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CPU Scheduling In Operating System


  
    ⇰  CPU SCHEDULING :-

 CPU scheduling is the basis of multiprogrammed operating systems. By switching the CPU among processes, the operating system can make the computer more productive. As we know in a single-processor system, only one process can run at a time and thers must wait until the CPU is free and can be rescheduled. The objective of multiprogramming is to have some process running at all times, to maximize CPU utilization.  
    
  In a simple computer system, the CPU just sits idle when a process goes for I/O works. All this waiting time is wasted,no useful work is accomplished. With multiprogramming, we try to use this time productively. Several processes are kept in memory at one time. when one process has to wait, the operating system takes the CPU away from that process and gives the CPU to another process. This pattern continues. Every time one process has to wait, another process can take over use of the CPU.

 CPU Scheduling is a process of determining which process will own CPU for execution while another process is on hold. The main task of CPU scheduling is to make sure that whenever the CPU remains idle, the OS at least select one of the processes available in the ready queue for execution. The selection process will be carried out by the CPU scheduler. It selects one of the processes in memory that are ready for execution.
    ⇰  SCHEDULING CRITERIA :-

 In choosing which algorithm to use in a particular situation, we must consider the properties of the various algorithms. Many criteria have been suggested for comparing CPU-scheduling algorithms. Basically CPU scheduling  algorithms depend on following criterias:-
   ∘ CPU utilization.
   ∘ Throughput.
   ∘ Turnaround Time.
   ∘ Waiting time.
   ∘ Response time.
  ∘ CPU utilization :-  In multiprogramming we have to keep the CPU  busy all time. CPU utilization can range from 0 to 100 percent. But normally it range from 40 percent to 90 percent.

 ∘ Throughput :- Throughput  is the number of processes that are completed per time unit. For larger processes, this rate may be one process per hour, for shorter processes it may be ten processes per second.

  ∘ Turnaround Time :-  Turnaround time is the sum of the periods spent waiting to get into memory, waiting in the ready queue, executing on the CPU, and doing I/O. So the turnaround time is interval from the time of submission of a process to the time of completion of process.

 ∘ Waiting time :- Waiting time is the sum of the periods spent waiting in the ready queue during the execution of process.

 ∘ Response time :- Response time is the duration of time from the submission of a request until the first response is produced. So response time is the time it takes to start responding.The turnaround time is generally limited by the speed of the output device.


     ⇰  SCHEDULING ALGORITHMS :-

 There are many types of scheduling algorithms but all those are basically divided into two types:-
 i > Nonpreemptive or Cooperative. 
ii > Preemptive.

  i > nonpreemptive or cooperative :- In nonpreemptive scheduling, once the CPU has been allocated to a process, the process keeps the CPU until it releases the CPU either by terminating or by switching to the waiting state.

  ii > Preemptive :- In preemptive scheduling the CPU can be preempted or taken from the process when a new process arrives with more priority then currently running process.

CPU Scheduling Algorithms in Operating Systems



From here we will watch different types of scheduling algorithms in next comming post.  

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