quartz/content/notes/19-cpu-scheduling.md

title	aliases	tags
19-cpu-scheduling		cosc204 lecture

why shedule?

• keep cpu busy (cpu utilisation)
• so processed dont hae to spend time waithing for the CPU
  • even if cpu is always bust executing processses in different orders can change the average time spent in queue
• cpu burst
  • 
  • (diagram is old the timing will have changed)
  • between burst - waiting for io
• so that interactive processes respond quickly
  • time spent waiting in total
  • average time between visits to CPU
  • 

criteria

• CPU utilisation - % time CPU is busy
• throughput - number of processes completed per time unit
• turnaround time - (for a single process) length of time from submitted to completed
• waiting time - (for a single process) time spent waiting for the CPU
  • does include time wating for IO
• response time - (for a signle process) average time from the submission of a request to a process until the first response is produced

Preemption

• 4 situations for scheduling
  1. a process switches from running to waiting
  2. a process switches from running to ready (due to interrupt)
  3. a process switches from wauting to ready (due to comletion of IO)
  4. a process terminates
• non preemptive - scheduling take splace only under 1 nd 4, which has no choice
• premptive - scheduling can take place under 2 and 3 as well. that means a running process has to give way to the processes with higher priority

if you allow interruption it is preemptive

Scheduling algorithms

Gantt charts

The operation of scheduling algortihm is commonly represented in a Gantt chart

First come first served (FCFS)

simplest method- executes processes in the ready queue on a first come first served basis

• when a proces become ready it is put at the tail of the queue
• when the currently executig process terminates, or waits for IO, the process at the front of the queue is selected next
• processes can come at any time - what if two at the same time? choose process with smallest ID to serve first

non-preemptive

pro

• easy to understand and implement con
• waiting time bad
• convoy effect: lots of small process can get stuck behind a big on
• bad response time

fair but not efficient

Shortest job first (SJF)

choose the job in the queue with the shortest burst time.

• equal burst time are executed FCFS (by process id)

pro

• prova