1. Outline Device Management Device Manager Design Buffering
Device Drivers
I/O Strategies
Polling vs. interrupt-driven I/O
Device Manager Design
Buffering
Optimizing Access for Rotating Devices
Device Management in Linux

2. Announcements
We are going to have a quiz at the end of the class on Sept. 18, 2003
It will be a closed book and closed notes quiz
It will cover
System calls and how to use them
Lecture notes up to Today (Sept. 16)
Related materials in the book
Tomorrow’s recitation session is optional
You are not required to come
I will mainly answer questions related to Lab 1
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3. Review: Device Drivers
The part of OS that implements device management is called collectively as device manager, which includes device drivers and device driver infrastructure
Device drivers can be complex
However, the drivers for different devices implement a similar interface so that a higher-level component has a uniform interface for different devices
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4. Device Management Organization
Application
Process
File
Manager
Device Controller
Command
Status
Data
Hardware Interface
System Interface
Device-Independent
Device-Dependent
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5. Review: Direct I/O vs. Direct-Memory Access
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6. Review: I/O Strategies – cont.
Direct I/O or DMA for data transferring
Polling or interrupt-driven
I/O strategies
Direct I/O with polling
DMA I/O with polling
Not supported in general
Direct I/O with interrupts
DMA I/O with interrupts
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7. Review: Direct I/O with Polling
read(device, …);
Data
Device Controller
Command
Status
read function
write function 1 2 3 4 5
Hardware Interface
System Interface
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8. Device Controllers – cont.
Command
Status
Data 0
Data 1
Data n-1
Logic
busy
done
Error code
. . .
busy done
idle
finished
working
(undefined)
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9. Review: Interrupt-Driven I/O
read(device, …);
Data
Device Controller
Command
Status
read driver
write driver 1 2 3 4 5
Hardware Interface
System Interface
Device Status Table
Device
Handler
Interrupt 6 7 8a 8b 9
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10. Review: Interrupt Handling
At the end of each instruction cycle, we check if an interrupt has occurred
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11. Interrupt-Driven Direct-Memory Access – cont.
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12. Interrupt-Driven Versus Polling
In general, an interrupt-driven system will have a higher CPU utilization than a polling-based system
The CPU time on polling by one process may be utilized by another process to perform computation
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13. Comparison of Polling and Interrupt-Driven I/O Performance
Polling is normally superior from the viewpoint of a single process
Because overhand of polling is less
Interrupt-driven I/O approach gives better overall system performance
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14. I/O-CPU Overlap
Through interrupt-driven I/O
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15. I/O-CPU Overlap – cont. Sequential operation and overlapped operation
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16. I/O-CPU Overlap – cont. - I/O-CPU overlap through overlapped operation
and polling
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17. I/O-bound and Compute-bound Processes
I/O-bound process
A process’s overall execution time is dominated by the time to perform I/O operations
Compute-bound process
A process’s time on I/O is small compared to the amount of time spent using the CPU
Many processes have I/O-bound and compute-bound phases
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18. Phases of a Process
Compute-bound
I/O-bound
Time
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19. Device Manager Design Device drivers
Application programming interface
Coordination among application processes, drivers, and device controllers
Performance optimization
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20. BSD UNIX Driver Functions
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21. The Kernel Interface
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22. Device-Independent Function Call
funci(…)
Trap Table
dev_func_i(devID, …) {
// Processing common to all devices …
switch(devID) {
case dev0: dev0_func_i(…);
break;
case dev1: dev1_func_i(…);
case devM: devM_func_i(…); }; }
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23. Re-configurable Device Drivers
Other
Kernel
services
Entry Points for Device j
open(){…}
read(){…}
etc.
System call interface
Driver for Device j
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24. Handling Interrupts Device driver J Device interrupt handler J
Device status table
int read(…) {
// Prepare for I/O
save_state(J);
out dev#
// Done (no return) }
void dev_handler(…) {
get_state(J);
//Cleanup after op
signal(dev[j]);
return_from_sys_call(); } J
Interrupt Handler
Device Controller
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25. Handling Interrupts – cont.
Device driver J
Device interrupt handler J
int read(…) { …
out dev#
// Return after interrupt
wait(dev[J});
return_from_sys_call(); }
void dev_handler(…) {
//Cleanup after op
signal(dev[j]); }
Interrupt Handler
Device Controller
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26. CPU-device Interactions
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27. Buffering
Buffering
A technique to keep slower I/O devices busy during times when a process is not requiring I/O operations
Input buffering
Output buffering
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28. The Pure Cycle Water Company
Customer Office
Water Company
Returning the Empties
Water Producer
Water Consumers
Delivering Water
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29. Buffering – cont.
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30. Double Buffering
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31. Multiple Buffers
- Producer-consumer problem
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32. Spooling
A spool is a buffer that holds output for a device that cannot accept interleaved data streams
Printer is an example
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33. Device Characteristics
Character devices vs. block devices
Communication devices
Sequentially accessed storage devices
Randomly accessed storage devices
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34. Communication Devices
Generic
Controller
Local
Device
Communications
Cabling connecting the
controller to the device
Printer
Modem
Network
Bus
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35. Randomly Accessed Devices
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36. Optimizing Access to Rotating Devices
Access time has three major components
Seek time – the time for the disk arm to move the heads to the cylinder containing the desired sector
Rotational latency – the additional time waiting for the disk to rotate the desired sector to the disk head
Transfer Time - time to copy bits from disk surface to memory
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37. Optimizing Seek Time
Multiprogramming on I/O-bound programs => set of processes waiting for disk
Seek time dominates access time => minimize seek time across the set
Tracks 0:99; Head at track 75, requests for 23, 87, 36, 93, 66
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38. Optimizing Access to Rotating Devices
First-Come-First-Served (75), 66, 87, 93, 36, 23
= 251 steps
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39. Optimizing Access to Rotating Devices – cont.
Shortest-Seek-First-First
- SSTF: (75), 66, 87, 93, 36, 23
= 107 steps
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40. Optimizing Access to Rotating Devices – cont.
-Scan/Look
- Scan: (75), 87, 93, 99, 66, 36, 23
= 100 steps
- Look: (75), 87, 93, 66, 36, 23
= 87 steps
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41. Optimizing Access to Rotating Devices – cont.
Circular Scan/
Circular Look
Circular Scan: (75), 87, 93, 99, 23, 36, 66
home = 90 + home
Circular Look: (75), 87, 93, 23, 36, 66
home = 84 + home
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42. Serial Port Printer Terminal Modem Mouse etc. CPU Serial Memory Device
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43. Serial Port RS-232 Interface Serial Device (UART) UART API
9-pin connector
4-wires
bit transmit/receive
...
Serial Device
(UART)
UART API
parity
bits per byte
etc.
Device Driver
Set UART parms
read/write ops
Interrupt hander
Software on the CPU
Device Driver API
Bus Interface
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44. Switched Telephone Network
Adding a Modem
Serial
Device
Memory
CPU
Modem
Phone
Switched Telephone Network
Dialing & connecting
Convert analog voice to/from digital
Convert bytes to/from bit streams
Transmit/receive protocol
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45. Serial Communication Device Driver Serial Device RS-232 Modem
Set UART parms
read/write ops
Interrupt hander
Driver-Modem Protocol
Dialing & connecting
Convert analog voice to/from digital
Convert bytes to/from bit streams
Transmit/receive protocol
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46. Exploiting the Phone Network
Comm
Device
Memory
CPU
Modem
Phone
Switched Telephone Network
Logical Communication
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47. Device Management in Linux
While it builds on the same basic principles, it is more complex and with more details
There are a lot of device drivers included and you can also develop your own device drivers for specific purpose devices
Linux divides into char-oriented devices and block oriented devices
fs/blk_dev.c
fs/char_dev.c
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48. Summary Device Management Organization I/O Strategies
Device is the bottleneck for I/O-bound processes
Overlap between I/O and CPU will increase the system’s throughput rate
Buffering
Access Optimization for Disks
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