UNIT-III
CPU Scheduling
• 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.
Categories of Scheduling
There are two categories of scheduling:
Non-preemptive: Here the resource can’t be taken from a process until the process
completes execution. The switching of resources occurs when the running process
terminates and moves to a waiting state.
Preemptive: Here the OS allocates the resources to a process for a fixed amount of time.
During resource allocation, the process switches from running state to ready state or from
waiting state to ready state. This switching occurs as the CPU may give priority to other
processes and replace the process with higher priority with the running process.
Process Scheduling Queues
The OS maintains all Process Control Blocks (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.
1
,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
Running
1
When a new process is created, it enters into the system as in the running state.
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.
2
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.
2
,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
3
, 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
It is a process swapping
1 It is a job scheduler It is a CPU scheduler
scheduler.
Speed is lesser than short Speed is fastest among Speed is in between both short
2
term scheduler other two and long term scheduler.
It provides lesser control
It controls the degree of It reduces the degree of
3 over degree of
multiprogramming multiprogramming.
multiprogramming
It is almost absent or minimal It is also minimal in time It is a part of Time sharing
4
in time sharing system sharing system systems.
It selects processes from pool It can re-introduce the process
It selects those processes
5 and loads them into memory into memory and execution
which are ready to execute
for execution can be continued.
Context Switching
A context switching 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.
4
CPU Scheduling
• 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.
Categories of Scheduling
There are two categories of scheduling:
Non-preemptive: Here the resource can’t be taken from a process until the process
completes execution. The switching of resources occurs when the running process
terminates and moves to a waiting state.
Preemptive: Here the OS allocates the resources to a process for a fixed amount of time.
During resource allocation, the process switches from running state to ready state or from
waiting state to ready state. This switching occurs as the CPU may give priority to other
processes and replace the process with higher priority with the running process.
Process Scheduling Queues
The OS maintains all Process Control Blocks (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.
1
,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
Running
1
When a new process is created, it enters into the system as in the running state.
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.
2
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.
2
,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
3
, 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
It is a process swapping
1 It is a job scheduler It is a CPU scheduler
scheduler.
Speed is lesser than short Speed is fastest among Speed is in between both short
2
term scheduler other two and long term scheduler.
It provides lesser control
It controls the degree of It reduces the degree of
3 over degree of
multiprogramming multiprogramming.
multiprogramming
It is almost absent or minimal It is also minimal in time It is a part of Time sharing
4
in time sharing system sharing system systems.
It selects processes from pool It can re-introduce the process
It selects those processes
5 and loads them into memory into memory and execution
which are ready to execute
for execution can be continued.
Context Switching
A context switching 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.
4
