# What Is The FCFS Algorithm?

## Is FCFS preemptive or Nonpreemptive?

First Come First Serve (FCFS) Jobs are executed on first come, first serve basis.

It is a non-preemptive, pre-emptive scheduling algorithm.

Easy to understand and implement.

Its implementation is based on FIFO queue..

## What is starvation OS?

Starvation is the problem that occurs when high priority processes keep executing and low priority processes get blocked for indefinite time. In heavily loaded computer system, a steady stream of higher-priority processes can prevent a low-priority process from ever getting the CPU.

## What are the disadvantages of FCFS scheduling?

Disadvantages: This scheduling method is nonpreemptive, that is, the process will run until it finishes. Because of this nonpreemptive scheduling, short processes which are at the back of the queue have to wait for the long process at the front to finish.

## Why is SJF optimal?

SJF is a scheduling algorithm that assigns to each process the length of its next CPU burst/execution time. SJF is provably optimal, in that for a given set of processes and their CPU bursts/execution times it gives the least average waiting time for each process. …

## How is FCFS calculated?

Calculating Average Waiting TimeHence, waiting time for P1 will be 0.P1 requires 21 ms for completion, hence waiting time for P2 will be 21 ms.Similarly, waiting time for process P3 will be execution time of P1 + execution time for P2, which will be (21 + 3) ms = 24 ms .More items…

## How is turn around time calculated SJF?

Waiting Time = Total Waiting Time / No. of Process = 41 / 5 = 8.2 mills. Total Turnaround Time : P1 = 28 + P2 = 7 + P3 = 12 + P4 = 19 + P5 = 3 = 69 mills.

## Which scheduling algorithm is best?

Sometimes FCFS algorithm is better than the other in short burst time while Round Robin is better for multiple processes in every single time. However, it cannot be predicted what process will come after. Average Waiting Time is a standard measure for giving credit to the scheduling algorithm.

## Is FCFS and FIFO same?

FCFS is also the jargon term for the FIFO operating system scheduling algorithm, which gives every process central processing unit (CPU) time in the order in which it is demanded. … A priority queue is neither FIFO or LIFO but may adopt similar behaviour temporarily or by default.

## How is FCFS Waiting time calculated?

For FCFS, the average waiting time is (0 + 10 + 39 + 42 + 49) / 5 = 28 ms. For nonpreemptive SJF scheduling, the average waiting time is (10 + 32 + 0 + 3 + 20) / 5 = 13 ms. For RR, the average waiting time is (0 + 32 + 20 + 23 + 40) / 5 = 23ms.

## What is a drawback of using preemptive systems?

Disadvantages of Preemptive Scheduling Takes a higher time by the scheduler to suspend the running task, switch the context, and dispatch the new incoming task. The process which has low priority needs to wait for a longer time if some high priority processes arrive continuously.

## Why is round robin algorithm considered better than first come first served algorithm?

First Come First Served Scheduling Algorithm provides high response time for the processes. In Round Robin Scheduling Algorithm, for the short processes there is very low response time. 3. FCFS is inconvenient to use in the time sharing system.

## What is FCFS and SJF?

First Come First Served (FCFS) executes the processes in the order in which they arrive i.e. the process that arrives first is executed first. Shortest Job First (SJF) executes the processes based upon their burst time i.e. in ascending order of their burst times.

## How does FCFS work?

First Come First Serve (FCFS) is an operating system scheduling algorithm that automatically executes queued requests and processes in order of their arrival. It is the easiest and simplest CPU scheduling algorithm. In this type of algorithm, processes which requests the CPU first get the CPU allocation first.

## What is shortest job first algorithm?

Shortest job next (SJN), also known as shortest job first (SJF) or shortest process next (SPN), is a scheduling policy that selects for execution the waiting process with the smallest execution time. SJN is a non-preemptive algorithm.

## What is turnaround time formula?

Turnaround time is the total amount of time spent by the process from coming in the ready state for the first time to its completion. Turnaround time = Burst time + Waiting time. or. Turnaround time = Exit time – Arrival time.