Chapter 6 CPU Scheduling. Processor Scheduling

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1 Chapter 6 CPU Scheduling Processor Scheduling Known as the dispatcher Invoked when an event occurs Clock interrupts I/O interrupts Operating system calls Signals 1

2 Aim of Scheduling Assign processes to be executed by the processor(s) Response time Throughput Processor efficiency Processor Scheduling Criteria User-oriented Response Time Elapsed time between the submission of a request until there is an output Turnaround Time The time between the submission and the completion of a process System-oriented Effective and efficient utilization of the processor 2

3 Nonpreemptive Decision Mode Once a process is in the running state, it will continue until it terminates or blocks itself for I/O Preemptive Decision Mode 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 3

4 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 Priority Queuing 4

5 First-Come-First-Served Each process joins the Ready queue When the current process ceases to execute, the process that waits the longest in the Ready queue is selected Process Scheduling Example 5

6 Process Scheduling Example First-Come-First-Served 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 Low utilization for I/O devices Possibly low utilization for processor 6

7 Round Robin Clock interrupt is generated at periodic intervals When an interrupt occurs, the currently running process is placed in the ready queue Next ready process is selected Known as time slicing Round Robin Uses preemption based on a clock 7

8 Round Robin Uses preemption based on a clock Time quantum is Round Robin The time interval between two clock interrupts The maximum time allocated to a process s execution in each turn 8

9 Round Robin Which type of processes are favored by round robin scheduling? CPU or I/O bound processes? Round Robin Example: A, C, D, E: CPU bound processes; B: I/O bound process. What should we do to treat them fairly in terms of CPU allocation? A B C D E 9

10 Round Robin Which type of processes are favored by round robin scheduling? CPU or I/O bound processes? Favors CPU-bound processes CPU-bound process gets more CPU time I/O process has to wait for CPU-bound processes to consume their quanta before its turn of executing on the CPU Queuing Diagram Virtual round robin Use an auxiliary queue to hold processes that have I/O requests serviced Process in auxiliary queue gets preference over those in the main ready queue Process from the auxiliary queue runs for remaining time from last time-slice 10

11 Shortest Process Next Process with shortest expected processing time is selected next Short process jumps ahead of longer processes Nonpreemptive policy Shortest Process Next 11

12 Shortest Process Next Shortest Remaining Time Preemptive version of shortest process next policy 12

13 Shortest Remaining Time Preemptive version of shortest process next policy Shortest Process Next Possibility of starvation for longer processes If estimated time for process not correct, the operating system may abort it How to estimate a process execution time? Average of history? Simple average: S n+1 = 1/n T i Exponential average: S n+1 = T n+1 + (1- )S n Greater weights to more recent instances 13

14 Highest Response Ratio Next Choose next process with the greatest ratio Non-preemptive scheduling algorithm shorter processes are favored aging without service increases the ratio so that a longer process will eventually be served Highest Response Ratio Next Choose next process with the greatest ratio 14

15 Highest Response Ratio Next Choose next process with the greatest ratio 15

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