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CPU SCHEDULING MAY BE CLASSIFIED AS:
1 . NON-PREEMPTIVE SCHEDULING
In Non-Preemptive Scheduling, the CPU cannot be taken away from the currently carrying out process. The sole time the CPU scheduler can give the PROCESSOR to another procedure in the all set queue is usually when the currently executing method terminates or perhaps enters into a Blocked state.
2 . PREEMPTIVE SCHEDULING
In Preemptive Scheduling, the CPU could be taken away from the currently carrying out process. The currently carrying out process can be sent back for the ready queue and the PROCESSOR scheduler designates the CPU to another process. This will happen if:
a. An disrupt occurred so the current procedure has to quit executing. The CPU can then be assigned to execute the ISR with the requesting device or method.
b. The priority with the process that enters the ready queue is higher than the presently executing procedure. The CENTRAL PROCESSING UNIT is then designated to execute the higher-priority process.
c. The time limit of a process for using the CENTRAL PROCESSING UNIT has been surpassed. The CPU is then designated to implement another method even though operating process has not yet completed its current CPU broken.
Preemptive scheduling is usually ideally intended for interactive or perhaps real-time computer systems. Non-preemptive scheduling is wonderful for batch digesting systems simply. However , non-preemptive scheduling has fewer circumstance switches than preemptive arranging; therefore the previous has much less overhead.
CPU SCHEDULING METHODS PERFORMANCE STANDARDS
The following efficiency criteria should be optimized by a good scheduler:
1 . CENTRAL PROCESSING UNIT UTILIZATION
The CPU has to be busy undertaking useful act on all occasions.
2 . THROUGHPUT
The amount of work done by the CPU should be maximized.
3. TURN-AROUND TIME вЂ“ the time between your point a process is posted and the period it coatings executing is definitely minimized.
4. RESPONSE TIME вЂ“ the time between the distribution of a obtain and the start of the systems 1st response can be minimized.
5. HOLDING OUT TIME вЂ“ the time a procedure has to dedicate inside the all set queue waiting to be performed by the CPU is reduced. Many looks at this as being a measure of how good a scheduling algorithm can be since holding out time in some way have an impact on the order of process delivery.
Note that a scheduling protocol cannot optimize all the efficiency criteria as a result of conflicts. Some criteria could possibly be optimized and some are sacrificed. The booking algorithm may well select to optimize just one or two criteria, which depends on the sort of computer system being used.
FAIRNESS, which means all techniques will be provided equal possibility to use the CENTRAL PROCESSING UNIT, is another overall performance criterion. It is difficult to measure so it had not been included in the over list.
DISTINCT CPU SCHEDULING ALGORITHMS
1 ) FIRST-COME, FIRST-SERVED ALGORITHM (FCFS)
It is just a non-preemptive booking algorithm where the one that gets into the Ready queue 1st gets to be carried out by the CENTRAL PROCESSING UNIT first. In choosing the next process to get executed, the CPU scheduler selects the process at the front from the Ready line. Processes enter in at the rear of the Ready queue.
A set of processes with the respective arrival times at the Ready line and the length of their next CPU burst are given listed below.
|Process ID |Arrival Time |CPU Burst | |A |0 |8 | |B |3 |4 | |C |4 |5 | |D |6 |3 | |E |10 |2
All principles are in milliseconds.
In the succeeding solutions, Gantt graphs are used to illustrate the time every single process starts and ends executing.