Let's kill -9 1

In a multi-programming environment, the CPU alternates between different programs, executing each for a duration ranging from tens to hundreds of milliseconds. Although the CPU is technically engaged with only one program at any given moment, over the span of one second, it may process several programs, thereby creating an illusion of parallelism for users.

The OS kernel and its core services, such as device drivers, system calls, and interrupt handlers, use kernel mode and in other side, user mode isolates user applications to prevent unauthorized access to critical system resources and memory.

PID 1, commonly referred to as "init", is the initial user-mode process in Linux, which persists until the system is powered down. The init process oversees various services, known as daemons in the Linux environment. Furthermore, an examination of the process tree on a Linux system reveals that init serves as the root of this hierarchy.

The creation of init is done by the kernel function called “rest_init”.

As users of Linux, we are accustomed to the concept of process identifiers (PID). The PID serves as the operating system's unique identifier for programs that are currently active. By executing a straightforward command to display the running processes, we can observe that the init process is assigned PID 1. This leads us to an intriguing inquiry: Which process is associated with PID 0?

A parent process ID is always assigned to every process ID. It tells us which process started a particular process. Therefore, the PPID value of 0 for the init process indicates that the init process has no parent.

If a parent process terminates prior to the completion of all its child processes, the init process will take over those child processes. Furthermore, only signals that have been specifically assigned a handler can be transmitted to init . Consequently, executing "kill -9 1" will typically have no effect in most distributions (attempting this will demonstrate that nothing occurs).

The init process, which holds PID 1, is exclusively responsible for the initiation and termination of the system.

Historically, older Unix systems relied on swapping rather than demand paging. Consequently, the swapper was tasked with managing the "Swap Process," which involved transferring all pages of a specific process to and from memory and the backing store, including the associated kernel data structures of the process. In the context of Linux, Process ID 0 was designated as the "idle process," which effectively performs no operations (similar to no-operations). Its presence ensured that the Linux system always had a task for the CPU to execute, particularly in scenarios where the CPU could not be halted to conserve power.

The process identified by PID 0 is tasked with managing paging and is commonly known as scheduler or the swapper process, which is often called the idle task. This is a special process that is part of the operating system kernel. This process is an integral component of the kernel and does not function as a standard user-mode process.

When there are no other processes that can run, the system executes the idle task, which effectively does nothing and allows the CPU to enter a low-power state. The idle task operates in kernel space, meaning it has full access to system resources and is not subject to the same restrictions as user-space processes.

Unlike user-space processes, PID 0 is not typically visible in standard process listings (like those produced by the ps command). The idle task is critical for efficient system resource management and plays a role in scheduling and CPU utilization. CPU Scheduling is a process that allows one process to use the CPU while another process is delayed (in standby) due to unavailability of any resources such as I / O etc, thus making full use of the CPU. The purpose of CPU Scheduling is to make the system more efficient, faster, and fairer. The idle task is created by the kernel during the system boot process and remains active as long as the system is running.

In summary, idle task is an essential part of the Linux kernel's process management system, ensuring that the CPU is efficiently utilized and that there is a task to execute when no other processes are ready to run.