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27 Cards in this Set
- Front
- Back
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Why is it impossible to completely swap out a blocked process?
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The page holding the buffer to receive input must not be swapped out to avoid deadlock
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When does 'deadlock' occur?
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When process is blocked on IO whilst IO is blocked on process swapping!
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Why must the 'receiving' page in memory be locked?
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To avoid deadlock.
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What are IO buffers?
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Memory reserves held to prevent deadlocking
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Describe the copying of an application's data for an output operation.
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Initially coped to the buffer.
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Describe the copying of an application's data for an input operation.
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Finally copied from the buffer.
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Name the three main buffering schemes.
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Single, Double, Circular.
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Through what does information from the IO reach the user process?
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OS
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Describe the Circular buffering schemes in terms of other schemes.
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Like the Double but with more than two IO buffers.
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Describe the motion of the disk head over the disk
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Positions over the correct track then waits for the correct sector to spin underneath
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Roughly how many heads are in a fixed-head drive?
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255
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How many heads are there in a moving-head drive?
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16
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For a moving-head drive, what is Seek Time?
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Time taken to position the head over the required track.
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Approximately how long is the seek time?
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8-12ms
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What is the Rotational Delay?
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Time taken for rotation of disk under head.
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What is the average rotational delay for a 5400rpm disk?
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90 rps, full rotation in 11ms, so mean rotational delay is 5.5ms
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What is the Access Time?
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Seek time + Rotational Delay
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If an IDE interface transfers data at a burst rate of 2MB/s, how long would a 4096B transfer take?
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4096/2000000 = 2ms
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What has a faster burst transfer rate, IDE or SCSI?
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SCSI
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Summarise the breakdown of speeds of steps in disk data retrieval.
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(slowest) Seek, rotation, Disk data to controller, driver software handling, controller data to memory (fastest)
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What is the problem with sharing one disk between multiple users on a system?
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Disk requests appear in a random sequence.
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How can we tackle this?
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Disk-scheduling strategies
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What are the four main disk-scheduling strategies?
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FIFO, SSTF, Scan, C-Scan
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What is FIFO?
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First-In-First-Out. No optimization.
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What is SSTF?
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Shortest Seek Time First - requires least movement of head (slowest step)
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What is the Scan policy?
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Keep moving disk head in same direction until no more requests in that direction, then change direction.
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What is C-Scan?
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Scan but one direction only. Reduces expected waiting time for all tracks.
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