Scheduling

✅ Scheduling

order plan to allocate CPU to thread
process on ReadyQueue will be scheduled
First in queue has CPU first

Overhead ⬇️ / Usage ⬆️/ Stavation ⬇️

• Batch System: throughout(처리량) ⬆️
• Interactive System: fast reaction time ⬆️ less waiting time ⬇️
• Real-time System: deadlines

Why do we need scheduling?

to use CPU efficiently
decide how to allocate CPU to process

✅ Types of Scheduling

1. Preemptive Scheduling(선점)

• preemtive: forestall, pre-empt

• OS can forcefully take control of CPU
• 👍🏻 process with priority can be executed
• 👍🏻 fast response time
• 👎🏻 if all processes have priority? overhead

• 👎🏻 difficult to expect runtime

  • Interrupt
  • I/O event completion
  • I/O event wait
  • exit
• Priority Scheduling
• Round Robin
• SRTF
• Multi level Queue
• Multi level Feedback Queue

2. Non Preemptive Scheduling(비선점)

• run until process terminates, event such as I/O occurs
• 👍🏻 fair to all processes
• 👍🏻 easier to except runtime

• 👎🏻 process that can end fast has to wait for long process to wait

  • I/O event wait
  • exit
• FCFS(First Come First Served)
• SJF(Shortest Job First)
• HRN(Highest Response-ratio Next): caculate priority

✅ Types of Scheduling based on algorithm

✔️ First come First Served

• first arrive in queue,
• then gets CPU first

✔️ Shortest Job First

• complement FCFS
• prioritize job that has shortest time
• If process has long duration time, might never be allocated CPU ➡️ starvation

✔️ Highest Response-ratio Next

• complement SJF(unfair CPU allocation based on time)
• make priority based on waiting time, time needed to perform
• priority = (waiting time + time to process)/time to process

✔️ Shortest Remaining Time First

• burst time: time that currently processing process needs before terminating
• If process that has shorter burst time, forcefully take CPU
• when new process arrives, new shceduling
• starvation

✔️ Round Robin

• Time Quantum: minimum time needed to process
• allocate equal Time Quantum to all process ordered by FCFS
• modern CPU scheduling method
• when Time Quantum ends, process goes to the back of Ready Queue to wait again
• 👍🏻 when process time is random
• 👍🏻 can save process context

✅ Measure Scheduling

• Response Time
• Turnaround Time