Protsessi plaanimine

Definitsioon - Protsessi planeermine on protsessi haldaja tegevus, mis haldab jooksvate protsesside eemaldamist CPUst ja mõne teise protsessi valimine kindla strateegia põhjal.
Plaanimise järjekorrad - Viitab järjekorrale protsessis või seadmes. Kui protsess siseneb süsteemi, siis protsess pannakse töö järjekorda. See järjekord koosneb kõikidest protsessidest süsteemis. OS omab ka teisi järjekordi nagu seadmejärjekord. Seadmejärjekord on järjekord, mitu protsessi ootavad kindlat I/O seadet. Igal seadmel on oma seamdejärjekord.

This figure shows the queuing diagram of process scheduling.

• Queue is represented by rectangular box.
• The circles represent the resources that serve the queues.
• The arrows indicate the process flow in the system.
  
  
  
  Queues are of two types
  
• Ready queue
• Device queue
    newly arrived process is put in the ready queue. Processes waits in ready queue for allocating the CPU. Once the CPU is assigned to a process, then that process will execute. While executing the process, any one of the following events can occur.
  
  • The process could issue an I/O request and then it would be placed in an I/O queue.
  • The process could create new sub process and will wait for its termination.
  • The process could be removed forcibly from the CPU, as a result of interrupt and put back in the ready queue.

  Two State Process Model

  -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 case, the dispatcher then selects a process from the queue to execute
  

  Schedulers

  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
  
• Long Term Scheduler
• Short Term Scheduler
• Medium Term Scheduler

• 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.
  

  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.
  

  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.
  
  

  Schedulerite võrdlus

  

Konteksti vahetus

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 context switcher saves the content of all processor registers for the process being removed from the CPU, in its process descriptor. The context of a process is represented in the process control block of a process.
Context switch time is pure overhead. Context switching can significantly affect performance as modern computers have a lot of general and status registers to be saved. Content switching times are highly dependent on hardware support. Context switch requires ( n + m ) bxK time units to save the state of the processor with n general registers, assuming b are the store operations are required to save n and m registers of two process control blocks and each store instruction requires K time units.

Some hardware systems employ two or more sets of processor registers to reduce the amount of context switching time. When the process is switched, the following information is stored.

• Program Counter
• Scheduling Information
• Base and limit register value
• Currently used register
• Changed State
• I/O State
• Accounting