1. Timing of process scheduling

Be careful ：

• Processes cannot be scheduled and switched in the critical area of the operating system kernel program .（√）
• Processor scheduling cannot be performed when the process is in a critical area .（×）

Critical resources ： Resources that only one process is allowed to use in a time period . Processes need mutually exclusive access to critical resources .
A critical region ： The code to access critical resources .

2. The way processes are scheduled

2.1 Non deprivation scheduling mode （ Non preemptive way ）

Only processes are allowed to voluntarily abandon the processor . During operation, even if more urgent tasks arrive , The current process will continue to use the processor , Until the process terminates or actively requests to enter the blocking state .

• Implement a simple
• Low system overhead, but unable to handle urgent tasks in time
• Suitable for early batch processing systems

2.2 Deprive scheduling mode （ Way of seizing ）

When a process is executing on the processor , If a more important or urgent process requires the use of a processor , The executing process is immediately suspended , Allocate processors to more important and urgent processes .

• Priority can be given to more urgent processes
• It can also realize the function of allowing each process to execute in turn according to the time slice （ Interrupt by clock ）
• Suitable for time-sharing operating system 、 Real-time operating system

3. Process switch and process

“ Process scheduling in a narrow sense ” and “ Process switching ” The difference between ：

• Process scheduling in a narrow sense ： Select a process to run from the ready queue .（ This process can be a process that has just been suspended , It can also be another process , In the latter case, process switching is required ）
• Process switching ： A process gives up the processor , A process in which a processor is occupied by another process
• Generalized process scheduling ： It includes two steps: selecting a process and process switching

Process switching is mainly completed ：

1. Save all kinds of data of the original running process
2. Recovery of various data of the new process

Be careful ： Process switching comes at a cost , If you schedule processes too often 、 Switch , It will inevitably reduce the efficiency of the whole system , So that the system spends most of its time on process switching , The time really spent executing the process is reduced .