Documentation< Previous | Contents | Next >
3.1.3. Managing Processes
A process is a running instance of a program, which requires memory to store both the program itself and its operating data. The kernel is in charge of creating and tracking processes. When a program runs, the kernel first sets aside some memory, loads the executable code from the file system into it, and then starts the code running. It keeps information about this process, the most visible of which is an identification number known as the process identifier (PID).
Like most modern operating systems, those with Unix-like kernels, including Linux, are capable of multi-tasking. In other words, they allow the system to run many processes at the same time. There is actually only one running process at any one time, but the kernel divides CPU time into small slices and runs each process in turn. Since these time slices are very short (in the millisecond range), they create the appearance of processes running in parallel, although they are active only during their time interval and idle the rest of the time. The kernel’s job is to adjust its scheduling mechanisms to keep that appearance, while maximizing global system performance. If the time slices are too long, the application may not appear as responsive as desired. Too short, and the system loses time by switching tasks too frequently. These decisions can be refined with process priorities, where high-priority processes will run for longer periods and with more frequent time slices than low-priority processes.
Multi-Processor Systems The limitation described above, of only one process running at a time, doesn’t always (and Variants) apply: the actual restriction is that there can be only one running process per pro- cessor core. Multi-processor, multi-core, or hyper-threaded systems allow several
processes to run in parallel. The same time-slicing system is used, though, to handle cases where there are more active processes than available processor cores. This is not unusual: a basic system, even a mostly idle one, almost always has tens of running processes.
Multi-Processor Systems The limitation described above, of only one process running at a time, doesn’t always (and Variants) apply: the actual restriction is that there can be only one running process per pro- cessor core. Multi-processor, multi-core, or hyper-threaded systems allow several
processes to run in parallel. The same time-slicing system is used, though, to handle cases where there are more active processes than available processor cores. This is not unusual: a basic system, even a mostly idle one, almost always has tens of running processes.
The kernel allows several independent instances of the same program to run, but each is allowed to access only its own time slices and memory. Their data thus remain independent.