1. I/O and Networking Fred Kuhns
Department of Computer Science and Engineering
Washington University in St. Louis

2. CS422 – Operating Systems Concepts
Overview
Two fundamental operations performed by a computing system
Processing of data (implement an algorithm)
Perform I/O (move data into and out of the system)
Large diversity in I/O devices, capabilities and performance
Common Categories: storage, transmission and user-interface devices.
Examples: display, keyboard, network, hard disks, tape drives, CDROM ...
OS Goal is to
provide simple and consistent interface to user
optimize I/O use for maximum concurrency
Mechanisms used by OS
device drivers provide standard interface to I/O devices
Fred Kuhns (4/1/2017)
CS422 – Operating Systems Concepts

3. CS422 – Operating Systems Concepts
I/O and Devices
Processor
Device table
Device Driver
dispatcher
(interrupt handler) X
Bus
command
status
data 0
data 1
Device Controller
(firmware and logic)
...
Device X
data N-1
Fred Kuhns (4/1/2017)
CS422 – Operating Systems Concepts

4. CS422 – Operating Systems Concepts
I/O Hardware
Common concepts
Port (device-machine communication point)
Bus (daisy chain or shared communication channel)
Controller (host adapter)
Devices have addresses, used by
Direct I/O instructions
Memory-mapped I/O
Common Techniques
Direct I/O with polling, aka Programmed I/O: Processor does all the work. Poll for results.
Interrupt Driven I/O: Device notifies CPU when I/O operation complete
Memory Mapped I/O: rather than reading/writing to controller registers the device is mapped into the OS memory space. increased efficiency
Direct memory access (DMA): DMA controller read and write directly to memory, freeing the CPU to do other things.CPU notified when DMA complete
Fred Kuhns (4/1/2017)
CS422 – Operating Systems Concepts

5. CS422 – Operating Systems Concepts
Programmed I/O
Busy-wait cycle to wait for I/O from device
Poll at select times: periodic, entering/leaving kernel etc.
Determines state of device: command-ready, busy, error
Processor transfer data to/from device.
Read/write directly to status and command registers
Processor polls device for status
Consumes a lot of processor time because every word read or written passes through the processor
Insert Read
command to
I/O Module
Read Status
of I/O
Module
Check
Status
Read word
from I/O
Write word
into memory
Done?
Yes No
Next Instruction
CPU Memory
I/O CPU
Error
Condition
CPU I/O
Ready
Not
Fred Kuhns (4/1/2017)
CS422 – Operating Systems Concepts

6. CS422 – Operating Systems Concepts
Interrupt-Driven I/O
Similar to direct I/O but processor not required to poll device.
Interrupt asserted to notify processor of a change in status
Interrupt handler receives interrupts
Maskable to ignore or delay some interrupts
Interrupt vector to dispatch interrupt to correct handler
Based on priority
Some unmaskable
Insert Read
command to
I/O Module
Read Status
of I/O
Module
Check
Status
Read word
from I/O
Write word
into memory
Done?
Yes No
Next Instruction
CPU Memory
I/O CPU
Error
Condition
CPU I/O
Ready
Do something
else
Interrupt
Interrupt mechanism also used for exceptions
Fred Kuhns (4/1/2017)
CS422 – Operating Systems Concepts

7. Direct Memory Access (DMA)
I/O exchanges to memory
Processor grants I/O module authority to read from or write to memory
Relieves the processor from the task
Processor is free to do other things
An interrupt is sent when the task is complete
The processor is only involved at the beginning and end of the transfer
Used to avoid programmed I/O for large data movement
Requires DMA controller
Next Instruction
CPU DMA
Interrupt
DMA CPU
Do something
else
Issue Read
block command
to I/O module
Read status
of DMA
module
Fred Kuhns (4/1/2017)
CS422 – Operating Systems Concepts

8. Six steps to perform DMA transfer
Fred Kuhns (4/1/2017)
CS422 – Operating Systems Concepts

9. Another way to Describe I/O Modes
Synchronous (Programmed I/O) - control returns to user program only upon I/O completion.
Idle CPU until the next interrupt
wait loop (contention for memory access).
Asynchronous (Interrupt driven I/O) - control returns to user program before I/O completion.
System call – request to the operating system to allow user to wait for I/O completion.
Device-status table contains entry for each I/O device indicating its type, address, and state.
Fred Kuhns (4/1/2017)
CS422 – Operating Systems Concepts

10. I/O Interfaces (Layers)
I/O system call layer encapsulates device behaviors in generic classes
Device-driver layer hides differences among I/O controllers from kernel
device dependent versus device independent layer
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
Fred Kuhns (4/1/2017)
CS422 – Operating Systems Concepts

11. 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
Fred Kuhns (4/1/2017)
CS422 – Operating Systems Concepts

12. Communication Endpoints
Varying enough from block and character oriented devices to have their own interface
Introduces new concepts such as connections, communications protocols
Unix and Windows/NT include socket interface
Separates network protocol from network operation
Includes select functionality
Approaches vary widely (pipes, FIFOs, streams, queues, mailboxes)
Fred Kuhns (4/1/2017)
CS422 – Operating Systems Concepts

13. CS422 – Operating Systems Concepts
Clocks and Timers
Provide current time, elapsed time, timer
Programmable interval time used for timings, periodic interrupts
ioctl (on UNIX) covers odd aspects of I/O such as clocks and timers
Fred Kuhns (4/1/2017)
CS422 – Operating Systems Concepts

14. Blocking versus Nonblocking I/O
Blocking - process suspended until I/O completed
Easy to use and understand
Insufficient for some needs
Nonblocking - I/O call returns what is 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
Fred Kuhns (4/1/2017)
CS422 – Operating Systems Concepts

15. CS422 – Operating Systems Concepts
Kernel I/O Subsystem
Scheduling
Efficiency, fairness, prioritized access
Buffering - store data in memory while transferring between devices
To cope with device speed mismatch, transfer size mismatch and to maintain “copy semantics”
Caching - fast memory holding copy of data
Always just a copy, Key to performance
Spooling - hold output for a device
Used when 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
Fred Kuhns (4/1/2017)
CS422 – Operating Systems Concepts

16. CS422 – Operating Systems Concepts
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
Fred Kuhns (4/1/2017)
CS422 – Operating Systems Concepts

17. Kernel Data Structures
Kernel keeps state info for I/O components, including open file tables, network connections, character device state
Many complex data structures to track buffers, memory allocation, “dirty” blocks
Some use object-oriented methods and message passing to implement I/O
Fred Kuhns (4/1/2017)
CS422 – Operating Systems Concepts

18. Example I/O Request to HW 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
Fred Kuhns (4/1/2017)
CS422 – Operating Systems Concepts

19. Life Cycle of an I/O Request
Fred Kuhns (4/1/2017)
CS422 – Operating Systems Concepts

20. Intercomputer communications
context switch
context switch
mode
switch
mode
switch
context switch
Fred Kuhns (4/1/2017)
CS422 – Operating Systems Concepts

21. Improving 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
Fred Kuhns (4/1/2017)
CS422 – Operating Systems Concepts