Process• Program (Passive Entity) is a sequence of instructions, written to

perform a specified task on a computer. A computer requires programs to function, typically executing the program's instructions in a central processor. Program stores in secondary memory.

• A process (active entity) is a program or an instance of a computer program that is being executed. It contains the program code and its current activity. Depending on the operating system (OS), a process may be made up of multiple threads of execution that execute instructions concurrently.“Program under execution or when program in main memory or running program”

Process State

• New (Job Queue or HDD): The process is being created.• Running (Processor): Instructions are being executed.• Waiting (device queue): The process is waiting for some event to occur (such as an

I/O completion or reception of a signal).• Ready (Ready Queue or Main Memory): The process is waiting to be assigned to a

processor.• Terminated: The process has finished execution.

Schedulers

• Long term scheduler or job scheduler: HDD to Main memory (between new state and ready state)

• Short term scheduler or CPU scheduler or Dispature: main memory to CPU (between Ready state and running state)

• Medium term scheduler

Long term scheduler• It is also called job scheduler. Long term scheduler

determines which programs are admitted to the system for processing.

• Job scheduler 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 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.

• Example any person waits for friends means not decided a time where this process will be completed.

Short term scheduler• It is also called CPU scheduler. Main objective is increasing 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 process among the processes that are ready to execute and allocates CPU to one of them.

• Short term scheduler also known as dispatcher, execute most frequently and makes the fine grained decision of which process to execute next. Short term scheduler is faster than long term scheduler.

• The dispatcher is the module that give control of the CPU to process selected by short term scheduler . This function involves– Switching context– Switching to user mode– Jumping to the proper location in the user program to restart the program

• the time it takes for the dispatcher to stop one process and start another running is known as the dispatch latency.

Medium term scheduler

• Medium term scheduling is part of the 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.

Difference

Process Control Block• Each process is represented

in the operating system by a process control block (PCB) also called a task control block. PCB is the data structure used by the operating system. Operating system groups all information that needs about particular process.

Context Switch• Switching the CPU to another process requires saving the state of

the old process and loading the saved state for the new process. This task is known as a context switch. The context of a process is represented in the PCB of a process.

• 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 context switcher saves the content of all processor registers for the process being removed from the CPU, in its process descriptor. The context of a process is represented in the process control block of a process.

Context Switching

Context Switching

When the process is switched, the following information is stored.

• Program Counter• Scheduling Information• Base and limit register value• Currently used register• Changed State• I/O State• Accounting

Process 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 system. Such operating systems allow more than one process to be loaded into the executable memory at a time and loaded process shares the CPU using time multiplexing.

• CPU schedule two type of process• CPU Bounded• I/O bounded process

CPU-I/O Burst cycle• The success of CPU scheduling

depends on the following observed property of processes: Process execution consists of a cycle of CPU execution and I/O wait. Processes alternate between these two states. Process execution begins with a CPU burst. That is followed by an I/O burst, then another CPU burst, then another I/O burst, and so on.

• Eventually, the last CPU burst will end with a system request to terminate execution, rather than with another I/O burst.

Scheduling AlgorithmsCPU scheduling deals with the problem of deciding which

of the processes in the ready queue is to be allocated the CPU.– First Come First Serve (FCFS) Scheduling– Shortest-Job-First (SJF) Scheduling

• Nonpreemptive SJF• Preemptive SJF

– Priority Scheduling• Nonpreemptive priority• Preemptive priority

– Round Robin(RR) Scheduling– Multilevel Queue Scheduling

Scheduling Criteria• CPU utilization – To keep the CPU as busy as possible.

– Lightly loaded system 40% busy– Heavily loaded system 90% busy

• Throughput – Number of processes that complete their execution per time unit.

• Turnaround time – Amount of time to execute a particular process; which is the interval from time of submission of a process to the time of completion (includes the sum of periods spent waiting to get into memory, waiting in ready queue, executing on the CPU, and doing I/O).

• Waiting time – Sum of the periods spent waiting in the ready queue. • Response time – The time from the submission of a request until the first

response is produced (i.e., the amount of time it takes to start responding, but not the time that it takes to output that response).

Optimization Criteria• Max CPU utilization • Max throughput • Min turnaround time • Min waiting time • Min response time• For interactive systems (such as time-sharing systems),

some analysts suggest that minimizing the variance in the response time is more important than minimizing the average response time. A system with reasonable and predictable response time may be considered more desirable than a system that is faster on the average, but is highly variable. However, little work has been done on CPU-scheduling algorithms to minimize variance.

• FCFS:– The process that requests the CPU first is allocated the CPU first.– Implementation is based on FIFO queue.– When a process enters the ready queue, its PCB is linked onto the tail of the

queue. When the CPU is free, it is allocated to the process at the head of the queue.

– The FCFS algorithm is nonpreemptive (i.e., the CPU keeps the process until either terminating or requesting I/O).

• SJF– Nonpreemptive– once CPU given to the process it cannot be preempted until completes

its CPU burst.• Under nonpreemptive scheduling, once the CPU has been allocated to a process,

the process keeps the CPU until it releases the CPU either by terminating or by switching to the waiting state. This scheduling method is used by the Microsoft Windows 3.1 and by the Apple Macintosh operating systems.

– Preemptive – if a new process arrives with CPU burst length less than remaining time of current executing process, preempt. This scheme is known as the Shortest-Remaining-Time-First (SRTF).

– If two processes have the same length next CPU burst, FCFS scheduling is used.– SJF is optimal – gives minimum average waiting time for a given set of processes.

Priority Scheduling• A priority number (integer) is associated with each process. • The CPU is allocated to the process with the highest priority (smallest

integer ≡ highest priority). • Equal priority processes are scheduled in FCFS.• Priority can be either preemptive or nonpreemptive. • A preemptive priority will preempt the CPU if the newly arrived process is

higher than the priority of the currently running process.• A nonpreemptive priority will simply put the new highest priority process

at the head of the ready queue.• Problem ≡ Starvation (indefinite blocking) – low priority processes may

never execute.• Solution ≡ Aging – as time progresses increase the priority of the process,

so eventually the process will become the highest priority and will gain the CPU (i.e., the more time is spending a process in ready queue waiting, its priority becomes higher and higher)

RR• The Round Robin is designed for time sharing systems. • The RR is similar to FCFS, but preemption is added to switch between

processes.• Each process gets a small unit of CPU time (time quantum or time slice ),

usually 10-100 milliseconds. After this time has elapsed, the process is preempted and added to the end of the ready queue (ready queue treated as a circular queue) . That is, after the time slice is expired an interrupt will occur and a context switch.

• If there are n processes in the ready queue and the time quantum is q, then each process gets 1/n of the CPU time in chunks of at most q time units at once. No process waits more than (n-1)q time units.

• The performance of RR depends on the size of the time slice q : – If q very large (infinite) FIFO⇒– If q very small RR is called processor sharing, and context switch increases. ⇒

So, q must be large with respect to context switch, otherwise overhead is too high.

• The performance of the RR algorithm depends heavily on the size of the time quantum. At one extreme, if the time quantum is very large (infinite), the RR policy is the same as the FCFS policy.

• If the time quantum is very small (say 1 microsecond), the RR approach is called processor sharing, and appears (in theory) to the users as though each of n processes has its own processor running at 1/n the speed of the real processor.

• This approach was used in Control Data Corporation (CDC) hardware to implement 10 peripheral processors with only one set of hardware and 10 sets of registers.

• Turnaround time depend on time quantum. (it will be larger with respect to context switching)

problems

• FCFS: Convoy effect• There is a convoy effect, as all the other processes wait for

the one big process to get off the CPU. This effect results in lower CPU and device utilization than might be possible if the shorter processes were allowed to go first.

• Priority schduling: Aging– Aging is a technique of gradually increasing the

priority of processes that wait in the system for a long time.

Multilevel Queue Scheduling• Multi-level queue scheduling algorithm is used in scenarios where

the processes can be classified into groups based on property like process type, CPU time, IO access, memory size, etc. One general classification of the processes is foreground processes and background processes.

• A multilevel queue-scheduling algorithm partitions the ready queue into several separate queues. The processes are permanently assigned to one queue, generally based on some property of the process, such as memory size, process priority, or process type. Each queue has its own scheduling algorithm.

• For example, separate queues might be used for foreground (interactive) and background (batch) processes. The foreground queue might be scheduled by an RR algorithm, while the background queue is scheduled by an FCFS algorithm.

• an example of a multilevel queue-scheduling algorithm with five queues:

1. System processes2. Interactive processes3. Interactive editing

processes4. Batch processes5. Student processes

Multilevel Feedback Queue Scheduling

• Normally, in a multilevel queue-scheduling algorithm, processes are permanently assigned to a queue on entry to the system. Processes do not move between queues (problem).

• If there are separate queues for foreground and background processes, for example, processes do not move from one queue to the other, since processes do not change their foreground or background nature.

• This setup has the advantage of low scheduling overhead, but the disadvantage of being inflexible.

• Multilevel feedback queue scheduling, however, allows a process to move between queues. The idea is to separate processes with different CPU-burst characteristics.

• If a process uses too much CPU time, it will be moved to a lower-priority queue. This scheme leaves I/O-bound and interactive processes in the higher-priority queues. Similarly, a process that waits too long in a lower priority queue may be moved to a higher-priority queue. This form of aging prevents starvation.

• In general, a multilevel feedback queue scheduler is defined by the following parameters:

• The number of queues• The scheduling algorithm for each

queue• The method used to determine

when to upgrade a process to a higher priority queue

• The method used to determine when to demote a process to a lower-priority queue

• The method used to determine which queue a process will enter when that process needs service

Process creation (Fork())

• Fork() is a system call which is required to create process in linux.

• User can create replication of current running process using fork().