Chapter 12: I/O Systems
Operating System Concepts – 10th Edition Silberschatz, Galvin and Gagne ©2018
Chapter 12: I/O Systems
Overview
I/O Hardware
Application I/O Interface
Kernel I/O Subsystem
Transforming I/O Requests to Hardware Operations
STREAMS
Performance
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Objectives
Explore the structure of an operating system’s I/O subsystem
Discuss the principles and complexities of I/O hardware
Explain the performance aspects of I/O hardware and software
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Overview
I/O management is a major component of operating system design and
operation
• Important aspect of computer operation
• I/O devices vary greatly
• Various methods to control them
• Performance management
• New types of devices frequent
Ports, busses, device controllers connect to various devices
Device drivers encapsulate device details
• Present uniform device-access interface to I/O subsystem
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I/O Hardware
Incredible variety of I/O devices
• Storage
• Transmission
• Human-interface
Common concepts – signals from I/O devices interface
with computer
• Port – connection point for device
• Bus - daisy chain or shared direct access
PCI bus common in PCs and servers, PCI Express
(PCIe)
expansion bus connects relatively slow devices
Serial-attached SCSI (SAS) common disk
interface
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I/O Hardware (Cont.)
• Controller (host adapter) – electronics that operate port,
bus, device
Sometimes integrated
Sometimes separate circuit board (host adapter)
Contains processor, microcode, private memory, bus
controller, etc.
– Some talk to per-device controller with bus controller,
microcode, memory, etc.
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A Typical PC Bus Structure
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I/O Hardware (Cont.)
Fibre channel (FC) is complex controller, usually separate
circuit board (host-bus adapter, HBA) plugging into bus
I/O instructions control devices
Devices usually have registers where device driver places
commands, addresses, and data to write, or read data from
registers after command execution
• Data-in register, data-out register, status register, control
register
• Typically 1-4 bytes, or FIFO buffer
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I/O Hardware (Cont.)
Devices have addresses, used by
• Direct I/O instructions
• Memory-mapped I/O
Device data and command registers mapped to
processor address space
Especially for large address spaces (graphics)
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Device I/O Port Locations on PCs (partial)
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Polling
For each byte of I/O
1. Read busy bit from status register until 0
2. Host sets read or write bit and if write copies data into data-out
register
3. Host sets command-ready bit
4. Controller sets busy bit, executes transfer
5. Controller clears busy bit, error bit, command-ready bit when
transfer done
Step 1 is busy-wait cycle to wait for I/O from device
• Reasonable if device is fast
• But inefficient if device slow
• CPU switches to other tasks?
But if miss a cycle data overwritten / lost
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Interrupts
Polling can happen in 3 instruction cycles
• Read status, logical-and to extract status bit, branch if not zero
• How to be more efficient if non-zero infrequently?
CPU Interrupt-request line triggered by I/O device
• Checked by processor after each instruction
Interrupt handler receives interrupts
• Maskable to ignore or delay some interrupts
Interrupt vector to dispatch interrupt to correct handler
• Context switch at start and end
• Based on priority
• Some nonmaskable
• Interrupt chaining if more than one device at same interrupt
number
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Interrupt-Driven I/O Cycle
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Interrupts (Cont.)
Interrupt mechanism also used for exceptions
• Terminate process, crash system due to hardware error
Page fault executes when memory access error
System call executes via trap to trigger kernel to execute
request
Multi-CPU systems can process interrupts concurrently
• If operating system designed to handle it
Used for time-sensitive processing, frequent, must be fast
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Latency
Stressing interrupt management because even single-user systems
manage hundreds or interrupts per second and servers hundreds of
thousands
For example, a quiet macOS desktop generated 23,000 interrupts
over 10 seconds
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Intel Pentium Processor Event-Vector Table
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Direct Memory Access
Used to avoid programmed I/O (one byte at a time) for large data
movement
Requires DMA controller
Bypasses CPU to transfer data directly between I/O device and
memory
OS writes DMA command block into memory
• Source and destination addresses
• Read or write mode
• Count of bytes
• Writes location of command block to DMA controller
• Bus mastering of DMA controller – grabs bus from CPU
Cycle stealing from CPU but still much more efficient
• When done, interrupts to signal completion
Version that is aware of virtual addresses can be even more efficient -
DVMA
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Six Step Process to Perform DMA Transfer
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Characteristics of I/O Devices
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Characteristics of I/O Devices (Cont.)
Subtleties of devices handled by device drivers
Broadly I/O devices can be grouped by the OS into
• Block I/O
• Character I/O (Stream)
• Memory-mapped file access
• Network sockets
For direct manipulation of I/O device specific characteristics, usually an
escape / back door
• Unix ioctl() call to send arbitrary bits to a device control register and
data to device data register
UNIX and Linux use tuple of “major” and “minor” device numbers to identify
type and instance of devices (here major 8 and minors 0-4)
% ls –l /dev/sda*
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Block and Character Devices
Block devices include disk drives
• Commands include read, write, seek
• Raw I/O, direct I/O, or file-system access
• Memory-mapped file access possible
File mapped to virtual memory and clusters brought via
demand paging
• DMA
Character devices include keyboards, mice, serial ports
• Commands include get(), put()
• Libraries layered on top allow line editing
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Network Devices
Varying enough from block and character to have own
interface
Linux, Unix, Windows and many others include socket
interface
• Separates network protocol from network operation
• Includes select() functionality
Approaches vary widely (pipes, FIFOs, streams, queues,
mailboxes)
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Clocks and Timers
Provide current time, elapsed time, timer
Normal resolution about 1/60 second
Some systems provide higher-resolution timers
Programmable interval timer used for timings, periodic
interrupts
ioctl() (on UNIX) covers odd aspects of I/O such as
clocks and timers
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Nonblocking and Asynchronous 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
• select() to find if data ready then read() or
write() to transfer
Asynchronous - process runs while I/O executes
• Difficult to use
• I/O subsystem signals process when I/O completed
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Two I/O Methods
Synchronous Asynchronous
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Vectored I/O
Vectored I/O allows one system call to perform multiple I/O
operations
For example, Unix readve() accepts a vector of multiple
buffers to read into or write from
This scatter-gather method better than multiple individual I/O
calls
• Decreases context switching and system call overhead
• Some versions provide atomicity
Avoid for example worry about multiple threads
changing data as reads / writes occurring
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Kernel I/O Subsystem
Scheduling
• Some I/O request ordering via per-device queue
• Some OSs try fairness
• Some implement Quality Of Service (i.e. IPQOS)
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”
• Double buffering – two copies of the data
Kernel and user
Varying sizes
Full / being processed and not-full / being used
Copy-on-write can be used for efficiency in some cases
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Device-status Table
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Common PC and Data-center I/O devices and Interface Speeds
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Kernel I/O Subsystem
Caching - faster device holding copy of data
• Always just a copy
• Key to performance
• Sometimes combined with buffering
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 de-allocation
• Watch out for deadlock
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Error Handling
OS can recover from disk read, device unavailable, transient
write failures
• Retry a read or write, for example
• Some systems more advanced – Solaris FMA, AIX
Track error frequencies, stop using device with
increasing frequency of retry-able errors
Most return an error number or code when I/O request fails
System error logs hold problem reports
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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
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Use of a System Call to Perform I/O
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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
• Windows uses message passing
Message with I/O information passed from user mode
into kernel
Message modified as it flows through to device driver
and back to process
Pros / cons?
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UNIX I/O Kernel Structure
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Power Management
Not strictly domain of I/O, but much is I/O related
Computers and devices use electricity, generate heat, frequently
require cooling
OSes can help manage and improve use
• Cloud computing environments move virtual machines
between servers
Can end up evacuating whole systems and shutting them
down
Mobile computing has power management as first class OS
aspect
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Power Management (Cont.)
For example, Android implements
• Component-level power management
Understands relationship between components
Build device tree representing physical device topology
System bus -> I/O subsystem -> {flash, USB storage}
Device driver tracks state of device, whether in use
Unused component – turn it off
All devices in tree branch unused – turn off branch
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Power Management (Cont.)
For example, Android implements (Cont.)
• Wake locks – like other locks but prevent sleep of device
when lock is held
• Power collapse – put a device into very deep sleep
Marginal power use
Only awake enough to respond to external stimuli (button
press, incoming call)
Modern systems use advanced configuration and power
interface (ACPI) firmware providing code that runs as routines
called by kernel for device discovery, management, error and
power management
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Kernel I/O Subsystem Summary
In summary, the I/O subsystem coordinates an extensive collection of
services that are available to applications and to other parts of the
kernel
• Management of the name space for files and devices
• Access control to files and devices
• Operation control (for example, a modem cannot seek())
• File-system space allocation
• Device allocation
• Buffering, caching, and spooling
• I/O scheduling
• Device-status monitoring, error handling, and failure recovery
• Device-driver configuration and initialization
• Power management of I/O devices
The upper levels of the I/O subsystem access devices via the uniform
interface provided by the device drivers
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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
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Life Cycle of An I/O Request
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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
• Flow control option to indicate available or busy
Asynchronous internally, synchronous where user process
communicates with stream head
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The STREAMS Structure
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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
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Intercomputer Communications
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Improving Performance
Reduce number of context switches
Reduce data copying
Reduce interrupts by using large transfers, smart controllers,
polling
Use DMA
Use smarter hardware devices
Balance CPU, memory, bus, and I/O performance for highest
throughput
Move user-mode processes / daemons to kernel threads
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Device-Functionality Progression
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I/O Performance of Storage (and Network Latency)
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End of Chapter 12
Operating System Concepts – 10th Edition Silberschatz, Galvin and Gagne ©2018
