Process scheduling is a critical aspect of operating system design, determining which processes run when and for how long. In Linux, the scheduler is responsible for allocating CPU time to various processes, ensuring efficient and fair use of system resources. Understanding process scheduling is essential for system administrators and developers to optimize performance and ensure smooth operation of applications. This article will explore the fundamentals of process scheduling in Linux, provide practical examples, and demonstrate how to manage and manipulate process priorities using command-line tools.

Examples:

1. Viewing Process Priorities with top and htop: The top and htop commands provide real-time views of running processes, including their priorities and CPU usage.

   top

   htop

   These commands display a list of processes with columns for PID, user, priority (PR), nice value (NI), and CPU usage (%CPU). The priority and nice values influence the scheduling of processes.

2. Changing Process Priority with nice and renice: The nice command starts a process with a specified priority, while renice changes the priority of an existing process.

   # Start a process with a lower priority (higher nice value)
   nice -n 10 my_process

   # Change the priority of an existing process
   renice -n 5 -p 1234

   In these examples, my_process is started with a nice value of 10, and the process with PID 1234 has its nice value changed to 5. Lower nice values result in higher priority.

3. Using chrt for Real-Time Scheduling: The chrt command sets or retrieves the real-time scheduling attributes of a process.

   # Start a process with a real-time priority
   chrt -f 10 my_realtime_process

   # Change the scheduling policy and priority of an existing process
   chrt -r -p 15 5678

   Here, my_realtime_process is started with a FIFO (First In, First Out) real-time priority of 10, and the process with PID 5678 is assigned a Round Robin (RR) policy with a priority of 15.

4. Scheduling Policies: Linux supports several scheduling policies, including SCHED_OTHER (default), SCHED_FIFO, SCHED_RR, and SCHED_BATCH. These policies can be set using the chrt command or programmatically via system calls.

   # Set a process to use the SCHED_BATCH policy
   chrt -b -p 0 9102

   In this example, the process with PID 9102 is set to use the SCHED_BATCH policy, suitable for batch processing tasks.