mirror of
https://github.com/jackyzha0/quartz.git
synced 2025-12-27 23:04:05 -06:00
134 lines
4.3 KiB
Markdown
134 lines
4.3 KiB
Markdown
---
|
|
title: "19-cpu-scheduling"
|
|
aliases:
|
|
tags:
|
|
- cosc204
|
|
- lecture
|
|
sr-due: 2022-11-04
|
|
sr-interval: 3
|
|
sr-ease: 250
|
|
---
|
|
|
|
why shedule?
|
|
- keep cpu busy (cpu utilisation)
|
|
- so processed dont hae to spend time waithing for the CPU
|
|
- even if cpu is always busy 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
|
|
- provably optimal average waiting time
|
|
|
|
con
|
|
- never know in advance what the length of next cpu burst will be
|
|
- possibility of long processes never being executed
|
|
- starvation of process
|
|
- not "fair"
|
|
|
|
### predicting next burst time
|
|
machine learning sorta - burst length predicted from previous CPU bursts using exponential average equation
|
|
|
|
|
|
preemption example
|
|
- p1 is executing
|
|
- p2 arrived with burst time shorter that remainging bust time of P1
|
|
- non preemptive - P1 finishes
|
|
- preemptive - P2 takes CPU
|
|
|
|
|
|
|
|
## Priority scheduling
|
|
- each process allocated a priotit when it arrives
|
|
- cpu is allocated to the process with the highest priority
|
|
|
|
low number is a higher priority
|
|
|
|
- non preemptive - run processes until finished
|
|
- preemptive - higher priority process should take over CPU when they arrive
|
|
|
|
SJF is a type of priority scheduling where the priority is based on the estimated next cpu burst
|
|
|
|
|
|
|
|
|
|
## Starvation and aging
|
|
starvation occurs when a proces waits indefinitely to be allocated the CPU
|
|
|
|
aging - as a process waits its priority increases
|
|
|
|
## Round Robin
|
|
commonly used today
|
|
|
|
designed for time sharing systems
|
|
- small unit of time (time quantum) if defined
|
|
- ready queue is treated as a circular list
|
|
- cpu schedular goes round the ready queue, allocating the CPU to each process for a time interval up to 1 time quantum
|
|
|
|
|
|
|
|
|
|
choosing time quantum
|
|
- if infinitely large - we have FCFS
|
|
- if very small - we have processor sharing - lots of overhead for switching
|
|
|
|
|