1. Input/Output Management and Disk Scheduling
Chapter 11
Chapter 11

2. Categories of I/O devices
Human-readable:
printers
video display
keyboard...
Machine Readable:
disk, tape
sensors...
Communication
line drivers
modems...
Chapter 11

3. Main Characteristics Data rate (bits or bytes?)
Unit of transfer (bit, byte, or block)
Data format
Error conditions
Interrupt signaling
Chapter 11

4. Varying data rates....
Chapter 11

5. Stages of Evolution No interrupts With 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 11

6. Direct 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 11

7. Possible DMA configuration 1 (communication unit/DMA involves bus: inefficient)
Chapter 11

8. Possible DMA configuration 2 (communication betw
Possible DMA configuration 2 (communication betw. DMA and unit does not involve bus)
Chapter 11

9. Possible DMA configuration 3 (very flexible, facilitates the inclusion of additional I/O units)
Chapter 11

10. OS Design Issues Efficiency Generality:
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 11

11. Lower levels hide details from higher ones
Layering Models
Lower levels hide details from higher ones
Chapter 11

12. I/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 11

13. I/O Buffering Block-Oriented Stream-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 11

14. Single 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 11

15. Shadow Double 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.
Chapter 11

16. Circular 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 11

17. Disk 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)
Chapter 11
Silberschatz

18. Disk 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 11

19. Chapter 11

20. Tracks and cylinders
Chapter 11

21. Chapter 11

22. Timing of a Disk I/O Transfer
Chapter 11

23. Disk 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 11

24. Evaluating the policies
To evaluate the policies, we will use a random sequence of track accesses (see book):
Starting at track 100 and then
and calculate how many track traversal each policy will require to finish the sequence
Chapter 11

25. FIFO policy: process requests in the order they arrive
FIFO policy: process 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: = 498 tracks
498 / 9 = 55.3 tracks traversed for the avg request
Chapter 11

26. 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: = 248 / 9 = 27.5 better!
Chapter 11

27. The SCAN (or elevator) 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 =
= 250 / 9 = a bit worse than SSTF but no starvation
Chapter 11

28. 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 11

29. C-SCAN (Circular SCAN)
If the return home is considered as =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
Chapter 11

30. 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 11

31. Chapter 11

32. 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 11

33. Important 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
Chapter 11