I/O Systems. Electrical and Computer Engineering Stephen Kim

Transcription

1 I/O Systems Electrical and Computer Engineering Stephen Kim Operating Systems 1 I/O Systems I/O Hardware Application I/O Interface Kernel I/O Subsystem Transforming I/O Requests to Hardware Operations Streams Performance Operating Systems 2 1

2 I/O Hardware (1) Incredible variety of I/O devices in functions and speed Storage devices disks, tapes Transmission devices network cards, modem human-interface devices monitor, keyboard, mouse Special devices sensor, motor Operating Systems 3 I/O Hardware (2) Port connection pointer b/w system and device eg) serial port Bus 2+ devices use a common set of wires eg) PCI, VME, SCSI (daisy chain) Controller (host adapter) electronic operating port, bus, or a device Complication is depending on devices serial port controller mostly embedded into multifunction controller SCSI bus controller mostly built as a separate circuit board (host adapter) Some controller has its own microprocessor and microcode Operating Systems 4 2

3 A Typical PC Bus Structure Operating Systems 5 Bus Daisy Chain devices are connected in a cascading fashion usually operate in a bus PCI Bus to connect memory sub-system to fast devices (monitor, hard disk) Expansion Bus to connect relatively slow devices such as keyboard Operating Systems 6 3

4 Device I/O Port Locations on PCs (partial) Operating Systems 7 I/O Hardware (3) How to give commands and data to a controller A controller has a set of registers for data and control signals. A CPU reads or writes data in the registers Typical IO registers status register control register data-in register data-out register Devices have addresses, used by Direct I/O instructions to use special IO instructions Memory-mapped I/O controller registers are mapped into the address space of the processor Operating Systems 8 4

5 Polling (1) Operation for Data Output 1. Host repeatedly reads a busy-bit in a controller status register until that bit becomes clear 2. Host sets a write-bit in the command register and writes a byte into data-out register 3. Host sets the command-ready bit 4. When the controller notices that the command-ready bit is set, it sets the busy-bit 5. Controller reads the command register and see the write command. It reads the data-out register to get the byte, and does the I/O to the device 6. Controller clears the command-ready bit, an error-bit in the status register Operating Systems 9 Polling (2) Need a buffer to adjust the speeds of consumer and producer Inefficient because of the busy waiting In multi-task operating systems, it is recommended not to use the polling, because of context switch overhead, but not because of the polling itself Operating Systems 10 5

6 Interrupts (1) CPU Interrupt request line triggered by I/O device 1. Device controller raise an interrupt 2. CPU catches the interrupt and dispatches to the interrupt handler 3. The handler clears the interrupt OS has to support 1. ability to defer interrupt handling during critical processing 2. effective dispatch to a proper handler without polling all devices 3. multilevel interrupt for prioritized interrupt handling Operating Systems 11 Interrupts (2) Two Interrupts Maskable to ignore or delay some interrupts Non-Maskable reserved for urgency Interrupt Chaining a small number of interrupt lines, but a large number of devices interrupt vector table contains a pointer to a list of interrupt handlers, which are called one by one until a proper handler is found Operating Systems 12 6

7 Interrupts (3) Interrupt Priority CPU delays handling of lower priority interrupts without masking all interrupts A high priority interrupt can preempt a lower priority interrupt Other Usages of Interrupt Exception handling Trap Operating Systems 13 Intel Pentium Processor Event-Vector Table Operating Systems 14 7

8 Direct Memory Access Programmed I/O CPU reads or writes 1 byte (or word) at a time Inefficient for a large data transfers DMA 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 Operating Systems 15 Six Step Process to Perform DMA Transfer Operating Systems 16 8

9 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 Operating Systems 17 A Kernel I/O Structure Operating Systems 18 9

10 Characteristics of I/O Devices Operating Systems 19 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 Operating Systems 20 10

11 Network Devices Varying enough from block and character to have own interface Unix and Windows include socket interface Separates network protocol from network operation Includes select functionality Approaches vary widely (pipes, FIFOs, streams, queues, mailboxes) Operating Systems 21 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 Operating Systems 22 11

12 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 Operating Systems 23 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 Operating Systems 24 12

13 Kernel I/O Subsystem 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 Operating Systems 25 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 Operating Systems 26 13

14 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 Operating Systems 27 UNIX I/O Kernel Structure Operating Systems 28 14

15 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 Operating Systems 29 Life Cycle of An I/O Request Operating Systems 30 15

16 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 Operating Systems 31 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 Operating Systems 32 16

17 Device-Functionality Progression Operating Systems 33 17