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CPU Scheduling: What it is and its roles?

CPU Scheduling

CPU Scheduling

CPU scheduling aims to optimize the utilization of CPU. It is a process that allows the system to carry out multiple processes at once. In case of scheduling, the CPU keeps a process on hold while the other one is being executed. This is done due to the unavailability of the resources. The scheduling ensures that all the process in the CPU are being executed in a timely manner and the system is utilizing the full capacity.

The CPU scheduling not only makes the system more efficient but it also increases the speed. When the scheduling is done, the operating system focuses on choosing and executing one of the processes available in the ready queue. A short-term scheduler or CPU scheduler is used to execute the selection process. It selects the processes which are ready to complete. The schedule also allocates the processes to the CPU. Now our assignment helpers from Australia will guide you through about the waiting time in CPU.

Waiting Time in CPU

In computing, almost every program is executed on the basis of some alternating cycle. When these programs are being executed, sometimes waiting for the input and the output becomes inevitable. It occurs due to the differences between the CPU and the memory speed. CPU is able to execute an instruction in a shorter time. On the contrary, fetching data from memory is a more time-consuming process.

So, the CPU needs to wait when the memory is fetching the data. During the time, the CPU remains idle. In other words, the waiting time and the CPU are wasted in the process. It makes the processes more time-consuming. Waiting for input and outputs for a long time affects the overall efficiency of the system. The scheduling solves the problem by keeping different programs in the queue.

 When the scheduling system is in operation, CPU becomes able to perform one process when the input or output for the others is not available. It prevents the loss of CPU cycles. Besides it also ensures full use of the CPU cycles. The efficiency of a system is affected due to the long waiting time and loss of CPU cycles.

So, enhancing the efficiency of the system becomes quite challenging. The systems should operate in a fair and efficient manner. It becomes difficult in varying dynamic condition. Additionally, prioritization of the tasks is another factor that needs to be considered while executing processes in the CPU. One of our programming assignment helper has explained below what is the concept of queues in CPU scheduling.

Queues Involved in Scheduling

 There are three types of queues which are involved with the CPU usage are—

  • Job queue: It includes all processes which are being or will be executed by the CPU. In other words, the processes which are once submitted to the CPU, reside in the job queue. The processes in the job queue are allocated by the long-term scheduler.
  • Ready queue: It includes the processes which are currently in memory only. These processes remain at the ready state. While being in the queue, the processes wait for execution. These processes are allocated by the CPU scheduler or the short-term CPU.
  • Device queue: It includes the processes which are waiting for a device. In a CPU, multiple processes can wait for the same device. In such situations, I/O completion sends the process back to the ready queue.

Components of CPU Scheduling

 The scheduling process is done with the help of CPU Burst Cycle, Dispatcher and the scheduler.

  • CPU Burst Cycle: Every process includes CPU burst cycle and I/O burst cycle. The duration of CPU burst cycle varies on the basis of the processes.
  • Scheduler: The scheduler operates when the processor becomes idle. It chooses another process which is ready to run from the queue. The storage structure of the ready queue plays a key role in determining which process needs to be executed. The algorithm is another factor that deals with the selection process. The scheduler works on the basis of these two factors and selects the most appropriate process accordingly.
  • Dispatcher: Dispatcher is another component that is involved in CPU scheduling. It is a module plays a significant part in the scheduling process. It transfers the control of the CPU to the next process which needs to be executed as chosen by the short-term scheduler. The function is done in the following steps—
  1. Switching of the processes
  2. The user mode is switched
  3.  The processor reaches the appropriate location in the user program. The destination, in this case, is the same location where the program was left last time.

The dispatcher requires operating at a faster speed to manage every process switch. The time needed by the dispatcher in order to terminate one process and switch to another one is known as the Dispatch Latency. 

There are some instances when there is a need for scheduling the CPU, these situations can be like: Switching from Running State to Ready State, Switching from Running State to Ready State, Switching from Running State to Ready State and Switching from Waiting State to Ready State.

Scheduling Algorithm

 The scheduling in CPU is done on the basis of different algorithms. The selection of the algorithm depends on a number of factors—

CPU Utilization: To ensure the best use of CPU, preventing the wastage of CPU cycle is necessary. It is achieved when the CPU works most of the time. Ideally, the CPU should utilize 100% of the time available to it. However, when a real system is considered, CPU utilization ranges from 40% in case of the lightly loaded systems to 90%, in case of the heavily loaded systems. It indicates that the workload of the system plays an important part to choose the scheduling algorithm.

Throughput: The total number of processes finished per unit time by the CPU is described as the throughput. In other words, the total amount of work completed by the CPU in a unit of time is considered as the throughput. The throughput level varies with the process. In some cases, it might be 10/second while in others it can reduce to 1/hour based on the requirement of specific processes.

Turnaround Time: It is the time which is required for executing the particular processes. It can also be described as the interval between submission of one process to the time when the process is completed.

Waiting Time: It is the cumulative amount of period which the process spends by waiting in the queue at ready state. In other words, it can be described by the time after which the process gains control over the CPU.

Load Average: It indicates the average number of processes which are residing in the ready queue and waiting for the turn to take control over the CPU.

Response Time: When CPU receives an instruction, it takes some time to respond. This duration is called the response time. While choosing the CPU scheduling, it is ensured that the Throughput and CPU utilization are maximized. It is also ensured that other factors are reduced to optimize utilization. It indicates that scheduling plays a key role to make the system faster and more efficient. The advantages obtained from the scheduling depends on the selection of the specific algorithms—

First Come First Serve: In the case of the “First come first serve” scheduling algorithm, the work is done in the same manner as the name suggests. It indicates that the process which is received by the CPU first is executed first. In other words, the process which sends a request to the CPU first gets the chance to be allocated first.

First Come First Serve algorithm is similar to the FIFO (First in First out) in the Queue data structure. In the case of FIFO also, the data which is included in the queue first is the one which is selected to leave the queue first. The application of this approach is found in the batch system.

The First Come First Serve algorithm is advantageous because is simple to interpret and implement to the system through programming. This approach also involves the use of a Queue data structure. In this way, a new process gets included at the tail end of the queue. On the contrary. the scheduler in the CPU chooses the process from the head of the queue structure. Purchasing tickets from the ticket counter is the real-life example of the scheduling.

 Despite the advantages such as ease of use, there are some issues associated with the algorithm also. Firstly, It is Non-Preemptive in nature. It indicates that the process priority does not have any significance in this case. Therefore, use of the FCFS scheduling might create the situations when the processes with the minimum priority are being executed. For example, the regular routine backup processes fall into the category of least priority. Such processes are time-consuming in nature. While executing these processes as per the FCFS system, the switching does not occur even if the high priority requests are received. It affects the effectiveness of the system. Optimal Average Waiting Time is not achieved in this case. It is another drawback of the FCFS system. The scheduling in this method also does not facilitate the system to utilize the resources in a parallel manner. It leads to poor resource utilization and causes Convoy effect. It is another disadvantage associated with FCFS scheduling.

Shortest Job First Scheduling: Reducing the waiting time is one of the key goals of the scheduling process. Shortest Job First Scheduling is one of the best approaches which minimize waiting time for the particular processes. This type of scheduling is also applied in the batch systems. This type of scheduling is categorized into two types. These are non-Pre-emptive and Pre-emptive. For implementing the method successfully, knowledge on the duration time or the burst time is necessary for the processor. However, practically, knowing the duration time or the burst time is not possible for all the processes. It also indicates that the processor should be aware of the processes before execution. It is also not feasible in every case. The shortest job first scheduling provides the optimum result when every process or job is available at the same time. It indicates that the highest efficiency is achieved when all the processes have the same arrival time.

Priority Scheduling: In this scheduling system, every process is assigned with a priority. The processes which are associated with the highest priority require being executed first. When the CPU encounters multiple processes with the same priority, the scheduling is done in FCFS manner. The priority of a task depends on different factors such as time requirements, memory requirements and other resource requirements.

Round Robin Scheduling: The round-robin is another schedule which is used to optimize the performance of the CPU. In this case, each process gets a fixed time to take control of the CPU for execution. The duration is called quantum. When a process gets the chance to be executed for a fixed time, it is pre-empted. After the time, other processes are executed. The Context switching saves the states of the pre-emptied processes.

Multilevel Scheduling: It is another type of algorithms. These algorithms are developed to deal with the situations in which the processes from the different class are present. For instance, batch or interactive processes fall into different classes. The response-time requirements for these processes are different. So, scheduling needs also vary. Additionally, the foreground processes are provided with higher priority than the background processes. When the multi-level queue scheduling algorithm is used, the ready queue is separated into different parts. Each process is allocated to one queue permanently. This allocation depends on the particular properties of the processes. For instance, the process priority, memory size and process type influence the allocation of sub-queues to the processes. The foreground and background processes are executed using separate queues.

Multilevel Feedback Queue Scheduling: In the case of a multilevel queue-scheduling algorithm, processes are assigned permanently to a queue after entering the system. Once assigned, the processes do not switch between queues. This algorithm is advantageous because of the low scheduling overhead. However, this is inflexible in nature. It is a disadvantage of the process. For more information, you can visit allassignmenthelp.com US and sign up to get your online academic assignment done.

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