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77 lines
2.6 KiB
Markdown
77 lines
2.6 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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---
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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 bust 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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- prova
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