Chapter 13: I/O Systems
Chapter 13: I/O Systems
13.2
Chapter 13: I/O Systems Chapter 13: I/O Systems
13.1 Overview 13.2 I/O Hardware
13.3 Application I/O Interface 13.4 Kernel I/O Subsystem
13.5 Transforming I/O Requests to Hardware Operations
13.6 STREAMS
13.7 Performance
13.3
Objectives Objectives
Explore the structure of an operating system’s I/O subsystem
Discuss the principles of I/O hardware and its complexity
Provide details of the performance aspects of I/O hardware and software
13.4
I/O Hardware I/O Hardware
Incredible variety of I/O devices
Common concepts
z Port
z Bus (daisy chain or shared direct access)
z Controller (host adapter)
I/O instructions control devices
Devices have addresses, used by
z Direct I/O instructions
z Memory-mapped I/O
13.5
A Typical PC Bus Structure
A Typical PC Bus Structure
13.6
Device I/O Port Locations on PCs (partial)
Device I/O Port Locations on PCs (partial)
13.7
Polling Polling
Determines state of device
z command-ready
z busy
z Error
Busy-wait cycle to wait for I/O from device
13.8
Interrupts Interrupts
CPU Interrupt-request line triggered by I/O device
Interrupt handler receives interrupts
Maskable to ignore or delay some interrupts
Interrupt vector to dispatch interrupt to correct handler
z Based on priority
z Some nonmaskable
Interrupt mechanism also used for exceptions
13.9
Interrupt
Interrupt - - Driven I/O Cycle Driven I/O Cycle
13.10
Intel Pentium Processor Event
Intel Pentium Processor Event - - Vector Table Vector Table
13.11
Direct Memory Access Direct Memory Access
Used to avoid programmed I/O for large data movement
Requires DMA controller
Bypasses CPU to transfer data directly between I/O device and memory
13.12
Six Step Process to Perform DMA Transfer
Six Step Process to Perform DMA Transfer
13.13
Application I/O Interface Application I/O Interface
I/O system calls encapsulate device behaviors in generic classes
Device-driver layer hides differences among I/O controllers from kernel
Devices vary in many dimensions
z Character-stream or block
z Sequential or random-access
z Sharable or dedicated
z Speed of operation
z read-write, read only, or write only
13.14
A Kernel I/O Structure
A Kernel I/O Structure
13.15
Characteristics of I/O Devices
Characteristics of I/O Devices
13.16
Block and Character Devices Block and Character Devices
Block devices include disk drives
z Commands include read, write, seek
z Raw I/O or file-system access
z Memory-mapped file access possible
Character devices include keyboards, mice, serial ports
z Commands include get, put
z Libraries layered on top allow line editing
13.17
Network Devices Network Devices
Varying enough from block and character to have own interface
Unix and Windows NT/9x/2000 include socket interface
z Separates network protocol from network operation
z Includes select functionality
Approaches vary widely (pipes, FIFOs, streams, queues, mailboxes)
13.18
Clocks and Timers Clocks and Timers
Provide current time, elapsed time, timer
Programmable interval timer used for timings, periodic interrupts
ioctl (on UNIX) covers odd aspects of I/O such as clocks and timers
13.19
Blocking and Nonblocking I/O Blocking and Nonblocking I/O
Blocking - process suspended until I/O completed
z Easy to use and understand
z Insufficient for some needs
Nonblocking - I/O call returns as much as available
z User interface, data copy (buffered I/O)
z Implemented via multi-threading
z Returns quickly with count of bytes read or written
Asynchronous - process runs while I/O executes
z Difficult to use
z I/O subsystem signals process when I/O completed
13.20
Two I/O Methods Two I/O Methods
Synchronous Asynchronous
13.21
Kernel I/O Subsystem Kernel I/O Subsystem
Scheduling
z Some I/O request ordering via per-device queue
z Some OSs try fairness
Buffering - store data in memory while transferring between devices
z To cope with device speed mismatch
z To cope with device transfer size mismatch
z To maintain “copy semantics”
13.22
Device
Device - - status Table status Table
13.23
Sun Enterprise 6000 Device
Sun Enterprise 6000 Device - - Transfer Rates Transfer Rates
13.24
Kernel I/O Subsystem Kernel I/O Subsystem
Caching - fast memory holding copy of data
z Always just a copy
z Key to performance
Spooling - hold output for a device
z If device can serve only one request at a time
z i.e., Printing
Device reservation - provides exclusive access to a device
z System calls for allocation and deallocation
z Watch out for deadlock
13.25
Error Handling Error Handling
OS can recover from disk read, device unavailable, transient write failures
Most return an error number or code when I/O request fails
System error logs hold problem reports
13.26
I/O Protection I/O Protection
User process may accidentally or purposefully attempt to disrupt normal operation via illegal I/O instructions
z All I/O instructions defined to be privileged
z I/O must be performed via system calls
Memory-mapped and I/O port memory locations must be protected too
13.27
Use of a System Call to Perform I/O
Use of a System Call to Perform I/O
13.28
Kernel Data Structures Kernel Data Structures
Kernel keeps state info for I/O components, including open file tables, network connections, character device state
Many, many complex data structures to track buffers, memory allocation, “dirty” blocks
Some use object-oriented methods and message passing to implement I/O
13.29
UNIX I/O Kernel Structure
UNIX I/O Kernel Structure
13.30
I/O Requests to Hardware Operations I/O Requests to Hardware Operations
Consider reading a file from disk for a process:
z Determine device holding file
z Translate name to device representation
z Physically read data from disk into buffer
z Make data available to requesting process
z Return control to process
13.31
Life Cycle of An I/O Request
Life Cycle of An I/O Request
13.32
STREAMS STREAMS
STREAM – a full-duplex communication channel between a user- level process and a device in Unix System V and beyond
A STREAM consists of:
- STREAM head interfaces with the user process - driver end interfaces with the device
- zero or more STREAM modules between them.
Each module contains a read queue and a write queue
Message passing is used to communicate between queues
13.33
The STREAMS Structure
The STREAMS Structure
13.34
Performance Performance
I/O a major factor in system performance:
z Demands CPU to execute device driver, kernel I/O code
z Context switches due to interrupts
z Data copying
z Network traffic especially stressful
13.35
Intercomputer Communications
Intercomputer Communications
13.36
Improving Performance Improving Performance
Reduce number of context switches
Reduce data copying
Reduce interrupts by using large transfers, smart controllers, polling
Use DMA
Balance CPU, memory, bus, and I/O performance for highest throughput
13.37
Device
Device - - Functionality Progression Functionality Progression
End of Chapter 13 End of Chapter 13
13.1 Overview 13.2 I/O Hardware
13.3 Application I/O Interface 13.4 Kernel I/O Subsystem
13.5 Transforming I/O Requests to Hardware Operations
13.6 STREAMS
13.7 Performance