lecture 9 chapter 2

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Lecture 9

Chapter 2

Processes and Threads

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Scheduling

The problem of determining when processors should be assigned and to which processes is called processor scheduling or CPU scheduling.

When more than one process is runable, the operating system must decide which one first. The part of the operating system concerned with this decision is called the scheduler, and algorithm it uses is called the

scheduling algorithm.

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Long-term Scheduling

Medium-term SchedulingShort-term Scheduling

Types of Scheduling

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Long-Term Scheduling:

1. The decision to add to the pool of processes to be executed.

2. Selects process and loads it into memory for execution.

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Medium-Term Scheduling:

1. The decision to add to the number of processes that are partially or fully in main memory.

2. The mid-term scheduler temporarily removes processes from main memory and places them on secondary memory (swapping)

3. Suspend processes for which adequate resources are not currently available.

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Short-Term Scheduling

 _{If a process requires a resource (or input) that it does not}

have, it is removed from the ready list (and enters the WAITING state)

 The decision as to which available process will be executed by the processor.

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Scheduling and process state transitions

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Queuing Diagram for Scheduling

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Class Work

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Summary

Long-term: which process to admit?

Medium-term: which process to swap in or out?

Short-term: which ready process to execute next?

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When to Schedule?

There are variety of situations in which scheduling is needed.

1. When a new process is created, a decision needs to be made whether to run the parent process or the child process.

2. When a process exits, so some other processes must be chosen from the set of ready processes.

3. When I/O interrupt occur and now be ready to run when I/ O interrupt finishes. The decision has to be made whether to run newly ready process or the process that was running at the time of interrupt.

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CPU Scheduling GOALS

Fairness- makes sure each process gets its fair share of the CPU.

Efficiency–keep the CPU busy 100 percent of the time.

Waiting time- minimize waiting time for job.

Turnaround – minimize the time, how long batch users must wait for output.

Throughput – maximize the number of jobs processed per hour.

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Summary

CPU utilization – keep the CPU as busy as possible

keep the CPU busy 100 percent of the time.

Throughput – # of processes that complete their execution per time unit.

Maximize the number of jobs processed per hour.

Turnaround time (TAT) – amount of time to execute a particular process. Time for each process to complete. Minimize the time, how long batch users must

wait for output. Time from submission to completion

Waiting time – amount of time a process has been waiting in the ready queue.

Response time – amount of time it takes from when a request was submitted until the first response is produced. Time used by the system to respond to the user job

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Non-preemptive Scheduling

A scheduling discipline is non-preemptive if, once a process has been given the CPU, the CPU cannot be taken away from that process.

Preemptive Scheduling

A scheduling discipline is preemptive if, once a process has been given the CPU can taken away.

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Types of Scheduler Types of Scheduler

Primary Scheduler Secondary Scheduler

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Primary Scheduler

1. Runs threads with the highest priority.

2. It then compares their priority and assigns resources to them, depending on their priority.

3. Threads with the highest priority will be executed for the current time slice.

4. With two or more threads with the same priority, they are put on a stack to allow each run on a given time slice.

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Secondary Scheduler

1. It is responsible for increasing the priority of non-executing threads.

2. It is important that those low-priority threads are given a chance to run on the operating system and this is the function of this type of scheduler.

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1. FCFS Scheduling

2. Round Robin (RR) Scheduling 3. SJF Scheduling

4. SRTN Scheduling 5. Priority Scheduling

6. Multilevel Queue Scheduling

CPU Scheduling Algorithm

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1. First-In-First-Out (FIFO) 2. Run-to-Completion

3. Run-Until-Finish

Processes are dispatched according to their arrival time on the ready queue. Being a non-preemptive discipline, once a process has a CPU, it runs to completion.

FCFS Scheduling

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1. Runs the processes in the order they arrive at the short-term scheduler

2. Removes a process from the processor only if it blocks (i.e., goes into the Wait state) or terminates

3. Wonderful for long processes when they finally get on.

4. Terrible for short processes if they are behind a long process.

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• _{Suppose we change the order of arriving}

job P2, P3, P1

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The problem with FCFS is that the average waiting time can be long. Consider the following processes

This gives us an average waiting time of 21ms (i.e. (0 + 27 + 36) /3 ).

Now consider if the processes had arrived in the order P2, P3, P1. The average waiting time would now be 6.6ms (i.e. (0 + 9 + 11) /3).

Process Burst Time

P1 27

P2 9

P3 2

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Burst Waiting Turnaround Process Time Time Time

P1 24 0 24

P2 3 24 27

P3 3 27 30

Average - 17 27

If the processes arrive in the order P1, P2, P3 and are served in FCFS order, we get the result in the following chart

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Process Wait Time : Service Time - Arrival Time

P0 0 - 0 = 0

P1 5 - 1 = 4

P2 8 - 2 = 6

P3 16 - 3 = 13

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10/07/2020 ₃₃ Process Burst P1 24 P2 3 P3 3

CLASS WORK-1

FCFS Scheduling

Assume processes arrive in this order P1, P2, P3

Calculate the average waiting time and turnaround time?

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Process Burst

P2 3

P3 3

P1 24

CLASS WORK-2

FCFS Scheduling

Assume processes arrive in this order P2, P3, P1

Calculate the average waiting time & turnaround time?

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Process P1 P2 P3 P4 P5 Arrival Time 12 8 10 9 7

Burst Time 5 6 11 11 9

Problem

Practice Sheet — CPU Scheduling

Consider the following set of processes with the arrival time and the length of the CPU burst time given in milliseconds:

(a) FCFS (First-Come-First-Served)

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Advantages and Disadvantages

Advantages:

1. simple

2. easy to understand & implement 3. first come first served

Disadvantages:

4. This scheduling method is non-preemptive, the process will run until it finishes.

5. Because of this non-preemptive scheduling, short processes which are at the back of the queue have to wait for the long process at the front to finish.

6. Poor in performance as average wait time is high.

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