LOOK and C-LOOK variants in Operating Systems - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the main idea behind the LOOK disk scheduling algorithm?

LOOK moves the disk arm towards the nearest request in one direction and reverses direction only when there are no more requests in that direction. It 'looks' ahead to avoid unnecessary travel to the disk's end.

beginner

How does C-LOOK differ from LOOK in handling disk requests?

C-LOOK moves the disk arm in one direction only, servicing requests until the last one in that direction, then jumps back to the first request at the other end without servicing requests on the return trip.

intermediate

Why does LOOK reduce unnecessary disk arm movement compared to SCAN?

LOOK stops moving the arm when there are no more requests in the current direction, instead of going all the way to the disk's end like SCAN, saving time and reducing wear.

intermediate

What is the advantage of C-LOOK over LOOK in terms of disk arm movement?

C-LOOK only moves the arm in one direction, which can lead to more uniform wait times and less total arm movement by skipping the return trip servicing.

advanced

In what scenario might LOOK be preferred over C-LOOK?

LOOK might be preferred when requests are evenly spread and servicing requests in both directions reduces wait time, as it services requests on the return trip as well.

What does the LOOK algorithm do when there are no more requests in the current direction?

A It stops and waits for new requests

B It continues to the end of the disk

C It reverses direction to service requests on the other side

D It jumps to the first request on the disk

LOOK reverses direction only when no more requests exist in the current direction.

How does C-LOOK handle the return trip after servicing the last request in one direction?

A Jumps directly to the first request without servicing

B Reverses direction gradually

C Stops and waits

D Services requests on the way back

C-LOOK jumps back to the first request without servicing requests on the return trip.

Which algorithm moves the disk arm all the way to the disk's end regardless of requests?

A SCAN

B C-LOOK

C LOOK

D FCFS

SCAN moves the arm to the disk's end even if no requests are there, unlike LOOK.

What is a key benefit of C-LOOK over LOOK?

A Services requests in both directions

B Reduces total arm movement by moving in one direction only

C Always moves to the disk's end

D Waits for new requests before moving

C-LOOK reduces arm movement by servicing requests in one direction and jumping back.

Which disk scheduling algorithm is more likely to provide uniform wait times?

A FCFS

B LOOK

C Random

D C-LOOK

C-LOOK tends to provide more uniform wait times by moving in one direction only.

Explain how the LOOK disk scheduling algorithm optimizes disk arm movement compared to SCAN.

Think about how LOOK 'looks' ahead before moving.

Describe the main difference between LOOK and C-LOOK algorithms and how it affects disk arm movement.

Consider the direction of arm movement and servicing on return.