1. Project 2: Preemption CS 4411 Spring 2017
xkcd.com/1542

2. Administrative Information
Project 1 is still being graded – results should be ready by beginning of next week.
Project 2 will be due Fri 3/3.

3. Fix Project 1 Bugs!!!! First Step
OS prac builds on each project, and P1 is literally the foundation of everything.

4. Purpose Made a major assumption in Project 1
Threads behave nicely and yield CPU
Issues?
Selfish Threads
Starvation
Need the ability to interrupt running threads…
Selfish Threads

5. P2 Overview Three main parts of P2: Receive Clock Interrupts
Add Alarms
Allow threads to sleep
Multilevel Feedback Queue Scheduler

6. Introducing Interrupts
Every clock tick the interrupt handler will be called.
Allows us to schedule a new thread to be run without relying on nice behavior.

7. Disabling Interrupts For operations that must be atomic
Typically when working with shared data structures.
E.g. Modifying the cleanup queue.
Trivial way of achieving correctness: disable interrupts for everything.
Why is this a bad idea?

8. Interrupt Handler - Reminder
Just a function that runs when an interrupt occurs.
Are there problems if the interrupt handler is interrupted?
Yes – accessing shared data structures
Solution – disable interrupt in the interrupt handler
CANNOT BLOCK

9. Alarms Alarms = Schedule a function for future execution
minithread_sleep()
Timeouts
Each alarms requires a call back function
Call back functions might not be executed by the thread that registered the alarm!

10. Performance Matters!
Remember, checking alarms will take place inside your interrupt handler.
How long does it take to find an element in your Project 1 queue?
We need a different approach. You may add extra functionality to your Project 1 queue
Some sort of queue where elements have different priorities

11. Scheduling: FIFO Now lets talk about scheduling
Suppose our strategy is FIFO (like in P1)
What problems can arise?
CPU bound vs IO bound
Ice cream analogy, or counting words vs moving your mouse

12. Shortest Job First Suppose our strategy is shortest job first
What problems can arise?

13. What we want
Goals:
- Short jobs added later don’t starve due to long jobs added earlier
- Long jobs don’t get perpetually starved if short jobs keep coming in
Also, how do we even know how long a job will take????

14. Introducing the Multi Level Queue!
Round Robin
Highest
Round Robin
Round Robin
Idea: Jobs start at highest priority because we have no idea how long it will take
Round Robin
Lowest

15. Scheduling Total 80 quanta – 1 quanta per thread1 2
Two numbers, First how long does the CPU spend in that level, then how long does the CPU run each individual thread for in that level 3
Do we achieve our goals?

16. Scheduling
If there are no threads in a given level, schedule threads from the next available level
Threads are demoted after using up their time for that level
Thread level starts at 0 and can only increase throughout its lifetime.

17. Testing There are a lot of parts to this project Common pitfalls
Multi-level queue
Interrupts
Alarms
Thread levels
Common pitfalls
Unnecessarily disabling interrupts
Not disabling interrupts when necessary
Multi-level queue corner cases

18. Questions
Questions?

19. Project 2 FAQ
All library calls are safe: interrupts are automatically disabled upon calling
interrupts will be restored to its original state (enabled/disabled) after the call.
Units of time
PERIOD is defined as 50 ms, which is as a constant.
Alarm and wakeup delays are specified in milliseconds.
You have to convert units; don’t blindly subtract PERIOD.
Irregular/random clock interrupts
This is normal
Be careful of introducing heisenbugs because of your debug statements.