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Full Discussion: Multi User Multi Task
Top Forums UNIX for Dummies Questions & Answers Multi User Multi Task Post 302070804 by amro1 on Friday 7th of April 2006 01:04:06 PM
Old 04-07-2006
here...

Multi task is ability to maintain schedule for more than one process. As normally number of processes exceeds by far number of CPUs in a system, the system scheduler becomes the core part of such multitasking. There is no real multitasking though, as only one process occupies processor's registers at time. However as it happens relatively fast it creates illusion of many processes flowing simultaneously. Multitasking systems is an opposition to mono tasking system as MS-DOS was. While in MS-DOS it was possible to have some processes to simulate simultaneous execution it was dome by using range of interruptions that is not a part of the system and worked more as a switch, while one process was working, until user would not designate manually recourses to the next process it would keep the current state.
Yes Windows XP is by all means a multitasking system.
Multi-User system implies that system can handle simultaneously more than one user. It means that system knows how to recognize the user, how to keep the environment and how to separate recourses between users.
Yes Windows XP is by all means a multi-user system.
The both Windows XP and UNIX may have many users connected using different hardware means and having sessions simultaneously.
I suspect you ask it as a point of comparison to Window/UNIX advantages/disadvantages. Let me tell you that the major advantage of any UNIX system is its simplicity. UNIX has very simple architecture compounded of many subsystems. If something is going wrong you always can trace problem to the specific subsystem. Each subsystem is mostly independed and may be stopped/started/ refreshed with no influence to anything else. Each subsystem is open, it means it may be discovered by regular mortals simply by reading man page; it has no "magic" involved. UNIX despite its evolution had some revolutionary steps in development and does not have to take increasing care of backward compatibility in its worst sense, because most of the programs can be recompiled with little efforts.
As system is modular it is easily upgradeable, many different teams of developers can coexist with little disturbance to one another, they don't have create “the system”, they are responsible for a little part and this way makes the system in general very reliable. There are many competing projects; each try to prove its module is the best.
Sorry for tedious port.
 

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getpriority(2)							System Calls Manual						    getpriority(2)

NAME
getpriority, setpriority - get or set process priority SYNOPSIS
DESCRIPTION
returns the priority of the indicated processes. sets the priority of the indicated processes to priority. The processes are indicated by which and who, where which can have one of the following values: Get or set the priority of the specified process where who is the process ID. A who of implies the process ID of the calling process. Get or set the priority of the specified process group where who is the process-group ID, indicating all processes belonging to that process-group. A who of implies the process-group ID of the calling process. Get or set the priority of the specified user where who is the user ID, indicating all processes owned by that user. A who of implies the user ID of the calling process. If more than one process is indicated, the value returned by is the lowest valued priority of all the indicated processes, and sets the priority of all indicated processes. priority is a value from to where lower values indicate better priorities. The default priority for a process is 0. If the calling process contains more than one thread or lightweight process (i.e., the process is multi-threaded) these functions shall apply to all threads or lightweight processes in the calling process. The priority specified (or retrieved) is the same for all threads or lightweight processes in a process. Negative priorities require appropriate privileges. Security Restrictions These system calls are subject to compartmental restrictions which restrict their access to processes in other compartments. This restric- tion covers for querying the priority of processes in other compartments, and for changing the priority of processes in other compartments. See compartments(5) for more information about compartmentalization on systems that support that feature. Compartmental restrictions can be overridden if the process has the privilege (PRIV_COMMALLOWED). Processes owned by the superuser may not have this privilege. Processes owned by any user may have this privilege, depending on system configuration. requires the privilege (PRIV_OWNER) to change the priority of a process whose uid does not match the caller's real or effective uid.. Pro- cesses owned by the superuser have this privilege. Processes owned by other users may have this privilege, depending on system configura- tion. requires the privilege (PRIV_LIMIT). Processes owned by the superuser have this privilege. Processes owned by other users may have this privilege, depending on system configuration. RETURN VALUE
returns the following values: Successful completion. n is an integer priority in the range to Failure. is set to indicate the error. See WARNINGS below. returns the following values: Successful completion. Failure. is set to indicate the error. ERRORS
If or fails, is set to one of the following values: [EACCES] The calling process does not have access rights to change one or more of the indicated processes. All processes for which access is allowed are still affected. [EINVAL] which is not one of the choices listed above, or who is out of range. [EPERM] The calling process attempted to change the priority of a process to a smaller priority value without having appro- priate privileges. [ESRCH] Processes indicated by which and who cannot be found. WARNINGS
can return both when it successfully finds a priority of and when it fails. To determine whether a failure occurred, set to before calling then examine after the call returns. AUTHOR
and were developed by the University of California, Berkeley. SEE ALSO
nice(1), renice(1M), nice(2). getpriority(2)
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