CS537 9/16

CPU Virtualization

Problem: What if process $P$ wants to do something restricted?
- See prev Note
Problem: Process may run for a long time
- Q: How does OS regain control of CPU?
- A: timer interrupt
  - @Boot:
    - set this up
    - interrupt the CPU every X milliseconds
      - Why not shorter?
      - A: Context switch overhead, Cache miss
  - OS responsibility
    - for each process: track current state
      - Running
      - Ready (Not running, but could be)
Problem: What if Process does something “Slow”?
- Desire: switch to other process when IO
- Mark $P_{a}$ as Blocked -> Switch to $P_{b}$ -> I/O completes -> Mark $P_{a}$ as Ready

Assumptions about “workload”
1. start: all jobs arrive at once
2. just use CPU (no I/O)
3. fixed length (runs for time $T$ )
4. length is known ahead of time
5. metric: turnaround time $T = T_{c o m p l e t e s} - T_{a r r i v e s}$
Algorithm #1: Fist In First Out, First Come First Serve
- $T_{A} = 10, T_{B} = 10, T_{C} = 10$
  - $T_{a v g} = 20$
- Relax Assumption #3 ( $T_{A} = 10, T_{B} = 100, T_{C} = 10$ )
  - $T_{a v g} = 80$
Algorithm #2: SJF (shortest job first)
- $T_{A} = 10, T_{B} = 100, C = T_{1} 0$
  - $T_{a v g} = 50$
- Relax Assumption #1 (All jobs don’t arrive at once)
Algorithm #3: STCF (Shortest time to completion first)