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The instruction cycle (also known as the fetch–decode–execute cycle, or simply the fetch-execute cycle) is the cycle that the central processing unit (CPU) follows from boot-up until the computer has shut down in order to process instructions. It is composed of three main stages: the fetch stage, the decode stage, and the execute stage.
Location of the "O(1) scheduler" (a process scheduler) in a simplified structure of the Linux kernel. An O(1) scheduler (pronounced "O of 1 scheduler", "Big O of 1 scheduler", or "constant time scheduler") is a kernel scheduling design that can schedule processes within a constant amount of time, regardless of how many processes are running on the operating system.
The base of each is exec (execute), followed by one or more letters: e – An array of pointers to environment variables is explicitly passed to the new process image. l – Command-line arguments are passed individually (a list) to the function. p – Uses the PATH environment variable to find the file named in the file argument to be executed.
The scheduler is an operating system module that selects the next jobs to be admitted into the system and the next process to run. Operating systems may feature up to three distinct scheduler types: a long-term scheduler (also known as an admission scheduler or high-level scheduler), a mid-term or medium-term scheduler, and a short-term scheduler.
They wanted to improve the software framework for expressing dependencies, to allow more processes to run concurrently or in parallel during system booting, and to reduce the computational overhead of the shell. In May 2011, Fedora Linux became the first major Linux distribution to enable systemd by default, replacing Upstart. The reasoning at ...
A thread may be viewed as a sub-process; that is, a separate, independent sequence of execution within the code of one process. Threads are becoming increasingly important in the design of distributed and client–server systems and in software run on multi-processor systems.
Specifically, the scheduler is permitted to forcibly perform a context switch (on behalf of a runnable and higher-priority process) on a driver or other part of the kernel during its execution, rather than co-operatively waiting for the driver or kernel function (such as a system call) to complete its execution and return control of the processor to the scheduler when done.
In the Linux operating system (prior to kernel 2.6.23), each CPU in the system is given a run queue, which maintains both an active and expired array of processes. Each array contains 140 (one for each priority level) pointers to doubly linked lists , which in turn reference all processes with the given priority.