Project 2
Project 2: Preemption
CS 4411 Spring 2017
xkcd.com/1542
Project 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.
Project 2
First Step
Fix Project 1 Bugs!!!!
Project 2
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…
Project 2
P2 Overview
 Three main parts of P2:
 Receive Clock Interrupts
 Add Alarms
 Allow threads to sleep
 Multilevel Feedback Queue Scheduler
Project 2
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.
Project 2
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?
Project 2
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
Project 2
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!
Project 2
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
Project 2
Scheduling: FIFO
 Now lets talk about scheduling
 Suppose our strategy is FIFO (like in P1)
 What problems can arise?
 CPU bound vs IO bound
Project 2
Shortest Job First
 Suppose our strategy is shortest job first
 What problems can arise?
Project 2
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????
Project 2
Introducing the Multi Level Queue!
Round Robin
Highest
Round Robin
Round Robin
Round Robin
Lowest
Project 2
Scheduling
0
Total 80 quanta – 1 quanta per thread
1
Total 40 quanta – 2 quanta per thread
2
Total 24 quanta – 4 quanta per thread
3
Total 16 quanta – 8 quanta per thread
Do we achieve our goals?
Project 2
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.
Project 2
Testing
 There are a lot of parts to this project




Multi-level queue
Interrupts
Alarms
Thread levels
 Common pitfalls
 Unnecessarily disabling interrupts
 Not disabling interrupts when necessary
 Multi-level queue corner cases
Project 2
Questions
Questions?
Project 2
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 50000 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.