3.1. Scheduling Algorithm
There are different types of scheduling algorithm. In this lesson, we will describe only about FCFS and SJF, the rest of the algorithm will be discussed in the following lesson.
There are different types of scheduling algorithm. In this lesson, we will describe only about FCFS and SJF, the rest of the algorithm will be discussed in the following lesson.
3.4.2. First-come First Served (FCFS)
First-Come First-Served (FCFS) is by far the simplest CPU scheduling algorithm. The work load is simply processed in the order of arrival, with no preemption. The process which request the CPU first is allocated the CPU first. Implementation
The implementation of FCFS is easily managed with a FIFO (First in-First out) queue. As the process becomes ready, it joins the ready queue when the current process finishes, the oldest process is selected next. Characteristics
First-Come First-Served (FCFS) is by far the simplest CPU scheduling algorithm. The work load is simply processed in the order of arrival, with no preemption. The process which request the CPU first is allocated the CPU first. Implementation
The implementation of FCFS is easily managed with a FIFO (First in-First out) queue. As the process becomes ready, it joins the ready queue when the current process finishes, the oldest process is selected next. Characteristics
i) Simple to implement
ii) Non-preemptive
iii) Penalize short and I/O bound process Implementation
The ready queue is used to contain all those processes that are ready to be placed on the CPU. The ready queue is treated as a circular queue. The CPU scheduler goes around the ready queue, allocating the CPU to each process for a time interval up to a quantum in length.
To implement round-robin scheduling, the ready queue is kept as a FIFO queue of processes. New processes are added to the tail of the ready queue. The CPU scheduler picks the first job from the ready queue, sets a timer to interrupt after one time quantum, and dispatches the process.
The process that is currently executing on the CPU will continue until either :
The ready queue is used to contain all those processes that are ready to be placed on the CPU. The ready queue is treated as a circular queue. The CPU scheduler goes around the ready queue, allocating the CPU to each process for a time interval up to a quantum in length.
To implement round-robin scheduling, the ready queue is kept as a FIFO queue of processes. New processes are added to the tail of the ready queue. The CPU scheduler picks the first job from the ready queue, sets a timer to interrupt after one time quantum, and dispatches the process.
The process that is currently executing on the CPU will continue until either :
i) its quantum expires
Each process has a fixed time limit (the quantum) during which they have the CPU and puts it on the operating system takes the process off the CPU and puts it on the end of the ready queue.
Each process has a fixed time limit (the quantum) during which they have the CPU and puts it on the operating system takes the process off the CPU and puts it on the end of the ready queue.
ii) it is blocked on some event. The process is awaiting for some event to occur not do any instruction execution until it is finished. The operating system will place the process onto the correct blocked queue. When the waited upon event occurs the process may be put onto either the CPU or the ready queue depending on the specific algorithm used.
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