Priority based scheduling

• Useful in limited circumstances
• Helpful as a framework for more advanced scheduling
• What if the OS could automatically adjust priorities?

Earliest Deadline First

• Useful in real-time applications with deadlines
• Assign highest priority to thread with the earliest deadline
• Improves upon rate-monotonic scheduling

Example Processes

Decay Usage Scheduling

• Suitable for systems without deadlines
• Lower priority for threads that run more
• Allows quick, interactive tasks to execute instantly while moving compute-intensive tasks to the background

Fairness

• Equal sharing of resources my be more important than other factors

Goal

• Provide tunable fair sharing of resources between threads and/or system users

Mechanisms

• Lottery Scheduling
• Weighted Round-robin
• Weighted Fair Queuing

Lottery Scheduling

• Uniform time slices for all threads
• No formal rotation
• Next thread to run selected randomly using assigned weights

Lottery Scheduling

• Simple
• Technically correct in the long run
• May produce undesirable behavior in the short run

Weighted Round Robin

• Threads with larger allocations are assigned larger time slices
• Reduces context switches

Example

• Three threads (T1, T2, T3)
• Allocated resources in the proportions 3:2:1

Weighted Fair Queuing

• Threads with smaller allocation have to sit out on some rounds
• Keeps accumulated runtimes very close to desired proportions

Linux Example

• nice values feed into scheduler
• renice can adjust niceness
• renice -n {niceness} -p {pid}