Several states of the process

What are the process scheduling strategies ？
First come, first served ： Non preemptive scheduling algorithm , Schedule in the order requested . It's good for long work , But it's not good for short work , Because the short job must wait for the previous long job to finish execution , And long assignments take a long time , Cause short work waiting time is too long . in addition , Yes I/O Intensive processes are also bad , Because this process goes on every time I/O After the operation, we have to queue up again .

Short job preferred ： Non preemptive scheduling algorithm , Schedule in the order with the shortest estimated run time . Long work may starve to death , In the state of waiting for the completion of the short job . Because if there are short assignments coming all the time , So long jobs can never be scheduled .

The shortest remaining time is preferred ： Preemptive version of the shortest job first , Schedule in the order of the remaining run time . When a new assignment arrives , Its entire run time is compared to the remaining time of the current process . If the new process takes less time , Suspend the current process , Run a new process . Otherwise new processes wait .

Time slice rotation ： Press all ready processes FCFS The principle of being in a line , Every time when scheduling , hold CPU Time allocated to the team leader process , The process can execute a time slice . When the time slice is used up , Clock interrupt by timer , The scheduler stops the execution of the process , And send it to the end of the ready queue , At the same time, continue to put CPU The process of allocating time to the team leader .

The efficiency of the time slice rotation algorithm has a lot to do with the size of the time slice ： Because process switching needs to save process information and load new process information , If the time slice is too small , Will cause the process to switch too often , Too much time will be spent on process switching . And if the time slice is too long , Then real time can't be guaranteed .

Priority scheduling ： Assign a priority to each process , Schedule by priority . To prevent low priority processes from never waiting to be scheduled , You can increase the priority of the wait process over time .

What are the states of the process ？
There is a total of 5 States , They are to create 、 be ready 、 function （ perform ）、 End 、 Blocking .

The running state is that the process is CPU Up operation . In a single processor environment , At most one process is running at any one time .
Ready state means that the process is ready to run , That is, the process obtains the division of CPU All the resources you need , Once you get it CPU You can run .
A blocking state is when a process is waiting for an event to stop running , For example, waiting for a resource to be available or waiting for I/O complete . Even if CPU Free , The process can't run either .
Running state → Blocking state ： It's often due to waiting for peripherals , Waiting for the allocation of resources such as main memory or waiting for manual intervention . Blocking state → The ready state ： The condition of waiting has been met , Just assign it to the processor and run . Running state → The ready state ： It's not for your own reasons , Instead, the process in running state gives up the processor for external reasons , Now it's ready . For example, the time slice runs out , Or a higher priority process to preempt the processor . The ready state → Running state ： The system selects a process in the ready queue to occupy the processor according to a certain policy , At this point, it becomes the running state .