of jobs submitted to the system that each require a maximum of four tape

drives to complete execution. Assume that each job starts running with only three

tape drives for a long period before requiring the fourth tape drive for a short period

toward the end of its operation. Also assume an endless supply of such jobs.

a. Assume the scheduler in the OS will not start a job unless there are four tape

drives available. When a job is started, four drives are assigned immediately and

are not released until the job finishes. What is the maximum number of jobs that

can be in progress at once? What are the maximum and minimum number of tape

drives that may be left idle as a result of this policy?

b. Suggest an alternative policy to improve tape drive utilization and at the same

time avoid system deadlock. What is the maximum number of jobs that can be in

progress at once? What are the bounds on the number of idling tape drives?

Explanation / Answer

These below answers are written based on my understanding and may help to solve
the given questions.

Answer for Question A:
If a conservative policy is used, at most 20/4 = 5 processes can be active simultaneously.
Because one of the drives allocated to each process can be idle most of the time, at most
5 drives will be idle at a time. In the best case, none of the drives will be idle.

Answer for Question B:
To improve drive utilization, each process can be initially allocated with three tape drives.
The fourth one will be allocated on demand. In this policy, at most floor(20/3) = 6 processes
can be active simultaneously. The minimum number of idle drives is 0 and the maximum number is 2.

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