1) Schedulers –
· Scheduler is a path of operating system that is responsible for changing the state of process through scheduling queue.
·
We can say that
the operating system must select process from queue for scheduling purpose this
selection is carried out by appropriate scheduler.
·
Three types of
scheduler are there –
I.
Long term
scheduler
II.
Short term
scheduler
III.
Medium term
scheduler
Comparison among Scheduler
S.N.
|
Long-Term Scheduler
|
Short-Term Scheduler
|
Medium-Term Scheduler
|
1
|
It is a job scheduler
|
It is a CPU scheduler
|
It is a process swapping scheduler.
|
2
|
Speed is lesser than short term scheduler
|
Speed is fastest among other two
|
Speed is in between both short and long term
scheduler.
|
3
|
It controls the degree of multi- programming
|
It provides lesser control over degree of multi-programming
|
It reduces the degree of multi-programming.
|
4
|
It is almost absent or minimal in time
sharing system
|
It is also minimal in time sharing system
|
It is a part of Time sharing systems.
|
5
|
It selects processes from pool and loads
them into memory for execution
|
It selects those processes which are ready
to execute
|
It can re-introduce the process into memory
and execution can be continued.
|
Schedulers are
special system software which handles process scheduling in various ways. Their
main task is to select the jobs to be submitted into the system and to decide
which process to run. Schedulers are of three types −
- Long-Term Scheduler
- Short-Term Scheduler
- Medium-Term Scheduler
Long Term Scheduler
It is also called
a job scheduler. A long-term scheduler determines which programs
are admitted to the system for processing. It selects processes from the queue
and loads them into memory for execution. Process loads into the memory for CPU
scheduling.
The primary objective
of the job scheduler is to provide a balanced mix of jobs, such as I/O bound
and processor bound. It also controls the degree of multiprogramming. If the
degree of multiprogramming is stable, then the average rate of process creation
must be equal to the average departure rate of processes leaving the system.
On some systems, the
long-term scheduler may not be available or minimal. Time-sharing operating
systems have no long term scheduler. When a process changes the state from new
to ready, then there is use of long-term scheduler.
Short Term Scheduler
It is also called
as CPU scheduler. Its main objective is to increase system
performance in accordance with the chosen set of criteria. It is the change of
ready state to running state of the process. CPU scheduler selects a process
among the processes that are ready to execute and allocates CPU to one of them.
Short-term
schedulers, also known as dispatchers, make the decision of which process to
execute next. Short-term schedulers are faster than long-term schedulers.
Medium Term Scheduler
Medium-term
scheduling is a part of swapping. It removes the processes from the
memory. It reduces the degree of multiprogramming. The medium-term scheduler is
in-charge of handling the swapped out-processes.
A running process may
become suspended if it makes an I/O request. A suspended process cannot make
any progress towards completion. In this condition, to remove the process from
memory and make space for other processes, the suspended process is moved to
the secondary storage. This process is called swapping, and the
process is said to be swapped out or rolled out. Swapping may be necessary to
improve the process mix.