Outline
• Device Management
– Device Drivers
– I/O Strategies
– Polling vs. interrupt-driven I/O
•
•
•
•
Device Manager Design
Buffering
Optimizing Access for Rotating Devices
Device Management in Linux
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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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 higherlevel component has a uniform interface for
different devices
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Device Management Organization
Application
Process
System Interface
File
Manager
Device-Independent
Device-Dependent
Hardware Interface
Command
Status
Data
Device Controller
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Review: Direct I/O vs. Direct-Memory Access
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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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Review: Direct I/O with Polling
read(device, …);
1
System Interface
Data
read function
5
write function
2
3
4
Hardware Interface
Command
Status
Data
Device Controller
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Device Controllers – cont.
...
busy
Command
done
Error code
Status
...
busy done
0
0 idle
0
1 finished
1
0 working
1
1 (undefined)
Data 0
Data 1
Logic
Data n-1
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Review: Interrupt-Driven I/O
read(device, …);
1
9
8b
Data
System Interface
Device Status Table
read driver
2
4
7
Device
Handler
write driver
6
3
Interrupt
Handler
8a
5
Hardware Interface
Command
Status
Data
Device Controller
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Review: Interrupt Handling
• At the end of each instruction cycle, we
check if an interrupt has occurred
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Interrupt-Driven Direct-Memory Access – cont.
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Interrupt-Driven Versus Polling
• In general, an interrupt-driven system will
have a higher CPU utilization than a pollingbased system
– The CPU time on polling by one process may be
utilized by another process to perform
computation
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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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I/O-CPU Overlap
Through interrupt-driven I/O
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I/O-CPU Overlap – cont.
• Sequential operation and overlapped
operation
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I/O-CPU Overlap – cont.
- I/O-CPU overlap through overlapped operation
and polling
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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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Phases of a Process
Compute-bound
Time
I/O-bound
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Device Manager Design
• Device drivers
– Application programming interface
• Coordination among application processes,
drivers, and device controllers
• Performance optimization
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BSD UNIX Driver Functions
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The Kernel Interface
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Device-Independent Function Call
funci(…)
Trap Table
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dev_func_i(devID, …) {
// Processing common to all devices
…
switch(devID) {
case dev0:
dev0_func_i(…);
break;
case dev1:
dev1_func_i(…);
break;
…
case devM:
devM_func_i(…);
break;
};
// Processing common to all devices
…
}
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Re-configurable Device Drivers
System call interface
open(){…}
read(){…}
Entry Points for Device j
etc.
Other
Kernel
services
Driver for Device j
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Handling Interrupts
Device driver J
int read(…) {
// Prepare for I/O
save_state(J);
out dev#
// Done (no return)
}
Device status table
J
Device interrupt handler J
void dev_handler(…) {
get_state(J);
//Cleanup after op
signal(dev[j]);
return_from_sys_call();
}
Interrupt Handler
Device Controller
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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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CPU-device Interactions
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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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The Pure Cycle Water Company
Customer Office
Water Company
Returning the Empties
Water Producer
Water Consumers
Delivering Water
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Buffering – cont.
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Double Buffering
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Multiple Buffers
- Producer-consumer problem
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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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Device Characteristics
• Character devices vs. block devices
• Communication devices
• Sequentially accessed storage devices
• Randomly accessed storage devices
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Communication Devices
Bus
Generic
Controller
Communications
Controller
Local
Device
Cabling connecting the
controller to the device
Device
•Printer
•Modem
•Network
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Randomly Accessed Devices
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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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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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Optimizing Access to Rotating Devices
-First-Come-First-Served (75), 66, 87, 93, 36, 23
52+ 64 + 51 + 57 + 27 = 251 steps
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Optimizing Access to Rotating Devices – cont.
-Shortest-Seek-First-First
- SSTF: (75), 66, 87, 93, 36, 23
–11 + 21 + 6 + 57 + 13 = 107 steps
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Optimizing Access to Rotating Devices – cont.
-Scan/Look
- Scan: (75), 87, 93, 99, 66, 36, 23
–12 + 6 + 6 + 33 + 30 + 13 = 100 steps
- Look: (75), 87, 93, 66, 36, 23
–12 + 6 + 27 + 30 + 13 = 87 steps
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Optimizing Access to Rotating Devices – cont.
- Circular Scan/
Circular Look
• Circular Scan: (75), 87, 93, 99, 23, 36, 66
–12 + 6 + 6 + home + 23 + 13 + 30 = 90 + home
• Circular Look: (75), 87, 93, 23, 36, 66
–12 + 6 + home + 23 + 13 + 30 = 84 + home
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Serial Port
CPU
Memory
Serial
Device
• Printer
• Terminal
• Modem
• Mouse
• etc.
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Serial Port
Device Driver API
Device Driver
Software on the CPU
• Set UART parms
•read/write ops
•Interrupt hander
UART API
•parity
•bits per byte
•etc.
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Bus Interface
Serial Device
(UART)
RS-232 Interface
• 9-pin connector
• 4-wires
• bit transmit/receive
• ...
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Adding a Modem
CPU
Memory
Serial
Device
Modem
• Dialing & connecting
• Convert analog voice to/from digital
• Convert bytes to/from bit streams
• Transmit/receive protocol
Phone
Switched Telephone Network
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Serial Communication
Device Driver
•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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Serial Device
RS-232
Modem
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Exploiting the Phone Network
Logical Communication
CPU
Memory
CPU
Comm
Device
Comm
Device
Modem
Modem
Phone
Phone
Memory
Switched Telephone Network
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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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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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