Question

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SYNCHRONIZATION or SCHEDUALING If you choose SCHEDUALING you have to do the following: define the SCHEDUALING Concept-1 discuss Why operating system needs process-2 SCHEDUALING 3-discuss the process SCHEDUALING algorithms - How it works - Advantage/disadvantage 4- support your presentation with two examples each has at least three processes and compare between the algorithms(using average waiting time ) |

about algorithm we need apply on all 4 type of algorithm

and apply of two example on 4 type of algorithm

and thank you

Answer 1

1.The process scheduling is the activity of the process manager that handles the removal of the running process from the CPU and the selection of another process on the basis of a particular strategy.

Process scheduling is an essential part of a Multiprogramming operating systems. Such operating systems allow more than one process to be loaded into the executable memory at a time and the loaded process shares the CPU using time multiplexing.

Why do we need scheduling?
A typical process involves both I/O time and CPU time. In a uni programming system like MS-DOS, time spent waiting for I/O is wasted and CPU is free during this time. In multi programming systems, one process can use CPU while another is waiting for I/O. This is possible only with process scheduling.

Objectives of Process Scheduling Algorithm

Max CPU utilization [Keep CPU as busy as possible]
Fair allocation of CPU.
Max throughput [Number of processes that complete their execution per time unit]
Min turnaround time [Time taken by a process to finish execution]
Min waiting time [Time a process waits in ready queue]
Min response time [Time when a process produces first response]

Objectives of Process Scheduling Algorithm

Max CPU utilization [Keep CPU as busy as possible]
Fair allocation of CPU.
Max throughput [Number of processes that complete their execution per time unit]
Min turnaround time [Time taken by a process to finish execution]
Min waiting time [Time a process waits in ready queue]
Min response time [Time when a process produces first response]

Process Scheduling Queues

The OS maintains all PCBs in Process Scheduling Queues. The OS maintains a separate queue for each of the process states and PCBs of all processes in the same execution state are placed in the same queue. When the state of a process is changed, its PCB is unlinked from its current queue and moved to its new state queue.

The Operating System maintains the following important process scheduling queues −

• Job queue − This queue keeps all the processes in the system.

• Ready queue − This queue keeps a set of all processes residing in main memory, ready and waiting to execute. A new process is always put in this queue.

• Device queues − The processes which are blocked due to unavailability of an I/O device constitute this queue.

Job queue Ready crueue (CPU) Exit Tlok o waiting - 1 Queue a

Different Scheduling Algorithms

First Come First Serve (FCFS): Simplest scheduling algorithm that schedules according to arrival times of processes. First come first serve scheduling algorithm states that the process that requests the CPU first is allocated the CPU first. It is implemented by using the FIFO queue. When a process enters the ready queue, its PCB is linked onto the tail of the queue. When the CPU is free, it is allocated to the process at the head of the queue. The running process is then removed from the queue. FCFS is a non-preemptive scheduling algorithm.

Note:First come first serve suffers from convoy effect.

Convoy Effect is a situation where many processes, who need to use a resource for short time are blocked by one process holding that resource for a long time.

This essentially leads to poort utilization of resources and hence poor performance.

Advantages:

• FCFS algorithm doesn't include any complex logic, it just puts the process requests in a queue and executes it one by one.
• Hence, FCFS is pretty simple and easy to implement.
• Eventually, every process will get a chance to run, so starvation doesn't occur.

Disadvantages:

• There is no option for pre-emption of a process. If a process is started, then CPU executes the process until it ends.
• Because there is no pre-emption, if a process executes for a long time, the processes in the back of the queue will have to wait for a long time before they get a chance to be executed.

Shortest Job First (SJF NON-preemptive): Process which have the shortest burst time are scheduled first.If two processes have the same bust time then FCFS is used to break the tie. It is a non-preemptive scheduling algorithm.

Shortest Remaining Time First (SRTF preemptive SJF): It is preemptive mode of SJF algorithm in which jobs are schedule according to shortest remaining time.

Advantages: of Shortest Job First scheduling algorithm.

• According to the definition, short processes are executed first and then followed by longer processes.
• The throughput is increased because more processes can be executed in less amount of time.

And the Disadvantages:

• The time taken by a process must be known by the CPU beforehand, which is not possible.
• Longer processes will have more waiting time, eventually they'll suffer starvation.

Note: Preemptive Shortest Job First(SRTF) scheduling will have the same advantages and disadvantages as those for SJF.

Round Robin Scheduling:

• A fixed time is allotted to each process, called quantum, for execution.
• Once a process is executed for given time period that process is preemptied and other process executes for given time period.
• Context switching is used to save states of preemptied processes.

Each process is assigned a fixed time(Time Quantum/Time Slice) in cyclic way.It is designed especially for the time-sharing system. The ready queue is treated as a circular queue. The CPU scheduler goes around the ready queue, allocating the CPU to each process for a time interval of up to 1-time quantum. To implement Round Robin scheduling, we keep the ready queue as a FIFO queue of processes. New processes are added to the tail of the ready queue. The CPU scheduler picks the first process from the ready queue, sets a timer to interrupt after 1-time quantum, and dispatches the process. One of two things will then happen. The process may have a CPU burst of less than 1-time quantum. In this case, the process itself will release the CPU voluntarily. The scheduler will then proceed to the next process in the ready queue. Otherwise, if the CPU burst of the currently running process is longer than 1-time quantum, the timer will go off and will cause an interrupt to the operating system. A context switch will be executed, and the process will be put at the tail of the ready queue. The CPU scheduler will then select the next process in the ready queue.

Advantages: of using the Round Robin Scheduling:

• Each process is served by the CPU for a fixed time quantum, so all processes are given the same priority.
• Starvation doesn't occur because for each round robin cycle, every process is given a fixed time to execute. No process is left behind.

Disadvantages:

• The throughput in RR largely depends on the choice of the length of the time quantum. If time quantum is longer than needed, it tends to exhibit the same behavior as FCFS.
• If time quantum is shorter than needed, the number of times that CPU switches from one process to another process, increases. This leads to decrease in CPU efficiency.

Priority Based scheduling (Non-Preemptive): In this scheduling, processes are scheduled according to their priorities, i.e., highest priority process is scheduled first. If priorities of two processes match, then schedule according to arrival time. Here starvation of process is possible.

Problem with Priority Scheduling Algorithm

In priority scheduling algorithm, the chances of indefinite blocking or starvation.

A process is considered blocked when it is ready to run but has to wait for the CPU as some other process is running currently.

But in case of priority scheduling if new higher priority processes keeps coming in the ready queue then the processes waiting in the ready queue with lower priority may have to wait for long durations before getting the CPU for execution.

Using Aging Technique with Priority Scheduling

To prevent starvation of any process, we can use the concept of aging where we keep on increasing the priority of low-priority process based on the its waiting time.

For example, if we decide the aging factor to be 0.5 for each day of waiting, then if a process with priority 20(which is comparitively low priority) comes in the ready queue. After one day of waiting, its priority is increased to 19.5 and so on.

Doing so, we can ensure that no process will have to wait for indefinite time for getting CPU time for processing.

Advantages of Priority Scheduling:

• The priority of a process can be selected based on memory requirement, time requirement or user preference. For example, a high end game will have better graphics, that means the process which updates the screen in a game will have higher priority so as to achieve better graphics performance.

Some Disadvantages:

• A second scheduling algorithm is required to schedule the processes which have same priority.
• In preemptive priority scheduling, a higher priority process can execute ahead of an already executing lower priority process. If lower priority process keeps waiting for higher priority processes, starvation occurs.

examples:

1. FC FS Burst Time Time Process Arrival 21 P 24 o The 2 24 30 32 aug. waiting time (WT) = 0 +21+24+30 18.75

Shortest Job First ( Non-Preemptive) Process Burst Time Arrival Time Pn 2 L 3.2 Aug. Waiting Time will be= (0 +² +5+11/4 = 4,5 SRTF ( preemptive SJF) Process Burst Time Arrival Time Rpy I 3 0 5 6 . 12 32 Any wating Time: 4,25

Page No. Priority Process Schauling Algorithm I Burst Time Pricerity 32 RIP Rih o 3 24 26 Aug waiting Time: 0+3+2 4+2614 = 13,25 Round Robin Process | Burst Time | Arrival Time o Pa Time quantum = 1 unit o 1 2 3 4 5 f waiting Time = 4 Py waiting Time = 6 Pg waiting Time = 6 6 7 8 4. Aug WT ir a u+6+6/3 = 5:33