MPE/iX is a multiprogramming environment in which one process has
control of the CPU at any given instant.
All other processes are suspended to allow time for this to
occur, and the processes take turns.
The frequency and duration of a turn is determined by the
MPE/iX Dispatcher and is dependent on the
priority assigned to the process.
Processes can voluntarily suspend for many reasons.
For example, a process may suspend because it is waiting for
input or output.
The code and data for suspended processes is retained in memory
until space is needed by the executing process.
Priority Levels |
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A priority number identifies the priority level for a process.
The lower the number, the higher the priority.
Priorities range from 1 to 255.
MPE/iX uses a mechanism of subqueues to determine
when to change the priority level of a process, if at all.
Operating system processes are given the highest priority to
promptly service requests.
The AS subqueue (used for most operating system processes) receives
the highest priorities, followed by the BS subqueue (usually used for
special user processes).
The CS, DS, and ES subqueues (used for the Command Interpreter, Editor,
compilers, and standard user processes), as a group receive the
lowest priorities.
Operating system processes are given the highest priority to
promptly service requests.
At the user level, processes are queued and given CPU control
based on rules for the types of queue used at the particular
level of the process.
The operating system always runs the highest priority process
that is ready to execute.
A system manager can assign to users and accounts
privileges or restrictions of one subqueue over another.
A system supervisor has some control over the priority levels in
CS, DS, and ES.
The AS and BS subqueues are linear queues.
The characteristics of a linear queue are:
MPE/iX does not change the priority level of a process.
Processes at a given priority level execute on a first come, first served basis.
An executing process controls the CPU until it voluntarily suspends or receives a higher priority interrupt.
The AS subqueue should be reserved for operating system processes. A user process running in AS can severely impair the operating
system's ability to function. The BS subqueue is for special high priority user processes.
It is a recommended practice to run a special user process at a priority of 140 or lower to avoid preempting important MPE/iX
functions, such as spooler operation.
The CS, DS, and ES subqueues are circular queues.
The characteristics of a circular queue are:
Priority levels are cycled.
An executing process controls the CPU until it voluntarily
suspends, receives a higher priority interrupt, or reaches the end of its
quantum (time slice).
A circular queue lowers the priority of a process at the end of
its quantum. After the drop, the process eventually gets another opportunity to execute.
The CS, DS, and ES subqueues differ in how low a priority can be
dropped.
