Scheduling

 Scheduling:

            Scheduling is a fundamental operating system function. All computer resources are scheduled before use. Since Microprocessor is one of the primary computer resources, its scheduling is central operating system design.

Scheduling refers to a set of policies and mechanisms supported by operating system that controls the order in which the work to be done is completed. A scheduler is an operating system program that selects the next job to be admitted for execution. The main objective of scheduling is to increase CPU utilization and higher throughput is the amount of work accomplished in a given time interval. CPU scheduling is the basis of operating system which supports multiprogramming concepts. By having a number of programs in computer memory at the same time, the CPU may be shared among them. This mechanism improves the overall efficiency of the computer system by getting more work done in less time. The advantage of multiprogramming is explained in the previous unit. There three different types of schedulers, which make concurrent in operating system.

 


1.      Long term scheduling:  Some time it is also known as job scheduling. The long term, or admission, scheduler decides which jobs or processes are to be admitted to the ready queue, that is when an attempt is made execute a program, its admission to the set of currently executing processes/ tasks is either authorized or delayed by the long-term scheduler. Newly submitted jobs are to be converted into processes and put into the READY QUEUE access to processor. The primary objective of long-term scheduling is balanced mix of jobs to the short-term scheduler. Long term scheduler takes a careful selection of process, that is process should combination of CPU and input/output bound types. If all processes are input/output bound, the READY queue will always be empty and short-term scheduler will have nothing to do. If all processes are CPU bound, no process will be waiting for input/output operations and again a system will be unbalanced. Therefore, long term scheduler provides good performance by selecting combination of CPU bound and input/output bound process.

2.      Medium term scheduler: The mid-term scheduler temporarily removes processes from main memory and places them on secondary memory (such as a disk drive) or vice versa. This is commonly referred to as “swapping out” or “swapping in” (“paging out” or “paging in”). The mid-term scheduler may decide to swap out a process which has not been active for some time, or a process which has a low priority, or a process which is page faulting frequently, or a process which is taking up a large amount of memory in order to free up main memory for other processes, swapping the process back in later when more memory is available, or when the process has been unlocked and is no longer waiting for a resource.

In many systems today (those that support mapping virtual address space to secondary storage other than the swap file), the mid-term scheduler may actually perform the role of the long-term scheduler, by treating pairs as “swapped out processes” upon their execution. In this way, when a segment of the binary is required, it can be swapped in on demand.

3.             Short term scheduler: It allocate processes READY queue to CPU for immediate processing. Its main objective is to maximize CPU requirement compared to other two scheduler it’s more frequent. It must select a new process for executes quite often because a CPU executes a process only for few mill-seconds, before it goes for input and output operation. There are two types of short-term scheduling policies- 1) Preemptive 2) Non-preemptive

1) Non-preemptive:  Non-preemptive algorithms are designed so that once a process enters the running state, it is not removed from the processor until it has completed its service time or explicitly yields the processor.

These are following characteristics of non-preemptive scheduling:

·         In non-preemptive system, short jobs are made to wait by longer jobs but the overall treatment of all processes is fair.

·         In non-preemptive system, response times are more predictable because incoming high priority jobs cannot displace waiting jobs.

·         In non-preemptive scheduling, a scheduler executes jobs in the following two situations.

1.      When a process switches from running state to the waiting state.

2.      When a process terminates.

2) Preemptive: The process with the highest priority should always be the one currently using the processer. If a process is currently using the processor and a new process with a higher priority enters, the ready list, the process on the processor should be removed and returned to the ready list until it is once again the highest priority process in the system.

 

Comments

Post a Comment

Thank you for message

https://asp-net22-23.blogspot.com/

classification of operating System

  Types of Operating System: There are many different Operating Systems. But operating systems are mainly classified in several categories. These are the following – 1)       Single user and single tasking operating system   2)       Single user and multitasking operating system 3)       Multiuser multitasking Operating System   1) Single user single tasking operating system: This operating system is very popular on personal computers. In this system one user works on a machine at a time. One program can be executed at a time. So this operating system is known as ‘Single user operating system’. If you have a large number of jobs to be executed on such a system, then you have to execute them one after another in sequence. For example: Input and output devices like the printer are very slow as compared to the speed of a CPU. When, you give the printing command a CPU reads a file ...
Read more

What is an operating System

Image
  What is an operating System?             An operating system is an essential component of a computer system. The primary objectives of an operating system are to make computer system suitable to use and utilized computer hardware. An operating system is a program designed to run other programs on a computer. A computer’s operating system is its most important program. It is considered the backbone of a computer, managing both software and hardware resources. Operating systems are responsible for everything from the control and allocation of memory to recognizing input from external devices and transmitting output to computer displays. They also manage files on computer hard drives and control peripherals, like printers and scanners. The operating system of a large computer system has even more work to do. Such operating systems monitor different programs and users, making sure everything runs smoothly, without interference,...
Read more

Functions of Operating System

  Functions of Operating System: There are many functions those are performed by the operating system, but the main goal of operating system is to provide the interface between the user and the hardware means provides the interface for working on the system by the user. The various functions those are performed by the operating system, these are the following:- 1)       Input/ Output device management: In a process program takes an input and then produces an output. It takes the input through input devices and produces output through output devices. Thus, it has to handle input and output devices during program execution. It is very tedious to write instructions to directly handle the input and output devices through the user program. Therefore operating system provides device driver to handle input and output devices. Programmers can user these derivers in their program to control the input/output devices. The interaction of the program wit...
Read more

Layered Systems

Image
  Layered Systems: In the layered system structure, operating system is organized as hierarchy of layers, each of the layers is constructed upon one another. The first system constructed in this way was the THE system built at the Technische Hogeschool Eindhoven in the Netherlands by E. W. Dijkstra and his students. The THE system was a simple batch system for a Dutch computer. Layer 0 assign with allocation of the processor, switching between processes when timers expired.    Above layer 0, the system consisted of sequential processes, each of which could be programmed without having to worry about the fact that multiple processes were running on a single processor. In other words, layer 0 provided the basic multiprogramming of the CPU. Layer 1 was used to the memory management. It distributed space for processes in main memory used for holding parts of processes. Layer 1 was taking care of making sure pages are transport into memory whenever they are needed....
Read more

operating system components

Image
Operating System components : An Operating system is an interface between users and the hardware of a computer system. It is system software that is viewed as an organized collection of software consisting of procedures and functions, providing an environment for the execution of programs. The operating system manages resources of system software and computer hardware resources. It allows computing resources to be used in an efficient way. Programs interact with computer hardware with the help of operating system. These are the important component of operating system as follows:               Process Management: The operating system manages many types of activities ranging from user programs to system programs like printer spoiler, name servers, file server etc. Each of these activities is summarize in a process. The five major activities of an operating system in regard to process management are : Creation and deletion of user and sys...
Read more

Services Provided by Operating System

Image
Services Provided by Operating System: An operating system provides an environment for the execution of user programs. It provides certain services to programs and to the users of those programs. The specific services provided, of course, differ from one operating system to another operating system. These operating system services are provided for the convenience of the programmer, to make the programming task easier and more efficiently. The following diagram is shows various operating system-                                Figure: Operating System Services These are the following services provided by operating system- User Interface : Almost all operating systems have a user interface. This interface can take several forms. One is a command-line interface, which uses text commands and a methods for entering them. Another is a batch interface, in which commands and di...
Read more

Batch Operating System

Image
Batch Operating System : In the early days this system was used. Those days,  the jobs were placed in a job queue and the memory allocate managed the primary memory space, when space was available in the main memory, a job was selected from the job queue and it was loaded into memory. Once the job loaded into primary memory, it competes for the processor. When the processor became available, the processor scheduler selects job that was loaded in the memory and execute it. In batch processing is implemented to provide a batch file processing. So in this approach files of the similar batch are processed to speed up the task. It resulted in improved system utilization and performance of machine,  due to reduced turnaround time of execution job.               In batch processing system, earlier the jobs were scheduled in the order of their arrival that first comes first out(FIFO) / FCFS.  This scheduling method was easy and simple to i...
Read more