1、Chapter 13: I/O Systems,Chapter 13: I/O Systems,I/O Hardware Application I/O Interface Kernel I/O Subsystem Transforming I/O Requests to Hardware Operations STREAMS Performance,Objectives,Explore the structure of an operating systems I/O subsystemDiscuss the principles of I/O hardware and its comple
2、xityProvide details of the performance aspects of I/O hardware and software,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 device
3、s frequentPorts, busses, device controllers connect to various devicesDevice drivers encapsulate device details Present uniform device-access interface to I/O subsystem,I/O Hardware,Incredible variety of I/O devices Storage Transmission Human-interface Common concepts signals from I/O devices interf
4、ace with computer Port connection point for device Bus - daisy chain or shared direct access 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 So
5、me talk to per-device controller with bus controller, microcode, memory, etc,A Typical PC Bus Structure,I/O Hardware (Cont.),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 ex
6、ecution Data-in register, data-out register, status register, control register Typically 1-4 bytes, or FIFO buffer 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 (graphi
7、cs),Device I/O Port Locations on PCs (partial),Polling,For each byte of I/O Read busy bit from status register until 0 Host sets read or write bit and if write copies data into data-out register Host sets command-ready bit Controller sets busy bit, executes transfer Controller clears busy bit, error
8、 bit, command-ready bit when transfer doneStep 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,Interrupts,Polling can happen in 3 instruction cycles Read status, logi
9、cal-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 di
10、spatch 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,Interrupt-Driven I/O Cycle,Intel Pentium Processor Event-Vector Table,Interrupts (Cont.),Interrupt mechanism also used for except
11、ions Terminate process, crash system due to hardware errorPage fault executes when memory access errorSystem call executes via trap to trigger kernel to execute requestMulti-CPU systems can process interrupts concurrently If operating system designed to handle itUsed for time-sensitive processing, f
12、requent, must be fast,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 mod
13、e Count of bytes Writes location of command block to DMA controller Bus mastering of DMA controller grabs bus from CPU When done, interrupts to signal completion,Six Step Process to Perform DMA Transfer,Application I/O Interface,I/O system calls encapsulate device behaviors in generic classes Device
14、-driver layer hides differences among I/O controllers from kernel New devices talking already-implemented protocols need no extra work Each OS has its own I/O subsystem structures and device driver frameworks Devices vary in many dimensions Character-stream or block Sequential or random-access Synch
15、ronous or asynchronous (or both) Sharable or dedicated Speed of operation read-write, read only, or write only,A Kernel I/O Structure,Characteristics of I/O Devices,Characteristics of I/O Devices (Cont.),Subtleties of devices handled by device driversBroadly I/O devices can be grouped by the OS into
16、 Block I/O Character I/O (Stream) Memory-mapped file access Network socketsFor 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,Block and Character Devices,
17、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(),
18、put() Libraries layered on top allow line editing,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,
19、 streams, queues, mailboxes),Clocks and Timers,Provide current time, elapsed time, timerNormal resolution about 1/60 secondSome 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 ti
20、mers,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 r
21、ead 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,Two I/O Methods,Synchronous,Asynchronous,Kernel I/O Subsystem,Scheduling Some I/O request ordering via per
22、-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 Ke
23、rnel and user Varying sizes Full / being processed and not-full / being used Copy-on-write can be used for efficiency in some cases,Device-status Table,Sun Enterprise 6000 Device-Transfer Rates,Kernel I/O Subsystem,Caching - faster device holding copy of data Always just a copy Key to performance So
24、metimes 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,Error Handling,OS can recover from disk re
25、ad, 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
26、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,Use of
27、a System Call to Perform I/O,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
28、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?,UNIX I/O Kernel Structure,I/O Requests to Hardware Operations,Consider reading a file from disk
29、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,Life Cycle of An I/O Request,STREAMS,STREAM a full-duplex communication channel between a user-level pro
30、cess and a device in Unix System V and beyondA STREAM consists of:- STREAM head interfaces with the user process- driver end interfaces with the device - zero or more STREAM modules between themEach module contains a read queue and a write queueMessage passing is used to communicate between queues F
31、low control option to indicate available or busyAsynchronous internally, synchronous where user process communicates with stream head,The STREAMS Structure,Performance,I/O a major factor in system performance: Demands CPU to execute device driver, kernel I/O code Context switches due to interrupts D
32、ata copying Network traffic especially stressful,Intercomputer Communications,Improving Performance,Reduce number of context switchesReduce data copyingReduce interrupts by using large transfers, smart controllers, pollingUse DMAUse smarter hardware devicesBalance CPU, memory, bus, and I/O performance for highest throughputMove user-mode processes / daemons to kernel threads,Device-Functionality Progression,End of Chapter 12,
