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 Switching
Switching the CPU to another process by saving the current state of running process in PCB. And loading the saved state from PCB for new process is called context switching.
It is a complementary part of a multitasking operating system because multitasking is also happens through it.
When the process is switched, the following information is stored for later use in PCB.
- Process ID
- Process State
- Pointer
- Priority
- PC
- CPU Registers
- I/O information
- Accounting Information
Above information’s are context of Process.
Switching Steps
- Save the context of running (First) in PCB.
- Place the PCB of the next(2nd) process, which has to execute, into the relevant queue i.e. ready queue, I/O queue etc.
- Select a new (2nd) process for execution.
- Update the the PCB of 2nd process by changing the state to running in PCB.
- Update the memory management data if required.
- After the execution of second process, restore the context of the first process on CPU.
When does switching occur?
The main reasons due to which switching is occur are given below
- Due to Trap or Interrupt.
- Higher priority process arrives (CPU scheduling Algorithm).
- Multitasking
Problems with Switching
Main factors due to which it leads to an overhead are
- TLB flushes
- sharing the cache between multiple tasks
- execution of task scheduler etc.
Note: Context switching in-between the threads of the a process is faster than in-between the different processes.Because threads have the same virtual memory maps and there is no need of flushing of TLB.
Is User and kernel mode switching is context switching?
Answer is No. When the control of CPU is transfer in-between user mode and kernel mode in OS then context switching is not necessary.Because context switching and User, kernel modes are different concept . However, depending on the OS, context switch may also take place at the time of user and kernel mode transition.
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