1. Scheduling and Fairness
Section 7
Scheduling and Fairness
March 3rd, 2017
Taught by Joshua Don

2. Scheduling
Scheduling: The process by which the kernel selects the next thread/process to run
Fairness: A measure of how equitably the kernel allocates system resources (ie. CPU usage to run each process)

3. First come, first serve FIFO; run in the order of arrival
Processes arrive: P2, P1, P3
We run: P2, P1, P3

4. Round robin
Each process gets a short amount of time to run, before it is pushed to the back of the queue
Cycle through all processes
Processes arrive: P2, P1, P3
We run: P2, P1, P3, P2, P1, P3, P2, P1, P3, ….

5. Simple priority scheduling
Run whatever process has the highest priority
If using preemption, can preempt the currently running process if a higher priority process arrives
Processes arrive: P2 – 5, P1 - 10
We start running: P1
P3 – 15 arrives
Overall order will look like: P1, P3, P1, P2

6. Shortest remaining time first
Run whatever process has the least amount of time left to completion
Processes arrive: P2 - 4 ticks, P1 – 3 ticks, P3 – 5 ticks
We run: P1, P2, P3

7. Other scheduling ideas
Run a random process at each interval
Decrease/increase priority based on how long a process has been waiting to get CPU time
Choose processes randomly, but with probability proportional to priority
Choose process with earliest deadline
Round-robin among the highest priority processes (ie. MLFQS in proj1)

8. Scheduling overview
There are many other scheduling algorithms, many are much more complicated.
Each algorithm has pros and cons; need to consider how they handle mixture of different processes – different run times, priorities, etc.
Some are harder to implement
Some require more overhead to figure out which process to schedule
Each algorithm tries to optimize its own definition of fairness