Definition
Scheduling Levels
OS scheduling operates at three levels, differing in frequency and scope: long-term, medium-term, and short-term.
Levels
Long-term Scheduling
Admission scheduler determines which programs are admitted to the system.
- Task: Controls degree of multiprogramming (number of processes)
- Goal: Balanced mix of CPU-intensive and I/O-intensive processes
- Frequency: Infrequent (seconds to minutes)
Medium-term Scheduling
Manages process residence in main memory.
- Task: Implements swapping between memory and secondary storage
- Goal: Manage memory pressure, improve process mix via Suspend states
- Frequency: Moderate
Short-term Scheduling
CPU scheduler (dispatcher) selects next process from Ready Queue.
- Task: Performs actual process switch
- Goal: Optimise performance criteria (response time, throughput)
- Frequency: Extremely frequent (milliseconds)