Chapter 111
Input/Output Management and Input/Output Management and Disk SchedulingDisk Scheduling
Chapter 11 Chapter 11
Chapter 112
Categories of I/O devicesCategories of I/O devices
Human-readable: printers video display keyboard...
Machine Readable: disk, tape sensors...
Communication line drivers modems...
Chapter 113
Main CharacteristicsMain Characteristics
Data rate (bits or bytes?) Unit of transfer (bit, byte, or block) Data format Error conditions Interrupt signaling
Chapter 114
Varying data rates....Varying data rates....
Chapter 115
Stages of EvolutionStages of Evolution No interrupts
Simple programmed I/O by CPU Add simple controller to this: channel;
busy waiting or polling needed CPU keeps asking channel whether it has finished
With interrupts Add interrupts to this
simultaneous operation CPU & I/O traffic I/O Memory still goes through CPU
Introduce DMA: a controller to minimize CPU involvement. CPU interrupted only after entire block has been transferred DMA has complex instruction set (e.g. “if”) DMA is complete separate CPU
Chapter 116
Direct Memory AccessDirect Memory Access
Takes control of the system from the CPU to transfer data to and from memory over the system bus
Cycle stealing is used to transfer data on the system bus CPU slowed down, but not as much as it would be
without DMA The CPU pauses one bus cycle to allow DMA
to do its work No interrupts occur until all data block
transferred
Chapter 117
Possible DMA configuration 1 Possible DMA configuration 1 (communication unit/DMA involves bus: inefficient)(communication unit/DMA involves bus: inefficient)
Chapter 118
Possible DMA configuration 2Possible DMA configuration 2(communication betw. DMA and unit does not involve bus)(communication betw. DMA and unit does not involve bus)
Chapter 119
Possible DMA configuration 3Possible DMA configuration 3(very flexible, facilitates the inclusion of additional I/O units)(very flexible, facilitates the inclusion of additional I/O units)
Chapter 1110
OS Design IssuesOS Design Issues
Efficiency I/O is usually the bottleneck! Extremely slow
with respect to CPU Generality:
Try to handle devices as much as possible in same manner
Use general-purpose primitives, hide peculiarities of devices from high-level modules every file can be treated in terms of read. write,
open, close, lock, unlock...
Chapter 1111
Layering Models
Lower levels hide details from higher ones
Chapter 1112
I/O BufferingI/O Buffering
Could buffer be in memory area of user process? Probably not. Process in execution is subject to
paging, suspension, etc. so I/O must occur in a separate memory area (next
solution)
Chapter 1113
I/O BufferingI/O Buffering
Block-Oriented Information is stored in fixed sized blocks Transfers are made a block at the time Used for disks and tapes
Stream-oriented Information unit is of variable size: a stream of bytes
special info, such as carriage return, will delimit its logical parts
Used for terminals, printers, communication ports, mouse and most other devices that are not secondary storage
Chapter 1114
Single BufferSingle Buffer
Block-oriented Input transfers made to buffer As soon as user takes data, new input can start
(similarly for output) Read ahead, output and go (no wait)
Chapter 1115
Double bufferingDouble buffering
More independence between I/O and processing: a process can use the content of one buffer while the I/O device works with the other buffer
Normally invisible to programmer.
Shadow
Chapter 1116
Circular bufferCircular buffer
Generalized scheme, such as in the producer-consumer problem
Sometimes the I/O device can be faster, other times the user process can be faster
Peak demands are smoothed out
Chapter 1117
Disk schedulingDisk scheduling
Cylinder: the set of tracks that are in the same position with respect to read/write head (but book only considers tracks, not cylinders)
Silberschatz
Chapter 1118
Disk performance parametersDisk performance parameters
To read or write, the disk head must be positioned at the desired track and at the beginning of the desired sector
Access time is the sum of: Seek time
time it takes to position the head at the desired track (or cylinder)
Rotational delay or rotational latency time its takes for the beginning of the sector to reach the
head Transfer time
Seek time >> Rotational Delay >> Transfer time
Chapter 1119
Chapter 1120
Tracks and cylinders
Chapter 1121
Chapter 1122
Timing of a Disk I/O TransferTiming of a Disk I/O Transfer
Chapter 1123
Disk Scheduling PoliciesDisk Scheduling Policies
Seek time is the reason for differences in performance Seek time >> Rotational latency
For a single disk there will be a number of I/O requests If requests are selected randomly, we will not get a
good performance we will assume that the requests for disk access of a
number of user will be random (but for a single user `locality of reference’ will again hold)
So it is important to devise better methods: I/O system sorts the requests in some way
Chapter 1124
Evaluating the policiesEvaluating the policies
To evaluate the policies, we will use a random sequence of track accesses (see book): Starting at track 100 and then
55 58 39 18 90 160 150 38 184
and calculate how many track traversal each policy will require to finish the sequence
Chapter 1125
FIFO policy: FIFO policy: process requests in the order they arriveprocess requests in the order they arrive
From 100 to 55: 45 tracks traversed From 55 to 58: 3 tracks traversed From 58 to 39: 19 tracks traversed etc etc.... In total: 45+3+19... = 498 tracks 498 / 9 = 55.3 tracks traversed for the avg request
55 58 39 18 90 160 150 38 184
Chapter 1126
The Shortest Service Time First policy (SSTF)The Shortest Service Time First policy (SSTF) This policy looks each time at the queue of the waiting
requests and chooses the one that can be served with the shortest seek from the current position.
From 100 to 90: 10 tracks From 90 to 58: 32 From 58 to 55: 3 In total: 10+32+3... = 248 / 9 = 27.5 better!
55 58 39 18 90 160 150 38 184
90 58 55 39 38 18 150 160 184
Chapter 1127
The SCAN (or The SCAN (or elevatorelevator) policy ) policy Problem with the previous policy: it is possible that some requests
will starve, because closer requests keep arriving!! Solution: keep going in one direction until all requests are
satisfied. Then change direction, and so on 100 150 = 50; 150 160 = 10; 160 184 = 24; 184 90 = 94.... 50+10+24+94+.... = 250 / 9 = 27.8 a bit worse than SSTF but no
starvation
55 58 39 18 90 160 150 38 184
150 160 184 90 58 55 39 38 18
Chapter 1128
C-SCAN (Circular SCAN)C-SCAN (Circular SCAN)
Similar behavior to SSTF Problem with the previous policy:
nothing to do immediately after the arm reverses direction (already done)
waiting requests will be at the other end C-SCAN: assumes that return trip to track is
rapid (as it is in many drivers) Disk is always scanned in one direction only When the scan is complete, arm goes back
to the beginning and restarts
Chapter 1129
C-SCAN (Circular SCAN)C-SCAN (Circular SCAN)
If the return home is considered as 184-18=166 then the average seek length is 35.8.
If it is considered 0, then 136 / 9 = 15 (only!) In practice, it will be closer to 35 than to 15.
return home
184
18
55 58 39 18 90 160 150 38 184
150 160 184 18 38 39 55 58 90
Chapter 1130
Last in, First Out (LIFO)Last in, First Out (LIFO)
Serve always the most recent request first! Rationale: keep serving the same user may
result in accessing nearby tracks certainly true if file is sequential applies the principle of locality to disk accesses
But may starve early users
Chapter 1131
Chapter 1132
In practice...In practice...
For a realistic evaluation of disk times, one cannot assume simply that traversal of one track takes one unit of time
One must consider real arm motion times, which include a start time when the head picks up speed
One must also consider rotational delay and read times
Such times vary from disk unit to disk unit
Chapter 1133
Important Concepts of Chapter 11Important Concepts of Chapter 11
Different types of I/O Devices Different types of I/O Processing Direct Memory Access Buffering Characteristics of disk units Access time Different disk scheduling algorithms
comparison
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