Uniprocessor Scheduling
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Transcript of Uniprocessor Scheduling
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Uniprocessor Scheduling
Chapter 9
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Aim of Scheduling
• Main Job: Assign processes to be executed by the processor(s) and processes to be loaded in main memory.
• Decision Methods (Processor)– Response time (Key in GUI systems)– Throughput (Not as important today)– Processor efficiency (Avoiding Thrashing)
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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
– System vs. User process preemptiability• 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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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.• Dealing with Load Control.
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Short-Term Scheduling
• Known as the dispatcher• Executes most frequently• Invoked when an event occurs
– Clock interrupts– I/O interrupts– Operating system calls– Signals
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Short-Tem Scheduling Criteria
• User-oriented– Turnaround Time – When your program
can finish– Deadlines – When a program has goals.– Response Time
• Elapsed time between the submission of a request until there is output.
– Predictability – consistent response times
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User Oriented Scheduling
• Decisecond Response Time – Instant response
• Subsecond Response Time – Near instant response
• Less than 2 seconds – Quick response
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User Oriented Scheduling
• 2 to 4 seconds - Slow• Greater than 4 seconds – Unbearable
Slow• Greater than 15 seconds – Trip to the
kitchen slow
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Short-Term Scheduling Criteria
• System-oriented– Throughput– Processor Utilization– Balancing Resources– Fairness– Enforcing Priorities
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Priority Scheduling
• 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 processes can starve– Upgrading a processes priority the longer it
doesn’t run.
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Different Short-Term Process Schedulers
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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.
• Bad for systems that need fast user response times.
• Not as much swapping of processes.• Favors CPU-bound processes
– I/O processes have to wait until CPU-bound process completes
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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
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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• A lot more swapping than FCFS• Better response times per process, but not
better throughput
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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
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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
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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 ratio
time spent waiting + expected service timeexpected 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 priority– Swapper– Block I/O device control– File manipulation– Character I/O device control– User processes
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Extra Slides
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