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Chapter 3 : Process Concept

Terms in this set (38)

A process migrates between the various scheduling queues throughout
its lifetime.
The OS must select a process from the different process queues in
some fashion. The selection process is carried out by a scheduler.
In a batch system the processes are spooled to mass-storage device.
Long-term scheduler (or job scheduler) - selects which processes
should be brought into the ready queue.
It may take long time
Short-term scheduler (or CPU scheduler) - selects which process
should be executed next and allocates CPU.
It is executed at least once every 100 msec.
If 10 msec is used for selection, then 9 % of CPU is used (or wasted)
The long-term scheduler executes less frequently.
The long-term scheduler controls degree of multiprogramming.
Multiprogramming: the number of processes active in the system.
If MPL is stable: average rate of process creation= average departure
rate of processes.
The long-term scheduler should make a careful selection.
Most processes are either I/O bound or CPU bound.
I/O bound process spends more time doing I/O than it spends
doing computation.
CPU bound process spends most of the time doing computation.
The LT scheduler should select a good mix of I/O-bound
and CPU-bound processes.
Example:
If all the processes are I/O bound, the ready queue will be empty
If all the processes are CPU bound, the I/O queue will be empty, the
devices will go unutilized and the system will be imbalanced.
Best performance: best combination of CPU-bound and I/O bound process.
Some OSs introduced a medium-term scheduler
using swapping.
Key idea: it can be advantageous, to remove
the processes from the memory and reduce the
multiprogramming.
Swapping: removal of process from main
memory to disk to improve the performance. At
some later time, the process can be reintroduced
into main memory and its execution can be
continued when it left off.
Swapping improves the process mix (I/O and
CPU), when main memory is unavailable