Scheduling Algorithm
To understand this topic in a better way I
must recommend you go through once from our last lecture which was about Process
Scheduling in Operating System so you may better understand the
concept.
The processes wait into the different
queues to get the process i.e. Job Queue, Ready Queue and Wait Queue.
This is the responsibility of algorithm to
get to check which process should be assigned to processor.
You might be thinking there is nothing like
responsibility here just let the process use the process number wise that is
why they are already in the queue.
NO! NO! A BIG NO!
Suppose there is a queue of vehicles in the
traffic jam and you have authority or say a kind of power by which you can make
any vehicle to go first or last. Why wouldn’t you allow an ambulance to go
first from that jam by using your powers or say authority.
The same case happens in the process queue
some process come with emergency or say they want to get processor on priority
basis (There are different cases for which a process wants to get processor
on priority basis.)
This is the job of the scheduler algorithm
to check which process should get the processor on priority basis that is why
the scheduling algorithms are important.
We know we have three different queues Job
Queue, Ready Queue and Wait Queue so we have three kind of scheduling
algorithms as well.
·
Long Term Scheduler
·
Short Term Scheduler
·
Medium Term Scheduler
Long Term Scheduler: This
is responsible for managing the process in the job queue which is in the
secondary memory.
Short Term Scheduler: This
is responsible for managing the process in the ready queue which is in the main
memory.
Medium Term Scheduler: This
is not responsible for managing the process in the waiting queue as your
thinking.
Why it is not responsible for waiting queue? Process have different states through
which it passes in the whole life one out of theme is waiting state when the
process is waiting for the I/O operation. After getting the I/O operating it
get backs into the ready state that means eventually it will be managed by the
short-term scheduler.
Medium Term scheduler is responsible to keep things in the smooth
flow. Smooth by keeping they ways of other process clear.
But who block the ways?
Some time a process resides into the main
memory and do nothing other than reserving resources short term scheduler swipe
it out of the main memory and make space for other process. In some cases a low
priority process use more memory in the main memory so to make space for
priority basis process this algorithm swipe out that low priority process.
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