Linux process scheduling

　　

The process provides two priorities, one is the normal process priority , and the second is the real-time priority . The former is suitable for sched_normal scheduling strategy, the latter can choose Sched_fifo or SCHED_RR scheduling strategy. At any time, real-time processes are prioritized higher than normal processes , and real-time processes are only preempted by higher-level real-time processes, which are scheduled in terms of FIFO (one-time-chance) or RR (multiple rotation) rules.

1. Scheduling of real-time processes

Real-time processes, with only static priority , because the kernel no longer adjusts its static priority based on factors such as hibernation, which ranges between 0~max_rt_prio-1. The default Max_rt_prio configuration is 100, that is, the default real-time priority range is 0~99. The nice value, in effect, is the process of prioritizing within the max_rt_prio~max_rt_prio+40 range.

Unlike normal processes, when a system is scheduled, a process with a high real-time priority is always preceded by a low-priority process . Real-time processes that know that real-time priorities are high cannot be executed. Real-time processes are always considered active. If there are several real-time processes with the same priority, the system chooses the process in the order in which the process appears on the queue. Assuming that the current CPU is running real-time process A with a priority of a, and a real-time process B with a priority of B enters the operational state, then as long as b<a, the system will interrupt the execution of a, and priority B should be executed until B is unable to execute (regardless of the real-time process of A/b).

Real-time processes of different scheduling strategies are comparable only at the same priority level:

1. For FIFO processes, it means that only the current process is completed before it is executed by another process;

2. For RR processes, once the time slice is consumed, the process is placed at the end of the queue, and then other processes of the same priority are run, and the process continues to execute if there are no other processes of the same priority level.

All in all, for a real-time process, a high-priority process executes first, and it executes until it is impossible to execute, before the low-priority process executes.

2. Scheduling of non-real-time processes

　　Linux is scheduled for normal processes, based on the dynamic priority. The dynamic priority is adjusted by the static priority (Static_prio). Under Linux, the static priority is invisible to the user and hidden in the kernel. The kernel provides the user with an interface that can affect the static priority, which is the nice value, and the relationship is as follows:

Static_prio = Max_rt_prio +nice+ 20

The range of nice values is -20~19, and thus the static priority range is between 100~139. The larger the nice value, the greater the Static_prio, and the lower the final process priority.

When the system is scheduled, other factors will be taken into account, which will result in the calculation of something called the dynamic priority of the process, according to which the scheduling is implemented. Because you should consider not only the static priority, but also the properties of the process. For example, if the process is an interactive process, it can be appropriately prioritized to make the interface more responsive and thus give users a better experience. Linux2.6 has been greatly improved in this respect. Linux2.6 that interactive processes can be judged from an measurement such as average sleep time. The more sleep the process has in the past, the more likely it is to be part of the interactive process. When the system is dispatched, more rewards (bonus) are given to the process so that the process has more opportunities to execute. Awards (bonus) range from 0 to 10.

Linux process scheduling