SSTF (Shortest Seek Time First) in Operating Systems - Step-by-Step Execution

Concept Flow - SSTF (Shortest Seek Time First)

Start at current head position

↓

Calculate distance to each request

↓

Select request with shortest distance

↓

Move head to selected request

↓

Remove serviced request

↓

Are requests left?

No→End

↩Back to Calculate distance

SSTF chooses the disk request closest to the current head position, moves there, then repeats until all requests are served.

Execution Sample

Operating Systems

Requests = [55, 58, 39, 18, 90, 160, 150, 38, 184]
Head = 50
while requests:
  Find closest request to Head
  Move Head to it
  Remove request

This simulates SSTF disk scheduling by moving the head to the nearest request each time.

Analysis Table

Step	Current Head	Requests	Distances to Requests	Selected Request	Action	New Head
1	50	[55,58,39,18,90,160,150,38,184]	[5,8,11,32,40,110,100,12,134]	55	Move head to 55	55
2	55	[58,39,18,90,160,150,38,184]	[3,16,37,35,105,95,17,129]	58	Move head to 58	58
3	58	[39,18,90,160,150,38,184]	[19,40,32,102,92,20,126]	39	Move head to 39	39
4	39	[18,90,160,150,38,184]	[21,51,121,111,1,145]	38	Move head to 38	38
5	38	[18,90,160,150,184]	[20,52,122,112,146]	18	Move head to 18	18
6	18	[90,160,150,184]	[72,142,132,166]	90	Move head to 90	90
7	90	[160,150,184]	[70,60,94]	150	Move head to 150	150
8	150	[160,184]	[10,34]	160	Move head to 160	160
9	160	[184]	[24]	184	Move head to 184	184
10	184	[]	[]	None	All requests serviced	184

💡 All requests have been serviced; no requests remain.

State Tracker

Variable	Start	After 1	After 2	After 3	After 4	After 5	After 6	After 7	After 8	After 9	Final
Head	50	55	58	39	38	18	90	150	160	184	184
Requests	[55,58,39,18,90,160,150,38,184]	[58,39,18,90,160,150,38,184]	[39,18,90,160,150,38,184]	[18,90,160,150,38,184]	[18,90,160,150,184]	[90,160,150,184]	[160,150,184]	[160,184]	[184]	[]	[]

Key Insights - 3 Insights

Why does SSTF sometimes cause longer wait times for some requests?

Because SSTF always picks the closest request next (see execution_table steps 3-5), requests far from the current head can wait longer, causing possible starvation.

How do we calculate which request is closest?

At each step (see execution_table column 'Distances to Requests'), we find the absolute difference between the current head and each request, then pick the smallest.

What happens when there are no requests left?

The algorithm stops (see execution_table step 10), as all requests have been serviced.

Visual Quiz - 3 Questions

Look at the execution_table at Step 4. What is the current head position before moving?

A 38

B 58

C 39

D 55

💡 Hint

Check the 'Current Head' column at Step 4 in execution_table.

At which step does the head move to request 90?

A Step 6

B Step 5

C Step 7

D Step 8

💡 Hint

Look at the 'Selected Request' column in execution_table to find when 90 is chosen.

If the initial head was at 100 instead of 50, which request would SSTF select first?

A 150

B 90

C 58

D 55

💡 Hint

Compare distances from 100 to each request as done in execution_table's 'Distances to Requests' column.

Concept Snapshot

SSTF (Shortest Seek Time First):
- Always pick the request closest to current head.
- Calculate absolute distance to all requests.
- Move head to nearest request and remove it.
- Repeat until all requests are served.
- Can cause starvation for far requests.

Full Transcript

SSTF is a disk scheduling method where the disk head moves to the closest pending request next. Starting from the current head position, it calculates the distance to all requests, picks the shortest one, moves the head there, and removes that request from the queue. This repeats until no requests remain. The execution table shows each step with the head position, requests left, distances, and chosen request. SSTF is efficient but can cause some requests to wait longer if they are far from the current head position.