2.7. Processes

Put most simply, a process is an instance of an executing program. When a program is executed, the kernel loads the code of the program into virtual memory, allocates space for program variables, and sets up kernel bookkeeping data structures to record various information (such as process ID, termination status, user IDs, and group IDs) about the process.

From a kernel point of view, processes are the entities among which the kernel must share the various resources of the computer. For resources that are limited, such as memory, the kernel initially allocates some amount of the resource to the process, and adjusts this allocation over the lifetime of the process in response to the demands of the process and the overall system demand for that resource. When the process terminates, all such resources are released for reuse by other processes. Other resources, such as the CPU and network bandwidth, are renewable, but must be shared equitably among all processes.