Self Doubt in Disk Scheduling

Question

I am presenting here 2 doubts in the form of questions.Plz give proper explanation to these :

Q1 : Out of the available disk scheduling algorithms , which can be considered as the best and most practical?

Q2 : In CSCAN or CLOOK , we consider the head movement when there is transition from one extreme to another extreme.But do we consider this movement for the waiting time as well.By waiting time I mean waiting time for those requests which are on the opposite side w.r.t. initial direction of head movement and hence not yet serviced ?

Comments

When we take a jump from one extreme to another extreme,

Do we add those head movements in total movements?

For example, if it is a C-LOOK algo,we are jumping from say 185th cylinder to 11th cylinder, do we add (185-11) movements ?

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Yes we take just as in the case of CSCAN .In fact this year only one question came from CSCAN..U can verify it there.

Answer 1

Q1)out of all disk scheduling algorithms SSTF is best.we cannot say alltime this is best it depends on th problem also.

because it search for nearest disk that come.

Q2)yes ,exactly we consider that disk head movements in waiting time

suppose there are total 200(0to199) cylinders,suppose head movements are 20,50,85,144,185.

in cscan we go until last cylinder and then came back to initial cylinder (199-185,199-0 these waiting times also included in cscan)

but in clook we reach until lst cylinder (185-144,185-20)

Comments

I do not think we should coclude something about this...I saw at many sites, this is still a area of research.

And also Cscan assumes that the arrangement is circular ...it is like 0 to 100 and again 0, although we will always consider this gap.

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well explained .