What is a scheduling policy and trying to optimize with shortest time-to-completion first (STCF).

1. What is a scheduling policy?

• On the context switch, which process to run next, from a set of ready processes?
• OS scheduler schedules the CPU requests (bursts) of processes
  – CPU burst = the CPU time used by a process in a continuous stretch
  – If a process comes back after I/O wait, it counts as a fresh CPU burst 2

2. What are we trying to optimize?

• Maximize (utilization = fraction of time CPU is used)
• Minimize average (turnaround time = time from process arrival to completion)
• Minimize average (response time = time from process arrival to first scheduling)
• Fairness: all processes must be treated equally
• Minimize overhead: run process long enough to amortize the cost of context switch (~1 microsecond)

3. Shortest Time-to-Completion First (STCF)

• Also called Shortest Remaining Time First (SRTF)
• Preemptive scheduler
• Preempts running task if time left is more than that of the new arrival