If removed forcefully, the process creates an interrupt. Thus, once this interrupt completes, the process goes back to the ready queue.<\/li>\n<\/ul>\nObjectives of Process Scheduling in OS<\/h3>\n
Following are the objectives of process scheduling:<\/p>\n
1. It maximizes the number of interactive users within acceptable response times.
\n2. It achieves a balance between response and utilization.
\n3. It makes sure that there is no postponement for an unknown time and enforces priorities.
\n4. It gives reference to the processes holding the key resources.<\/p>\n
Two-State Process Model<\/h3>\n
There are two states in the two-state process model, namely, running state and non-running state.<\/p>\n
1. Running:<\/strong> A new process enters into the system in running state, after creation.<\/p>\n2. Not Running:<\/strong> The not running processes are stored in a queue until their turn to get executed arrives and each entry in the queue points to a particular process. The queue can be implemented using a linked list.<\/p>\nIf a process has been completed or aborted, the OS discards it and if it is interrupted, then the OS transfers it to the waiting queue. Irrespective of either case, the dispatcher then selects a process from the queue to execute.<\/p>\n
Schedulers in OS<\/h3>\n
A scheduler is a special type of system software that handles process scheduling in numerous ways. It mainly selects the jobs that are to be submitted into the system and decides whether the currently running process should keep running or not. If not then which process should be the next one to run. A scheduler makes a decision:<\/p>\n
\n- When the state of the current process changes from running to waiting due to an I\/O request or some unsatisfied OS.<\/li>\n
- If the current process terminates.<\/li>\n
- When the scheduler needs to move a process from running to ready state as it has already run for its allotted interval of time.<\/li>\n
- When the requested I\/O operation is completed, a process moves from the waiting state to the ready state. So, the scheduler can decide to replace the currently-running process with a newly-ready one.<\/li>\n<\/ul>\n
Types of Schedulers in operating System<\/h3>\n
There are three types of schedulers:<\/p>\n
1. Long Term Scheduler:<\/h4>\n
A long-term scheduler also known as a job scheduler determines which program should be admitted to the system for processing. It selects and loads the processes into the memory for execution with the help of CPU scheduling. It provides a balanced combo of jobs, such as I\/O bound and processor bound and controls the degree of multiprogramming.<\/p>\n
A stable degree of multiprogramming means that the average rate of process creation and the average departure rate of processes leaving the system is equal. Many systems like time-sharing OS, do not have a long term scheduler as it is only required when a process changes its state from new to ready.<\/p>\n
2. Short Term Scheduler:<\/h4>\n
A short-term scheduler also known as a CPU scheduler increases system performance as per the chosen set of criteria. This is the change of ready state to running state of the process.<\/p>\n
It selects a process from the multiple processes that are in ready state in order to execute it and also allocates the CPU to one of them. It is faster than long-term schedulers and is also called a dispatcher as it makes the decision on which process will be executed next.<\/p>\n
3. Medium Term Scheduler:<\/h4>\n
Medium-term scheduling removes processes from the memory and is a part of swapping. It reduces the degree of multiprogramming and is in-charge of handling the swapped out processes. Swapping is necessary to improve the process mix.<\/p>\n
When a running process makes an I\/O request it becomes suspended i.e., it cannot be completed. Thus, in order to remove the process from the memory and make space for others, the suspended process is sent to the secondary storage. This is known as swapping, and the process that goes through swapping is said to be swapped out or rolled out.<\/p>\n
Comparison of OS Schedulers<\/h3>\n
\n\n\n| S.No.<\/b><\/td>\n | Long-Term Scheduler<\/b><\/td>\n | Short-Term Scheduler<\/b><\/td>\n | Medium-Term Scheduler<\/b><\/td>\n<\/tr>\n\n| 1.<\/span><\/td>\n | A job scheduler<\/span><\/td>\n | A CPU scheduler<\/span><\/td>\n | A process swapping scheduler<\/span><\/td>\n<\/tr>\n\n| 2.<\/span><\/td>\n | Slowest speed<\/span><\/td>\n | Fastest Speed<\/span><\/td>\n | Speed is between the other two<\/span><\/td>\n<\/tr>\n\n| 3.<\/span><\/td>\n | Controls the degree of multiprogramming<\/span><\/td>\n | Provides less control over the degree of multiprogramming<\/span><\/td>\n | Reduces the degree of multiprogramming<\/span><\/td>\n<\/tr>\n\n| 4.<\/span><\/td>\n | Absent or minimal in the time-sharing OS<\/span><\/td>\n | Minimal in time-sharing OS<\/span><\/td>\n | Part of time-sharing OS<\/span><\/td>\n<\/tr>\n\n| 5.<\/span><\/td>\n | Selects a process from pool and loads it into memory for execution<\/span><\/td>\n | Selects a process that is ready for execution<\/span><\/td>\n | Re-introduces processes into memory for continued execution<\/span><\/td>\n<\/tr>\n<\/tbody>\n<\/table>\nContext Switch in OS<\/h3>\nA context switch is an important feature of multitasking OS that can be used to store and restore the state or context of a CPU in a PCB, so that the execution of a process can be resumed from that very point at a later time. A context switch allows multiple processes to share a single CPU. Some hardware systems even employ two or more sets of processor registers, in order to avoid context switching time.<\/p>\n When the scheduler switches the CPU from one process to another, the state of the current running process is stored into the PCB and the state for the next process is loaded from its own PCB. This is then used to set the PC, registers, etc. Since the register and memory state is saved and restored in context switches, they are computationally intensive. Following information is stored during switching:<\/p>\n | | | | | |
![\"Process](\"https:\/\/data-flair.training\/blogs\/wp-content\/uploads\/sites\/2\/2021\/07\/Process-Scheduling-Queues.jpg\")
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