Process Scheduling
Definition
The
process scheduling is the activity of the process manager that handles the
removal of the running process from the CPU and the selection of another
process on the basis of a particular strategy.
Process
scheduling is an essential part of a Multiprogramming operating systems. Such
operating systems allow more than one process to be loaded into the executable
memory at a time and the loaded process shares the CPU using time multiplexing.
Process Scheduling Queues
The OS
maintains all PCBs in Process Scheduling Queues. The OS maintains a separate
queue for each of the process states and PCBs of all processes in the same
execution state are placed in the same queue. When the state of a process is
changed, its PCB is unlinked from its current queue and moved to its new state
queue.
The
Operating System maintains the following important process scheduling queues −
· Job queue − This queue keeps all the processes in the
system.
· Ready queue − This queue keeps a set of all processes
residing in main memory, ready and waiting to execute. A new process is always
put in this queue.
· Device queues − The processes which are blocked due to
unavailability of an I/O device constitute this queue.
The OS
can use different policies to manage each queue (FIFO, Round Robin, Priority,
etc.). The OS scheduler determines how to move processes between the ready and
run queues which can only have one entry per processor core on the system; in
the above diagram, it has been merged with the CPU.
Two-State Process Model
Two-state
process model refers to running and non-running states which are described
below −
|
S.N.
|
State & Description
|
|
1
|
Running
When
a new process is created, it enters into the system as in the running state.
|
|
2
|
Not Running
Processes
that are not running are kept in queue, waiting for their turn to execute.
Each entry in the queue is a pointer to a particular process. Queue is
implemented by using linked list. Use of dispatcher is as follows. When a
process is interrupted, that process is transferred in the waiting queue. If
the process has completed or aborted, the process is discarded. In either
case, the dispatcher then selects a process from the queue to execute.
|
Schedulers
Schedulers
are special system software which handle 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
processes 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.
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 multiprogramming
|
It
provides lesser control over degree of multiprogramming
|
It
reduces the degree of multiprogramming.
|
|
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.
|
Context Switch
A
context switch is the mechanism to store and restore the state or context of a
CPU in Process Control block so that a process execution can be resumed from
the same point at a later time. Using this technique, a context switcher
enables multiple processes to share a single CPU. Context switching is an
essential part of a multitasking operating system features.
When
the scheduler switches the CPU from executing one process to execute another,
the state from the current running process is stored into the process control
block. After this, the state for the process to run next is loaded from its own
PCB and used to set the PC, registers, etc. At that point, the second process
can start executing.
Context
switches are computationally intensive since register and memory state must be
saved and restored. To avoid the amount of context switching time, some
hardware systems employ two or more sets of processor registers. When the
process is switched, the following information is stored for later use.
- Program Counter
- Scheduling
information
- Base and limit
register value
- Currently used
register
- Changed State
- I/O State
information
- Accounting
information
