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Chapter 13: I/O Systems - 6 th ed. I/O Hardware Application I/O Interface Kernel I/O Subsystem Transforming I/O Requests to Hardware Operations Streams Performance. Review Chapters 2 and 3, and instructors notes on: “Interrupt schemes and DMA”

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chapter 13 i o systems 6 th ed
Chapter 13: I/O Systems- 6th ed
  • I/O Hardware
  • Application I/O Interface
  • Kernel I/O Subsystem
  • Transforming I/O Requests to Hardware Operations
  • Streams
  • Performance

Review Chapters 2 and 3, and instructors notes on:

“Interrupt schemes and DMA”

This chapter gives more focus to these chapters and topics.

Instructor’s annotations in blue

Updated 12/5/03

Operating System Concepts

i o hardware
I/O Hardware
  • Incredible variety of I/O devices
  • Common concepts
    • Port - basic interface to CPU - status, control, data
    • Bus (daisy chain or shared direct access) - main and specialized local (ex: PCI for main and SCSI for disks)
    • Controller (host adapter) - HW interface between Device and Bus - an adapter card or mother board moduleController has special purposes registers (commands, etc.) which when written to causes actions to take place - may be memory mapped
  • I/O instructions control devices - ex: in, out for Intel
  • Devices have addresses, used by
    • Direct I/O instructions - uses I/O instructions
    • Memory-mapped I/O - uses memory instructions

Operating System Concepts

a typical pc bus structure
A Typical PC Bus Structure

Operating System Concepts

device i o port locations on pcs partial
Device I/O Port Locations on PCs (partial)

Various ranges for a device includes both control and data ports

Operating System Concepts

  • Handshaking
  • Determines state of device
    • command-ready
    • busy
    • Error
  • Busy-wait cycle to wait for I/O from deviceWhen not busy - set data in data port, set command in control port and let ‘er rip
  • Not desirable if excessive - since it is a busy wait which ties up CPU & interferes with productive work
  • Remember CS220 LABs

Operating System Concepts

  • CPU Interrupt request line (IRQ) 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 unmaskable
  • Interrupt mechanism also used for exceptions
  • Application can go away after I/O request, but is til responsible for transferring data to memory when it becomes available from the device.
  • Can have “nested” interrupts (with Priorities)
  • See Instructors notes: “Use of Interrupts and DMA”
  • Soft interrupts or “traps” generated from OS in system calls.

Operating System Concepts

interrupt driven i o cycle
Interrupt-Driven I/O Cycle

Go away & do

Something else ==>

Operating System Concepts

intel pentium processor event vector table
Intel Pentium Processor Event-Vector Table

Interrupts 0-31 are non-maskable - cannot be disabled

Operating System Concepts

direct memory access
Direct Memory Access
  • With pure interrupt scheme, CPU was still responsible for transferring data from controller to memory (on interrupt) when device mad it available.
  • Now DMA will do this - all CPU has to do is set up DMA and user the data when the DMA-complete interrupt arrives. … Interrupts still used - but only to signal DMA Complete.
  • 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
  • Cycle stealing: interference with CPU memory instructions during DMA transfer. - DMA takes priority - CPU pauses on memory part of word.

Operating System Concepts

application i o interface
Application I/O Interface
  • The OS software interface to the I/O devices (an API to the programmer)
  • Attempts to abstract the characteristics of the many I/o devices into a few general classes.
  • 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
      • units for data transfer bytes vs blocks
    • Sequential or random-access - access methods
    • Synchronous (predictable response times) vs asynchronous (unpredictable response times)
    • Sharable or dedicated - implications on deadlock
    • Speed of operation - device/software issue
    • read-write, read only, or write only - permissions

Operating System Concepts

a kernel i o structure
A Kernel I/O Structure

System calls ==>

… “user” API ==>

Example: ioctl(…) generic call

(roll your own)

in UNIX (p. 468), and other more specific commands or calls

open, read, ...

Fig. 13.6

Operating System Concepts

characteristics of i o devices
Characteristics of I/O Devices

Device driver must deal with these at a low level

Use of I/O buffering

Operating System Concepts

block and character devices
Block and Character Devices
  • Block devices include disk drives
    • example sectors or sector clusters on a disk
    • Commands/calls include read, write, seek
    • Access is typically through a file-system interface
    • Raw I/O or file-system access - “binary xfr” of file data - interpretation is in application (personality of file lost)
    • Memory-mapped (to VM) file access possible - use memory instructions rather than I/O instructions - very efficient (ex: swap space for disk).
    • Device driver xfr’s blocks at a time - as in paging
    • DMA transfer is block oriented
  • Character devices include keyboards, mice, serial ports
    • Device driver xfr’s byte at a time
    • Commands include get, put - character at a time
    • Libraries layered on top allow line editing- ex: keyboard input
    • could be beefed up to use a line at a time (buffering)
  • Block & character devices also determine the two general device driver catagories

Operating System Concepts

network devices
Network Devices
  • Varying enough from block and character to have own interface - OS makes network device interface distinct from disk interface - due to significant differences between the two
  • Unix and Windows NT/9i/2000 include socket interface
    • Separates network protocol from network operation
    • Encapsulates details of various network devices for application … analogous to a file and the disk???
    • Includes select functionality - used to manage and access sockets - returns info on packets waiting or ability to accept packets - avoids polling
  • Approaches vary widely (pipes, FIFOs, streams, queues, mailboxes) … you saw some of these!

Operating System Concepts

clocks and timers
Clocks and Timers
  • Provide current time, elapsed time, timer
  • If programmable, interval time used for timings, periodic interrupts
  • ioctl (on UNIX) covers odd aspects of I/O such as clocks and timers - a back door for device driver writers (roll your own). Can implement “secret” calls which may not be documented in a users or programming manual

Operating System Concepts

blocking and nonblocking i o
Blocking and Nonblocking I/O
  • Blocking - process (making the request blocks - lets other process execute) suspended until I/O completed
    • Easy to use and understand
    • Insufficient for some needs
    • multi-threading - depends on role of OS in thread management
  • 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 - ex: read a “small” portion of a file very quickly, use it, and go back for more, ex: displaying video “continuously from a disk”
    • Asynchronous - process (making the asynch request)runs while I/O executes
    • Difficult to use - can it continue without the results of the I/O?
    • I/O subsystem signals process when I/O completed - via interrupt (soft), or setting of shared variable which is periodically tasted.

Operating System Concepts

kernel i o subsystem
Kernel I/O Subsystem
  • See A Kernel I/O Structure slide - Fig 13.6
  • 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 - de-couples application from device action
    • To cope with device transfer size mismatch
    • To maintain “copy semantics” - guarantee that the version of data written to device from a buffer is identical to that which was there at the time of the “write call” - even if on return of the system call, the user modifies buffer - OS copies data to kernel buffer before returning control to user.
    • Double or “ping-pong” buffers - write in one and read from another - decouples devices and applications… idea can be extended to multiple buffers accesses in a circular fashion

Operating System Concepts

kernel i o subsystem continued
Kernel I/O Subsystem - (continued)
  • Caching - fast memory holding copy of data
    • Always just a copy
    • Key to performance
    • How does this differ from a buffer?
  • Spooling - a buffer holding output/(input too) for a device
    • If device can serve only one request at a time
    • Avoids queuing applications making requests.
    • Data from an application is saved in a unique file associated with the application AND the particular request. Could be saved in files on a disk, or in memory.
    • Example: Printing
  • Device reservation - provides exclusive access to a device
    • System calls for allocation and deallocation
    • Watch out for deadlock - why?

Operating System Concepts

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
  • CRC checks - especially over network transfers of a lot of data, for example video in real time.

Operating System Concepts

kernel data structures
Kernel Data Structures
  • Kernel keeps state info for I/O components, including open file tables, network connections, character device state
    • used by device drivers in manipulating devices and data transfer, and in for error recovery
    • data that has images on the disk must be kept in synch with disk copy.
  • Many, many complex data structures to track buffers, memory allocation, “dirty” blocks
  • Some use object-oriented methods and message passing to implement I/O
    • Make data structures object oriented classes to encapsulate the low level nature of the “device” - UNIX provides a seamless interface such as this.

Operating System Concepts

unix i o kernel data structure
UNIX I/O Kernel Data Structure

Refer to chapter 11 and 12 on files

Fig. 13.9

Operating System Concepts

mapping i o requests to hardware operations
Mapping I/O Requests to Hardware Operations
  • Consider reading a file from disk for a process:How is connection made from file-name to disk controller:
    • 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
  • See the 10 step scenario on pp. 479-481 (Silberschatz, 6th ed.) for a clear description.

Operating System Concepts

life cycle of an i o request
Life Cycle of An I/O Request

Data already in buffer

Ex read ahead

Operating System Concepts

  • STREAM – a full-duplex communication channel between a user-level process and a device
  • 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

Operating System Concepts

the streams structure
The STREAMS Structure

Operating System Concepts

performance sect 13 7
Performancesect 13.7
  • I/O a major factor in system performance:
    • Places demands on CPU to execute device driver, kernel I/O code
      • resulting in context switching
      • interrupt overhead
    • Data copying - loads down memory bus
    • Network traffic especially stressful
    • See bulleted list on page 485(Silberschatz, 6th ed.)
  • Improving PerformanceSee bulleted list on page 485(Silberschatz, 6th ed.)
    • Reduce number of context switches
    • Reduce data copying
    • Reduce interrupts by using large transfers, smart controllers, polling
    • Use DMA
    • Move proccessing primitives to hardware
    • Balance CPU, memory, bus, and I/O performance for highest throughput

Operating System Concepts

device functionality progression
Device-Functionality Progression

Where should I/O functionality be implemented? Application level … device hardware

Decision depends on trade-offs in the design layers:

Operating System Concepts