CPU scheduling

Question

Which of the following CPU scheduling leads to convey effect?
i) SJF
ii) Priority
iii) FCFS

the answer is all .

is it true ?

Also , i read somewhere .. that for SJF convoy effect is usually confused with priority inversion .

kindly clear this doubt .

Comments

I dont think answer will be all. Answer should be SJF only. @Bikram Sir can you please verify?

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Yes.. all three CPU scheduling algo leads to convey effect . 

All these 3 cpu scheduling algo are known as highest priority first schedulers .. so all 3 cases convoy effect can happen.

Answer 1

Convoy effect is like you are going through road by car and suddenly you have to stop and wait for few minutes because minister have to pass through that road .  This is convoy effect ..where a high burst time cpu bound process is executing and low burst time I/O bound process waiting in ready queue ..

all these given scheduling algo - FCFS, Priority and SJF are Highest priority First Scheduling algorithm  means high priority process execute first then low priority process can execute.. so low priority process is starving .

FCFS is non preemptive, FCFS  assigns priorities according to their order of arrival or arrival time.  Non preemptive Shortest-job-first (SJF) assigns priorities according to the burst time. In preemptive shortest-job-first the scheduler is always running the job with the highest priority. hence in all 3 cases convoy effect possible.

Comments

By that logic every algorithm would have convoy effect. Won't it? Because all the algorithms prioritize a process based on one or other criteria like arrival time, priority or  burst time.

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Round Robin don't have convoy effect.

Convoy effect mainly deals with FCFS.

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But convoy effect results in unnecessary increase in average waiting time which isn't a case in SJF. How could SJF lead to convoy effect?

As far as I remember SJF is literally the solution to avoid the convoy effect.

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In SJF, if a process with large burst time arrives first then no other process can preempt it causing convoy effect. see the Non preemptive example https://stackoverflow.com/questions/27679120/can-shortest-job-first-scheduling-be-subject-to-convoy-effect

In preemptive priority, if all processes arrive at time 0 and if a high priority process has very large burst time, it can lead to convoy effect as no other process can preempt this process because they all have lesser priority.