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Operating System

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This document discusses different types of process schedulers in an operating system: 1. Long term schedulers determine which programs are admitted to the system and load them into memory. 2. Short term (CPU) schedulers select ready processes and allocate the CPU to one of them. 3. Medium term schedulers handle swapped out processes and can reintroduce processes back into memory. Context switching allows multiple processes to share the CPU by storing and restoring a process's state from its process control block when switching between processes. This switching is computationally expensive so some hardware uses multiple register sets.

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BY G.GOWRILATHA,M.SC(INFO TECH) DEPARTMENT OF CS & IT NADAR SARASWATHI COLLEGE OF ARTS AND SCIENCE, THENI. PROCESS SCHEDULING

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An operating system executes a variety of programs: ✦ batch system – jobs ✦ time-shared systems – user programs or tasks textbook uses the terms job and process almost interchangeably. Process – a program in execution; process execution must progress in sequential fashion. A process includes: ✦ program counter ✦ stack ✦ data section PROCESS SCHEDULING

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As a process executes, it changes state ✦ new: the process is being created. ✦ Running: instructions are being executed. ✦ Waiting: the process is waiting for some event to occur. ✦ Ready: the process is waiting to be assigned to a process. ✦ Terminated: the process has finished execution.

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Process termination process executes last statement and asks the operating system to decide it (exit). ✦ Output data from child to parent (via wait). ✦ Process’ resources are deallocated by operating system. Parent may terminate execution of children processes (abort). ✦ Child has exceeded allocated resources. ✦ Task assigned to child is no longer required. ✦ Parent is exiting. ✔ Operating system does not allow child to continue if its parent terminates. ✔ Cascading termination.

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Cooperating processes independent process cannot affect or be affected by the execution of another process. Cooperating process can affect or be affected by the execution of another process advantages of process cooperation ✦ information sharing ✦ computation speed-up ✦ modularity ✦ convenience

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Definition: The process scheduling is the activity of the process manager that handles the removal of the running process from the CPU and the selection of another process on the basis of a particular strategy. Process scheduling is an essential part of a Multiprogramming operating system. Such operating systems allow more than one process to be loaded into the executable memory at a time and loaded process shares the CPU using time multiplexing.

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PROCESS SCHEDULING QUEUES The OS maintains a separate queue for each of the process states and PCBs of all processes in the same execution state are placed in the same queue. When the state of a process is changed, it's PCB is unlinked from its current queue and moved to its new state queue.  Job queue - This queue keeps all the processes in the system. Ready queue - This queue keeps set of all processes residing in main memory, ready and waiting to execute. A new process is always put in this queue.  Device queues - The processes which are blocked due to unavailability of an I/O device constitute this queue.

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Operating System

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Two State Process Model 1 . 2. Running When new process is created by Operating System that process enters into the system as in the running state. Not Running Processes that are not running are kept in queue, waiting for their turn to execute. Each entry in the queue is a pointer to a particular process. Queue is implemented by using linked list. Use of dispatcher is as follows. When a process is interrupted, that process is transferred in the waiting queue. If the process has completed or aborted, the process is discarded. In either

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schedulers are special system softwares which handles process scheduling in various ways.their main task is to select the jobs to be submitted into the system and to decide which process to run. schedulers are of three types 1.long term scheduler 2.short term scheduler 3.medium term scheduler Schedulers

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long Term Scheduler: it is also called job scheduler. long term scheduler determines which programs are admitted to the system for processing. job scheduler selects processes from the queue and loads them into memory for execution. process loads into the memory for cpu scheduling. the primary objective of the job scheduler is to provide a balanced mix of jobs, such as i/o bound and processor bound. it also controls the degree of multiprogramming. if the degree of multiprogramming is stable, then the average rate of process creation must be equal to the average departure rate of processes leaving the system. on some systems, the long term scheduler may not be available or minimal. time-sharing operating systems have no long term scheduler. when process changes the state from new to ready, then there is use of long term scheduler.

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Short Term Scheduler: It Is Also Called CPU Scheduler. Main Objective Is Increasing System Performance In Accordance With The Chosen Set Of Criteria. It Is The Change Of Ready State To Running State Of The Process. CPU Scheduler Selects Process Among The Processes That Are Ready To Execute And Allocates CPU To One Of Them. Short Term Scheduler Also Known As Dispatcher, Execute Most Frequently And Makes The Fine Grained Decision Of Which Process To Execute Next. Short Term Scheduler Is Faster Than Long Term Scheduler.

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Medium Term Scheduler: Medium Term Scheduling Is Part Of The Swapping. It Removes The Processes From The Memory. It Reduces The Degree Of Multiprogramming. The Medium Term Scheduler Is In-charge Of Handling The Swapped Out-processes. Running Process May Become Suspended If It Makes An I/O Request. Suspended Processes Cannot Make Any Progress Towards Completion. In This Condition, To Remove The Process From Memory And Make Space For Other Process, The Suspended Process Is Moved To The Secondary Storage. This Process Is Called Swapping, And The Process Is Said To Be Swapped Out Or Rolled Out. Swapping May Be Necessary To Improve The Process Mix.

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S.N. Long Term Scheduler Short Term Scheduler Medium Term Scheduler 1. It is a job scheduler It is a CPU scheduler It is a process swapping scheduler. 2. Speed is lesser than short term scheduler Speed is fastest among other two Speed is in between both short and long term scheduler. 3. It controls the degree of multiprogramming It provides lesser control over degree of multiprogramming It reduces the degree of multiprogramming. 4. It is almost absent or minimal in time sharing system It is also minimal in time sharing system It is a part of Time sharing systems. 5. It selects processes from pool and loads them into memory for execution It selects those processes which are ready to execute It can re-introduce the process into memory and execution can be continued.

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CONTEXT SWITCH: A context switch is the mechanism to store and restore the state or context of a cpu in process control block so that a process execution can be resumed from the same point at a later time. using this technique a context switcher enables multiple processes to share a single cpu. context switching is an essential part of a multitasking operating system features. when the scheduler switches the cpu from executing one process to execute another, the state from the current running process is stored into the process control block. after this completes, the state for the process to run next is loaded from its own pcb and used to set the pc, registers, etc. at that point, the second process can begin executing.

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context switches are computationally intensive since register and memory state must be saved and restored and to avoid the amount of context switching time, some hardware systems employ two or more sets of processor registers. when the process is switched, the following information is stored for later use. ->program counter ->scheduling information ->base and limit register value ->currently used register ->changed state ->i/o state information ->accounting information

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Thank You..

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Operating System

  • 1. BY G.GOWRILATHA,M.SC(INFO TECH) DEPARTMENT OF CS & IT NADAR SARASWATHI COLLEGE OF ARTS AND SCIENCE, THENI. PROCESS SCHEDULING
  • 2. An operating system executes a variety of programs: ✦ batch system – jobs ✦ time-shared systems – user programs or tasks textbook uses the terms job and process almost interchangeably. Process – a program in execution; process execution must progress in sequential fashion. A process includes: ✦ program counter ✦ stack ✦ data section PROCESS SCHEDULING
  • 3. As a process executes, it changes state ✦ new: the process is being created. ✦ Running: instructions are being executed. ✦ Waiting: the process is waiting for some event to occur. ✦ Ready: the process is waiting to be assigned to a process. ✦ Terminated: the process has finished execution.
  • 4. Process termination process executes last statement and asks the operating system to decide it (exit). ✦ Output data from child to parent (via wait). ✦ Process’ resources are deallocated by operating system. Parent may terminate execution of children processes (abort). ✦ Child has exceeded allocated resources. ✦ Task assigned to child is no longer required. ✦ Parent is exiting. ✔ Operating system does not allow child to continue if its parent terminates. ✔ Cascading termination.
  • 5. Cooperating processes independent process cannot affect or be affected by the execution of another process. Cooperating process can affect or be affected by the execution of another process advantages of process cooperation ✦ information sharing ✦ computation speed-up ✦ modularity ✦ convenience
  • 6. Definition: The process scheduling is the activity of the process manager that handles the removal of the running process from the CPU and the selection of another process on the basis of a particular strategy. Process scheduling is an essential part of a Multiprogramming operating system. Such operating systems allow more than one process to be loaded into the executable memory at a time and loaded process shares the CPU using time multiplexing.
  • 7. PROCESS SCHEDULING QUEUES The OS maintains a separate queue for each of the process states and PCBs of all processes in the same execution state are placed in the same queue. When the state of a process is changed, it's PCB is unlinked from its current queue and moved to its new state queue.  Job queue - This queue keeps all the processes in the system. Ready queue - This queue keeps set of all processes residing in main memory, ready and waiting to execute. A new process is always put in this queue.  Device queues - The processes which are blocked due to unavailability of an I/O device constitute this queue.
  • 9. Two State Process Model 1 . 2. Running When new process is created by Operating System that process enters into the system as in the running state. Not Running Processes that are not running are kept in queue, waiting for their turn to execute. Each entry in the queue is a pointer to a particular process. Queue is implemented by using linked list. Use of dispatcher is as follows. When a process is interrupted, that process is transferred in the waiting queue. If the process has completed or aborted, the process is discarded. In either
  • 10. schedulers are special system softwares which handles process scheduling in various ways.their main task is to select the jobs to be submitted into the system and to decide which process to run. schedulers are of three types 1.long term scheduler 2.short term scheduler 3.medium term scheduler Schedulers
  • 11. long Term Scheduler: it is also called job scheduler. long term scheduler determines which programs are admitted to the system for processing. job scheduler selects processes from the queue and loads them into memory for execution. process loads into the memory for cpu scheduling. the primary objective of the job scheduler is to provide a balanced mix of jobs, such as i/o bound and processor bound. it also controls the degree of multiprogramming. if the degree of multiprogramming is stable, then the average rate of process creation must be equal to the average departure rate of processes leaving the system. on some systems, the long term scheduler may not be available or minimal. time-sharing operating systems have no long term scheduler. when process changes the state from new to ready, then there is use of long term scheduler.
  • 12. Short Term Scheduler: It Is Also Called CPU Scheduler. Main Objective Is Increasing System Performance In Accordance With The Chosen Set Of Criteria. It Is The Change Of Ready State To Running State Of The Process. CPU Scheduler Selects Process Among The Processes That Are Ready To Execute And Allocates CPU To One Of Them. Short Term Scheduler Also Known As Dispatcher, Execute Most Frequently And Makes The Fine Grained Decision Of Which Process To Execute Next. Short Term Scheduler Is Faster Than Long Term Scheduler.
  • 13. Medium Term Scheduler: Medium Term Scheduling Is Part Of The Swapping. It Removes The Processes From The Memory. It Reduces The Degree Of Multiprogramming. The Medium Term Scheduler Is In-charge Of Handling The Swapped Out-processes. Running Process May Become Suspended If It Makes An I/O Request. Suspended Processes Cannot Make Any Progress Towards Completion. In This Condition, To Remove The Process From Memory And Make Space For Other Process, The Suspended Process Is Moved To The Secondary Storage. This Process Is Called Swapping, And The Process Is Said To Be Swapped Out Or Rolled Out. Swapping May Be Necessary To Improve The Process Mix.
  • 14. S.N. Long Term Scheduler Short Term Scheduler Medium Term Scheduler 1. It is a job scheduler It is a CPU scheduler It is a process swapping scheduler. 2. Speed is lesser than short term scheduler Speed is fastest among other two Speed is in between both short and long term scheduler. 3. It controls the degree of multiprogramming It provides lesser control over degree of multiprogramming It reduces the degree of multiprogramming. 4. It is almost absent or minimal in time sharing system It is also minimal in time sharing system It is a part of Time sharing systems. 5. It selects processes from pool and loads them into memory for execution It selects those processes which are ready to execute It can re-introduce the process into memory and execution can be continued.
  • 15. CONTEXT SWITCH: A context switch is the mechanism to store and restore the state or context of a cpu in process control block so that a process execution can be resumed from the same point at a later time. using this technique a context switcher enables multiple processes to share a single cpu. context switching is an essential part of a multitasking operating system features. when the scheduler switches the cpu from executing one process to execute another, the state from the current running process is stored into the process control block. after this completes, the state for the process to run next is loaded from its own pcb and used to set the pc, registers, etc. at that point, the second process can begin executing.
  • 16. context switches are computationally intensive since register and memory state must be saved and restored and to avoid the amount of context switching time, some hardware systems employ two or more sets of processor registers. when the process is switched, the following information is stored for later use. ->program counter ->scheduling information ->base and limit register value ->currently used register ->changed state ->i/o state information ->accounting information