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

2. 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 = finish – request)

3. 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?

4. 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

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.

6. 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)

7. 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.