Operating systems may feature up to three distinct scheduler types : a long-term scheduler also known as an admission scheduler or high-level scheduler , a mid-term or medium-term scheduler , and a short-term scheduler. The names suggest the relative frequency with which their functions are performed. What is the role of scheduler? The Scheduling role involves assembling and coordinating the information, people, materials, equipment, along with all the other necessary resources to get the job done.
Planning comes first and scheduling is done after the plan is finalised. A Scheduler makes the plan happen. What is deadlock explain? Deadlock is a situation where a set of processes are blocked because each process is holding a resource and waiting for another resource acquired by some other process. Hold and Wait: A process is holding at least one resource and waiting for resources. What is a device queue?
Answered Nov 16, The short-term scheduler also known as CPU scheduling selects a process from the ready queue and yields control of the CPU to the process. How do schedulers work? Job scheduling is the process of allocating system resources to many different tasks by an operating system OS.
The system handles prioritized job queues that are awaiting CPU time and it should determine which job to be taken from which queue and the amount of time to be allocated for the job. What is mean by scheduling? Scheduling is the process of arranging, controlling and optimizing work and workloads in a production process or manufacturing process.
Scheduling is used to allocate plant and machinery resources, plan human resources, plan production processes and purchase materials. What is semaphore OS?
In computer science, a semaphore is a variable or abstract data type used to control access to a common resource by multiple processes in a concurrent system such as a multitasking operating system. The job of the long-term scheduler is very important and directly affects the system for a long time.
The short-term scheduler selects one of the processes from the ready queue and schedules them for execution. A scheduling algorithm is used to decide which process will be scheduled for execution next. The short-term scheduler executes much more frequently than the long-term scheduler as a process may execute only for a few milliseconds.
The choices of the short term scheduler are very important. If it selects a process with a long burst time, then all the processes after that will have to wait for a long time in the ready queue. This is known as starvation and it may happen if a wrong decision is made by the short-term scheduler. In these cases, special-purpose job scheduler software is typically used to assist these functions, in addition to any underlying admission scheduling support in the operating system.
Some operating systems only allow new tasks to be added if it is sure all real-time deadlines can still be met. The specific heuristic algorithm used by an operating system to accept or reject new tasks is the admission control mechanism. The medium-term scheduler temporarily removes processes from main memory and places them in secondary memory such as a hard disk drive or vice versa, which is commonly referred to as "swapping out" or "swapping in" also incorrectly as "paging out" or "paging in".
The medium-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 unblocked 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 medium-term scheduler may actually perform the role of the long-term scheduler, by treating binaries 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, or "lazy loaded", also called demand paging. Thus the short-term scheduler makes scheduling decisions much more frequently than the long-term or mid-term schedulers — a scheduling decision will at a minimum have to be made after every time slice, and these are very short.
This scheduler can be preemptive, implying that it is capable of forcibly removing processes from a CPU when it decides to allocate that CPU to another process, or non-preemptive also known as "voluntary" or "co-operative" , in which case the scheduler is unable to "force" processes off the CPU.
A preemptive scheduler relies upon a programmable interval timer which invokes an interrupt handler that runs in kernel mode and implements the scheduling function. Another component that is involved in the CPU-scheduling function is the dispatcher, which is the module that gives control of the CPU to the process selected by the short-term scheduler. It receives control in kernel mode as the result of an interrupt or system call.
The functions of a dispatcher mop the following:. The dispatcher should be as fast as possible, since it is invoked during every process switch. During the context switches, the processor is virtually idle for a fraction of time, thus unnecessary context switches should be avoided.
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