Cpu scheduling pdf

P1 runs for 4ms. P2 arrived at 2 ms but P1 continued as burst time of P2 is longer than P1. After P1 completes, P0 is scheduled again as the remaining time for P0 is less than the burst time of P2. P0 waits for 4 ms, P1 waits for 0 ms and P2 waits for 11 ms. What is the average turnaround time for these processes with the preemptive shortest remaining processing time first SRPT algorithm? Skip to content. Change Language. Related Articles. CPU Scheduling. Process Synchronization. Memory Management.

Disk Management. Disk Scheduling. Operating System Quizes. Table of Contents. Improve Article. Save Article. Like Article. Previous Difference between Process and Thread. Next Preemptive and Non-Preemptive Scheduling.

Recommended Articles. Article Contributed By :. Easy Normal Medium Hard Expert. Writing code in comment? Please use ide. Load Comments. The dispatcher is the module that gives control of the CPU to the process selected by the short-term scheduler. Process with highest priority is to be executed first and so on.

The interval from time of submission of the process to the time of completion of the process Wall clock time. It is the only method that can be used on certain hardware platforms, because It does not require the special hardware for example: It is the total number of processes completed per unit time or rather say total amount of work done in a unit of time. It is the average number of processes residing in the ready queue waiting for their turn to get into the CPU.

When a process switches from the running state to the ready state for example, when an interrupt occurs. Amount of time it takes from when a request was submitted schdeuling the first response is produced. The time taken by the dispatcher zcheduling stop one process and start another process is known as the Dispatch Latency. There is a choice, however in circumstances 2 and 3. At times it is necessary to run a certain task that has a higher priority before another task although it is running. Operating System Scheduling algorithms Advertisements.

Another component involved in the CPU scheduling function is the Dispatcher. The scheduler selects from among the processes in memory that are ready to execute, and allocates the CPU to one of them.

The sum of the periods spent waiting in the ready queue amount of time a process has been waiting in the ready queue to acquire scheduing control on the CPU.

Multiple-level queues are not an independent scheduling algorithm. Switching context Switching to user mode Jumping to the proper location in the user program to restart that program from numerical it left last time.

We will be discussing all the scheduling algorithms, one by one, in detail in the next tutorials. What is CPU Scheduling? Whenever the CPU becomes idle, the operating system must select one of the processes in the ready queue to be executed. Each process is assigned a priority.

A Process Scheduler schedules different processes to be assigned to the CPU based on particular scheduling algorithms. Response times are more expectable because incoming process with higher priority cannot move process in waiting. Shortest job first scheduling gives the shortest average waiting time for the given processes so this is probably optimal. If processor executes short process first rather than long process it may take a less waiting time for execution of the short process and more waiting time for execution of the long process.

Therefore, the waiting time decreases. The real problem in the shortest job first is getting the length of the CPU burst of the next process. Shortest job first is used in long-term scheduling but is not applied at short term CPU scheduling because the processor has no information about the CPU Burst size of next process. Every process has its own priority. A queue is designed in which highest priority processes are queued firstly. The processor is issued to that process that has the highest priority from all given processes.

Priority of each process is calculated by the user or by using some predefined methods. The processor may define the priority based on how much memory or time is required to each process for execution. And which type of resources will be used by each process. Different processes that have equal priority are queued as the order of first come first served. Priority scheduling is a scheduling algorithm in which we deal processes with high priority and low priority.

Minimum numbers are used by systems to signify low priority and other systems use minimum numbers for representing high priority. This difference can create confusion.

Now we assume that low numbers represent high priority. Priorities can be based on internal factors and external factors. Internal factors may include time limits and how much memory is required to that process. Priority scheduling can be either preemptive or non-preemptive. When a task reaches in the ready queue from the state of waiting, its priority is matched with the priority of that process that in the state of execution.

A Non- Preemptive Scheduling is a scheduling in which one process that is in processing state cannot be separated from the processor. A small element of time is known as the time slice.

For the implementation of this scheduling, the ready queue is treated as first in first out queue of processes. A new process is added at the end of the ready queue. CPU Scheduler nominates the first process from the ready queue, then sets the interrupt time after one-time slice and then the process is dismissed. A ready queue is made for completion of the processes.

A process may have its CPU burst less than the one-time slice. This process will free the CPU willingly after the completion. After the release of Central Processing Unit, the Scheduler moves to the process which is next in ready queue.

An interrupt will be created to the operating system of the computer by stopping the timer if the process in execution has higher CPU burst length than a one-time slice. A process is loaded at the end of the ready queue by applying context switching Process of loading and reloading the state is applied and the. One time the Scheduler starts its working again and takes the process which is next in the queue. In round robin scheduling the average waiting time for the processes is higher than other scheduling algorithms.

But it needs more 20 milliseconds for completing its execution, it is restricted after the time slice used by the first process, and CPU is provided to the next process in the queue. Then the CPU is provided to the next process. When the CPU is provided to every process in the ready queue for the one- time slice then the CPU is provided for an additional time slice for the process P1.

The average waiting time for the processes is 7. In the Round Robin scheduling algorithm, CPU is not provided to a process for besides one- time slice straightly except it is the one and only process for execution.

If a CPU burst length of a process goes above one-time slice, that particular process is restricted and is return at last of the ready queue. So this Scheduling algorithm is preemptive scheduling. The waiting time for a process is not more than n-1 x q. For example, in the ready queue there are four processes and for the processing, there is a time slice of 25 milliseconds, 25 milliseconds will be taken by each process every milliseconds.

But there will be a lot of shifting of processes if the time slice is too small. For example, if the process time units are 6 and the time slice are 8-time units than the process will be executed in a shorter time than 1-time slice having no context switching. If the process time units are 12 and the time slice is shorter, i. If the process needs 6 times unit and a time slice is a 1-time unit, there will be 5 contexts switching.

So the time slice should be larger than the process. The context switching time is less than ten microseconds.