Processes Just as the file is the base unit of data storage on a UNIX system, the process is the base unit of software. A process has its own execution and memory address space on the system, and is treated as an individual entity by the UNIX operating system. It's important to note that a program (like ls or cd) is not the same thing as a process - a program can contain many processes. If you can find a grizzled old timer who used computer systems in the 1960s and early 1970s, they can tell you about the batch-oriented operating systems that predated UNIX. A batch operating system could only execute one task at a time, so users literally had to stand in line and wait their turn to use the processor. UNIX is a multitasking operating system, which means it can perform multiple tasks simultaneously. At any given time, an average UNIX system may be performing anywhere from 5 to 50,000 tasks simultaneously. This magic is actually rather simple - UNIX quickly switches between every process on the system, giving each one a short period of time (called a time slice) to execute. Some very simple processes can finish in one time slice; more complex ones can take millions upon millions of time slices to complete. More important processes can be given time slices more often than less important processes through a system called priority scheduling. For example, a process that writes memory buffers to disk is much more important than the process that transmits an email message. For the most part UNIX takes care of itself when it comes to priority scheduling - the system is smart enough to know which processes should be given a higher priority. A user can reduce (but not increase) the priority of a process that they have started. Usually this is done by more advanced users on a system who are running a process that can take hours or even days to complete. Instead of slowing down the system for other users, the long-running process only runs during periods of relative system inactivity. While it may take the process an additional hour or two to complete because of its reduced priority level, it keeps the system running at peak efficiency for other users. The root user has the privileges to alter the priority of any process on the system. Most often this is used when a rogue process tries to take much more than its fair share of execution time. By reducing the process's priority, the system administrator can keep the system usable for other users. On occasion, the system administrator may be performing an important time-sensitive operation that needs to be completed as quickly as possible. When the situation warrants it, the system administrator can increase the priority of a process to speed up its execution. For information on how to change the priority of a process, see the section on managing processes in Chapter 8. |
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