HP 3000 Manuals HP 3000 Manuals
Multi-programming Environment
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
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.
Linear Subqueues.
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.
Circular Subqueues.
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.