Scheduling
Chapter Uniprocessor 9
Aim of Scheduling
Response time Throughput Processor efficiency
1
Types of Scheduling
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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
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Medium-Term Scheduling
Part of the swapping function Based on the need to manage the degree of multiprogramming
Short-Term Scheduling
Known as the dispatcher Executes most frequently Invoked when an event occurs
Clock interrupts I/O interrupts Operating system calls Signals
4
Short-Tem Scheduling Criteria
User-oriented
Response Time
Elapsed time between the submission of a request until there is output.
System-oriented
Effective and efficient utilization of the processor
Short-Term Scheduling Criteria
Performance-related
Quantitative Measurable such as response time and throughput
Not performance related
Qualitative Predictability
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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 starvation
allow 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
First-Come-First-Served (FCFS)
0 5 10 15 20 1 2 3 4 5
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
Round-Robin
0 5 10 15 20
1 2 3 4 5
Uses preemption based on a clock An amount time of is determined that allows each process to use the processor for that length of time
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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 queue
Next ready job is selected
Known as time slicing
Shortest Process Next
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Nonpreemptive policy Process with shortest expected processing time is selected next Short process jumps ahead of longer processes
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Shortest Process Next
Predictability of longer processes is reduced If estimated time for process not correct, the operating system may abort it Possibility of starvation for longer processes
Shortest Remaining Time
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1 2 3 4 5
Preemptive version of shortest process next policy Must estimate processing time
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Highest Response Ratio Next (HRRN)
0 5 10 15 20 1 2 3 4 5
Choose next process with the lowest ratio
time spent waiting + expected service time expected service time
Feedback
0 5 10 15 20 1 2 3 4 5
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 Ne...