Operating Systems
COMP 3430 Guderian
Compare Drive scheduling algos
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Good workloads Bad workloads Is it fair?
(Not in the book) Much the same as CPU scheduling! First-come, first served… super egalitarian!
(Not in the book) Much the same as CPU scheduling! First-come, first served… super egalitarian!
Also, terribly inefficient
Shortest seek time first
Where are we now? Can we opportunistically just move to the next closest block requested?
Shortest seek time first
Where are we now? Can we opportunistically just move to the next closest block requested?
Shortest seek time first
Where are we now? Can we opportunistically just move to the next closest block requested?
Efficient, but can starve data on the edges
Shortest Positioning Time First
¡°Where is the closest track¡± + ¡°where in the track¡± Greedy algorithm to fetch that data
AKA Elevator algorithm
AKA Elevator algorithm Up and down, up and down….
AKA Elevator algorithm
Up and down, up and down…. Scan from cylinder 0 to N, back
AKA Elevator algorithm
Up and down, up and down….
Scan from cylinder 0 to N, back
Wait time is predictable, if not efficient
AKA Elevator algorithm
Up and down, up and down….
Scan from cylinder 0 to N, back
Wait time is predictable, if not efficient Unfair to cylinder 0 and N
Circular-SCAN Go from cylinder 0 to N, ¡°Fast return¡± (no reads) back to 0
Circular-SCAN Go from cylinder 0 to N, ¡°Fast return¡± (no reads) back to 0
Circular-SCAN Go from cylinder 0 to N, ¡°Fast return¡± (no reads) back to 0
Keeps predictability!
LOOK Algorithm
Extension of SCAN algorithm, with one optimization
Question: What if we are on cylinder x, and there are no read requests >x?
LOOK Algorithm
Extension of SCAN algorithm, with one optimization
Question: What if we are on cylinder x, and there are no read requests >x?
Don¡¯t go there! There¡¯s nothing there for us
LOOK Algorithm
Extension of SCAN algorithm, with one optimization
Question: What if we are on cylinder x, and there are no read requests >x?
Don¡¯t go there! There¡¯s nothing there for us Return from x, scan back to 0.
LOOK Algorithm
Extension of SCAN algorithm, with one optimization
Question: What if we are on cylinder x, and there are no read requests >x?
Don¡¯t go there! There¡¯s nothing there for us Return from x, scan back to 0.
Circular Look, same with Hard return
Solid State Drives….
Solid State Drives…. What differences do we have?
Solid State Drives….
What differences do we have?
How does this affect our I/O scheduling?
Solid State Drives….
What differences do we have?
How does this affect our I/O scheduling? Is it an improvement?
If it¡¯s not open and read then it¡¯s a queue… amirite?
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