# Difference between Priority Scheduling and Longest Job First (LJF)

1. Priority Scheduling Algorithm :
Priority scheduling algorithm executes the processes depending upon their priority. Each process is allocated a priority and the process with the highest priority is executed first. Priorities can be defined internally as well as externally. Internal priorities are decided by the system depending upon the number of resources required, time needed etc. whereas external priorities are based upon the time in which the work is needed or the amount being paid for the work done or the importance of process. Priority scheduling can be preemptive or non- preemptive.

Note –

• If two processes have the same priority then tie is broken using FCFS.
• The waiting time for the highest priority process is always zero in preemptive mode while it may not be zero in case of non preemptive mode.

The major problem is the starvation or indefinite blocking. It may so happen that in stream of processes, the system keeps executing the high priority processes and the low priority processes never get executed.

2. Longest Job First :
Longest Job First is a non-preemptive scheduling algorithm. This algorithm is based upon the burst time of the processes. The processes are put into the ready queue based on their burst times i.e. in a descending order of the burst times. As the name suggests this algorithm is based upon the fact that the process with the largest burst time is processed first. The burst time of only those processes is considered that have arrived in the system until that time. Its preemptive version is called Longest Remaining Time First (LRTF) algorithm.

Note –
If two processes have the same burst time then the tie is broken using FCFS i.e., the process that arrived first is processed first.

• This algorithm gives very high average waiting time and average turn-around time for a given set of processes.
• This may lead to convoy effect.
• It may happen that a short process may never get executed and the system keeps on executing the longer processes.
• It reduces the processing speed and thus reduces the efficiency and utilisation of the system.

The difference between Longest job first (LJF) and Priority Scheduling algorithm are as follows:

Longest job first (LJF) Priority scheduling
Longest Job First (LJF) executes the processes based upon their burst time i.e. in descending order of their burst times. Priority scheduling executes the processes based upon their priorities i.e. in descending order of their priorities. A process with higher priority is executed first.
LJF is non-preemptive but its preemptive version is also there called Shortest Remaining Time First (SRTF) algorithm. Priority scheduling is both preemptive and non preemptive in nature.
The average waiting time for given set of processes is quite long which reduces the effectiveness of the system. There is no idea of average waiting time and response time.
A long process may never get executed and the system may keep executing the short processes. The problem of blocking of a process can be solved through aging which means to gradually increase the priority of a process after a fixed interval of time by a fixed number.

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