CS 111 – Sept. 24 Operating Systems Commitment: Scheduling

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CS 111 – Sept. 24 Operating Systems Commitment: Scheduling File permissions Commitment: Please read section 4.1

Scheduling OS may need to decide the order in which to do jobs Many ways to create a schedule. We’ll look at 2. First-come, first-served Do the jobs in the order in which they are requested Shortest Job Next Give priority to short/easy tasks. Evaluating schedules People are interested in how long for their requested jobs to complete. Compute the average turnaround time. Turnaround time of a job = (time @ finish – time @ request)

Example 1 First-come, first-served Process number Time of request Execution time needed 1 20 2 5 30 3 10 40 4 First-come, first-served Process 1 can execute from t=0 to t=20 Process 2 can execute from t=20 to t=50 Process 3 can execute from t=50 to t=90 Process 4 can execute from t=90 to t=100 We can enter this info as extra columns in the table. What is the average turnaround time? What if we tried Shortest Job Next?

Example 2 Note that it’s possible to have idle time. Process number Time of request Execution time needed 1 10 2 30 3 40 20 4 50 5

System load A measure of how “busy” the CPU is At an instant: how many tasks are currently running or ready. If load > 1, the system is “overloaded”, and work is backing up. Typically reported as an average of the last 1, 5, or 15 minutes. Based on the schedule, can calculate average load as well as maximum load.

File permissions 3 levels: owner, group, rest of world For each level: ‘r’ = Can I read the file? ‘w’ = Can I write to (or delete) the file? ‘x’ = Can I execute the file? Examples rw-rw-r-- (664) rwxr-xr-- (754) rw-r----- (640)

Common permissions On many systems, there are no groups, so the group permission is the same as “everybody else”. Examples 644 600 755 700 Only a file/folder’s owner or the administrator may change permissions.