what is dispatcher? three functions of it and scheduling criteria and its algorithms.?

DISPATCHER

·        It is a component of operating system involved in CPU scheduling.

·        Dispatcher is a program responsible for assigning the CPU to the process, which has been selected by the short-term scheduler.

·        Dispatching a process involves three functions

1.      Context switching

2.      Switching to user mode from monitor mode

3.      Jumping to the proper location in  he user program to restart that program.

1.       Context Switching : It means switching of CPU from one process to another.

·        This involves saving the state of old process (one that was running previously) and loading the saved state, if any, for this ready process (scheduled to be run now).

·        There will be no saved state if this ready process has been newly created and is being allocated to CPU for the first time .

·        If this ready process was previously in the blocked state (because of /O), then the process registers and memory must be loaded with all the previously saved data.

2.      Switching to user mode from monitor mode .

·        A user process must be run in user mode. For this it is necessary to change its mode from monitor to user.

3.      Restart the execution of process

·        The execution of process should be restarted by jumping to the instruction that was supposed to be executed when this process was last interrupted.

·        In case, if the process is being executed for the first time after creation, jump should be made to the first instruction.

SCHEDULING CRITERIA

·        The goal of a scheduling algorithm is to identify the process whose selection will result in the best possible system performance.

·        Best performance is a subjective evaluation and depends upon the number of criteria of different relative importance. .

·        The commonly used criteria can be grouped into two categories user oriented and system oriented criteria.

·        User Oriented criteria relate to the behaviour of the system as perceived by the individual user or process. For example, response time, turnaround time.

·        System Oriented criteria relate to the effective and efficient use of processor. For example : throughput, processor utilization, fairness, priorities.

·        User Oriented criteria are important on all system, system oriented criteria are generally of minor importance on single-user system.

·        The various scheduling criteria for evaluating an algorithm are: .

1.      CPU Utilization

·        CPU utilization is the average fraction of time during which the processor is busy.

·        Being busy usually refers to the processor not being idle and includes both the time i.e. time spent in executing user program as well as the time spent in executing the operating system.

·        The level of CPU utilization depends on the load on the system.

·        CPU utilization may range from 0 to 100 percent.

·        In a real system, it should range from 40 percent (for light loaded system) to 90 percent (for a heavily used system).

2.      Throughput

·        Throughput refers to the number of processes the system can execute in a period of time.

·        For long processes, this rate may be 1 process per hour, for short processes, throughput may be 10 processes per second.

·        Thus evaluation of throughput depends on the average length of a process.

·        Throughput is also dependent on the job-mix i.e. the number of CPU bound & /O bound processes. If all the processes are CPU bound, throughout will be less as compared to I/O based processes (which requires CPU for es time and hence more of them can be finished in a given time). Thus two pes of jobs should be properly mixed to improve the throughput.

3.       Turnaround time

·        This is the interval of time between the submission of a process and its completion.

·        Thus turnaround time is an average period of time it takes a process to execute.

·        A process's turnaround time includes all the time it spends in the system and can be computed by subtracting the time the process was created from the time it terminated.

·        Therefore, turnaround time includes actual execution time plus time spent waiting for resources (i.e. time spent waiting to get into memory, waiting in ready queue and doing I/O).

·        Turnaround time is inversely proportional to throughput.

·        It is the metrics used for batch jobs .

4.       Waiting time .

·        It is the average period of time a process spends waiting.

·        It is a time spent in waiting for a resource allocation. Therefore, waiting time is the penalty imposed for sharing resources with others.

·        Waiting time presents a more accurate measure of performance as compared to turnaround time because it does not include the time a process is executing on the CPU or performing I/O. It includes only the time a process spends waiting

·        Waiting time can be expressed as turnaround time minus the actual execution time.

W(x)=T(x)-x

where x is the service time, W(x) is the waiting time of job requiring x units of service and T(x) is the job's turnaround time.

5.      Response Time

·        Response time is defined as the time interval between the job submission and the first response produced by the job.

·        Thus, it is the average time it takes the system to start responding to user input. It is not the time taken to output the response.

·        Response time is an important metric used in the interactive system.

·        In order to obtain better performance response time should be low.

6.      Predictability .

·        A given job should run in about the same amount of time and at about the same cost regardless of the load on the system.

·        A wide variation in response time or turnaround time is distracting to users.

·        For example, an interactive system that routinely responds within a second but on occasion takes 10 sec. to respond, may be viewed more negative than a system that consistency responds in 2 secs. Although average respons time in the latter system is greater, user may prefer the system with greater predictability.

7.      Fairness

·        Fairness refers to the degree to which all processes are given equal opportunity to execute

·        In the absence of guidance from the user or other system supplied guidance, processes should be treated the same and no process should suffer starvation.

·        A process is a victim of starvation if it becomes struck in a scheduling queue indefinitely

8.      Priorities

·        When the processes are assigned priorities, the scheduling policy should give preference to the higher priority processes.

·        Higher priority processes should be given preference and the lower priority processes should be preempted.

The main purpose of scheduling is to maximize CPU utilization and throughput and to minimize turnaround time, waiting time and response time.