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134 lines
4.3 KiB
Markdown
134 lines
4.3 KiB
Markdown
---
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title: "19-cpu-scheduling"
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aliases:
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tags:
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- cosc204
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- lecture
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sr-due: 2022-11-27
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sr-interval: 16
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sr-ease: 250
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---
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why shedule?
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- keep cpu busy (cpu utilisation)
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- so processed dont hae to spend time waithing for the CPU
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- even if cpu is always busy executing processses in different orders can change the average time spent in queue
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- cpu burst
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- 
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- (diagram is old the timing will have changed)
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- between burst - waiting for io
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- so that interactive processes respond quickly
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- time spent waiting in total
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- average time between visits to CPU
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- 
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criteria
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- CPU utilisation - % time CPU is busy
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- throughput - number of processes completed per time unit
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- turnaround time - (for a single process) length of time from submitted to completed
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- waiting time - (for a single process) time spent waiting for the CPU
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- does include time wating for IO
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- response time - (for a signle process) average time from the submission of a request to a process until the first response is produced
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Preemption
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- 4 situations for scheduling
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1. a process switches from running to waiting
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2. a process switches from running to ready (due to interrupt)
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3. a process switches from wauting to ready (due to comletion of IO)
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4. a process terminates
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- non preemptive - scheduling take splace only under 1 nd 4, which has no choice
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- 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
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if you allow interruption it is preemptive
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# Scheduling algorithms
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## Gantt charts
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The operation of scheduling algortihm is commonly represented in a Gantt chart
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## First come first served (FCFS)
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simplest method- executes processes in the ready queue on a first come first served basis
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- when a proces become ready it is put at the tail of the queue
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- when the currently executig process terminates, or waits for IO, the process at the front of the queue is selected next
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- processes can come at any time - what if two at the same time? choose process with smallest ID to serve first
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non-preemptive
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pro
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- easy to understand and implement
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con
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- waiting time bad
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- convoy effect: lots of small process can get stuck behind a big on
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- bad response time
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fair but not efficient
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## Shortest job first (SJF)
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choose the job in the queue with the shortest burst time.
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- equal burst time are executed FCFS (by process id)
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pro
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- provably optimal average waiting time
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con
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- never know in advance what the length of next cpu burst will be
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- possibility of long processes never being executed
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- starvation of process
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- not "fair"
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### predicting next burst time
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machine learning sorta - burst length predicted from previous CPU bursts using exponential average equation
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preemption example
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- p1 is executing
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- p2 arrived with burst time shorter that remainging bust time of P1
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- non preemptive - P1 finishes
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- preemptive - P2 takes CPU
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## Priority scheduling
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- each process allocated a priotit when it arrives
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- cpu is allocated to the process with the highest priority
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low number is a higher priority
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- non preemptive - run processes until finished
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- preemptive - higher priority process should take over CPU when they arrive
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SJF is a type of priority scheduling where the priority is based on the estimated next cpu burst
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## Starvation and aging
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starvation occurs when a proces waits indefinitely to be allocated the CPU
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aging - as a process waits its priority increases
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## Round Robin
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commonly used today
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designed for time sharing systems
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- small unit of time (time quantum) if defined
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- ready queue is treated as a circular list
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- cpu schedular goes round the ready queue, allocating the CPU to each process for a time interval up to 1 time quantum
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choosing time quantum
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- if infinitely large - we have FCFS
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- if very small - we have processor sharing - lots of overhead for switching
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