[IOS] I/O Systems

Ch 13 I/O Systems

Ch 13.1 Overview

• 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
  

Ch 13.2 I/O Hardware

• Incredible variety of I/O devices
• Common concepts
  
  • Port
  • Bus (daisy chain or shared direct access)
  • Controller (host adapter)
• I/O instructions control devices
• Devices have addresses, used by
  
  • Direct I/O instructions
  • Memory-mapped I/O

A Typical PC Bus Structure

13.2.1 Polling

• Determines state of device
  
  • command-ready
  • busy
  • Error
• Busy-wait cycle to wait for I/O from device

13.2.2 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
  
  • Based on priority
  • Some nonmaskable
• Interrupt mechanism also used for exceptions
  

13.2.3 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

Six Step Process to Perform DMA Transfer

Ch 13.3 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
  
  • Character-stream or block
  • Sequential or random-access
  • Sharable or dedicated
  • Speed of operation
  • read-write, read only, or write only

13.3.1 Block and Character Devices

• Block devices include disk drives
  
  • Commands include read, write, seek
  • Raw I/O or file-system access
  • Memory-mapped file access possible
• Character devices include keyboards, mice, serial ports
  
  • Commands include get, put
  • Libraries layered on top allow line editing

13.3.2 Network Devices

• Varying enough from block and character to have own interface
• Unix and Windows NT/9x/2000 include socket interface
  
  • Separates network protocol from network operation
  • Includes select functionality
• Approaches vary widely (pipes, FIFOs, streams, queues, mailboxes)

13.3.3 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

13.3.4 Blocking and Nonblocking I/O

• Blocking - process suspended until I/O completed
  
  • Easy to use and understand
  • Insufficient for some needs
• Nonblocking - I/O call returns as much as available
  
  • User interface, data copy (buffered I/O)
  • Implemented via multi-threading
  • Returns quickly with count of bytes read or written
• Asynchronous - process runs while I/O executes
  
  • Difficult to use
  • I/O subsystem signals process when I/O completed

Ch 13.4 Kernel I/O Subsystem

• Scheduling
  
  • Some I/O request ordering via per-device queue
  • Some OSs try fairness
• Buffering - store data in memory while transferring between devices
  
  • To cope with device speed mismatch
  • To cope with device transfer size mismatch
  • To maintain “copy semantics”
• Caching - fast memory holding copy of data
  
  • Always just a copy
  • Key to performance
• Spooling - hold output for a device
  
  • If device can serve only one request at a time
  • i.e., Printing
• Device reservation - provides exclusive access to a device
  
  • System calls for allocation and deallocation
  • Watch out for deadlock

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

I/O Protection

• User process may accidentally or purposefully attempt to disrupt normal operation via illegal I/O instructions
  
  • All I/O instructions defined to be privileged
  • I/O must be performed via system calls
    
• Memory-mapped and I/O port memory locations must be protected too

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

Ch 13.5 Transforming I/O Requests to Hardware Operations

• Consider reading a file from disk for a process:
  
  • Determine device holding file
  • Translate name to device representation
  • Physically read data from disk into buffer
  • Make data available to requesting process
  • Return control to process

Ch 13.6 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
  

Ch 13.7 Performance

• I/O a major factor in system performance:
  
  • Demands CPU to execute device driver, kernel I/O code
  • Context switches due to interrupts
  • Data copying
  • Network traffic especially stressful

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

Reference: Operating System Concepts 8th, by Silberschatz, Galvin, Gagne