uniprocessor scheduling
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Uniprocessor Scheduling. Chapter 9. Aim of Scheduling. Assign processes to be executed by the processor(s) Response time Throughput Processor efficiency. Long-Term Scheduling. Determines which programs are admitted to the system for processing Controls the degree of multiprogramming - PowerPoint PPT PresentationTRANSCRIPT
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Uniprocessor Scheduling
Chapter 9
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Aim of Scheduling
Assign processes to be executed by the processor(s)
Response time Throughput Processor efficiency
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Long-Term Scheduling Determines which programs are admitted to the
system for processing Controls the degree of multiprogramming More processes, smaller percentage of time
each process is executed Many possible approaches
First-come-first-served Prioritized Time-share: accept next until full
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Medium-Term Scheduling
Part of the swapping function Based on the need to manage the degree
of multiprogramming
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Short-Term Scheduling
Known as the dispatcher Executes most frequently Invoked when an event occurs
Clock interrupts I/O interruptsOperating system callsSignals
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Short-Term Scheduling Criteria
User-orientedResponse Time
Elapsed time between the submission of a request until there is output.
System-orientedEffective and efficient utilization of the
processor
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Short-Term Scheduling Criteria
Performance-relatedQuantitativeMeasurable such as response time and
throughput
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What’s the definition of … and what should they be? Service Time (TS) Waiting Time (TW) Turnaround Time (TR = TW + Ts) (also called residence time) Response Time Deadlines Predictability Throughput Processor Utilization Balancing Resources Fairness What is the range of TR / TS and what do we want it to be?
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Priorities
Scheduler will always choose a process of higher priority over one of lower priority
Have multiple ready queues to represent each level of priority
Lower-priority may suffer starvationAllow a process to change its priority based
on its age or execution history
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Decision Mode
Nonpreemptive Once a process is in the running state, it will continue
until it terminates or blocks itself for I/O Preemptive
Currently running process may be interrupted and moved to the Ready state by the operating system
Allows for better service since any one process cannot monopolize the processor for very long
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Process Scheduling Example
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First-Come-First-Served(FCFS)
Each process joins the Ready queue When the current process ceases to execute, the
oldest process in the Ready queue is selected
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First-Come-First-Served(FCFS) A short process may have to wait a very
long time before it can execute Favors CPU-bound processes
I/O processes have to wait until CPU-bound process completes
Favors longer processes (see table p402)
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FCFS Tr/Ts Performance (table p 402)
Proc. Arr. T Svc Ts
Start Finish Tr Tr/Ts
W 0 1 0 1 1 1
X 1 100 1 101 100 1
Y 2 1 101 102 100 100
Z 3 100 102 202 199 1.99
Avg. 100 26
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Round-Robin
Clock interrupt is generated at periodic intervals
When an interrupt occurs, the currently running process is placed in the read queueNext ready job is selected
Known as time slicing
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Round-Robin
Uses preemption based on a clock An amount of time is determined that allows each
process to use the processor for that length of time Tends to be unfair for IO bound processes
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Shortest Process Next
Nonpreemptive policy Process with shortest expected processing
time is selected next Short process jumps ahead of longer
processes Must estimate processing time
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Shortest Process Next
Needs estimate of Ts Programmer or statistics
Predictability of longer processes is reduced If estimated time for process not correct, the
operating system may abort it Prevent cheating
Possibility of starvation for longer processes
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Shortest Remaining Time
Preemptive version of shortest process next policy
Must estimate processing time
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Highest Response Ratio Next (HRRN)
Choose next process with the greatest ratiotime spent waiting + expected service time
expected service time
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Feedback
Penalize jobs that have been running longer Don’t know remaining time process needs to execute
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Fair-Share Scheduling
User’s application runs as a collection of processes (threads)
User is concerned about the performance of the application
Need to make scheduling decisions based on process sets
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Traditional UNIX Scheduling Multilevel feedback using round robin
within each of the priority queues If a running process does not block or
complete within 1 second, it is preempted Priorities are recomputed once per second Base priority divides all processes into
fixed bands of priority levels
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Bands
Decreasing order of prioritySwapperBlock I/O device controlFile manipulationCharacter I/O device controlUser processes
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