Operating system

Question

A multiprocessor with 8 processors has 20 attached tape drives. There is a large number of jobs submitted to the system, and each requires 4 tape drives to complete execution. Assume that each job starts running with only 3 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. The maximum number of jobs that can execute at once are _______.

Comments

Must be 6

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@gauravkc

Can you explain what question actually means

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There are 20 instances of the resource tape drive.

Each process requires exactly 4 tape drives.

It starts with 3 and requires 4th for short period of time.

Say N=5. For 5 jobs, we'll require 5*4=20 tape drives. But more jobs can be executed as 4th tape drive is only required for some time, thus we are making poor utilization of 5 tape drives here.

Say N=6. For 6 job, atleast 18 will be required. For the 4th tape drive, the 6 process can compete for remaining 2 tape drives. Two of them will acquire, complete its execution and leave the tape drive for another job. Hence, there is no deadlock.

N=7 would require atleast 21 resources which are not available.

Answer 1

Answer is 3 ..but not able to get the logic