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I/O

I/O. CS 416: Operating Systems Design, Spring 2001 Department of Computer Science Rutgers University http://remus.rutgers.edu/cs416/F01/. I/O Devices. So far we have talked about how to abstract and manage CPU and memory

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I/O

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  1. I/O CS 416: Operating Systems Design, Spring 2001 Department of Computer ScienceRutgers University http://remus.rutgers.edu/cs416/F01/

  2. I/O Devices • So far we have talked about how to abstract and manage CPU and memory • Computation “inside” computer is useful only if some results are communicated “outside” of the computer • I/O devices are the computer’s interface to the outside world (I/O  Input/Output) • Example devices: display, keyboard, mouse, speakers, network interface, and disk CS 416: Operating Systems

  3. Disk Basic Computer Structure CPU Memory Memory Bus (System Bus) Bridge I/O Bus NIC CS 416: Operating Systems

  4. OS: Abstractions and Access Methods • OS must virtualizes a wide range of devices into a few simple abstractions: • Storage • Hard drives, Tapes, CDROM • Networking • Ethernet, radio, serial line • Multimedia • DVD, Camera, microphones • Operating system should provide consistent methods to access the abstractions • Otherwise, programming is too hard CS 416: Operating Systems

  5. User/OS method interface • The same interface is used to access devices (like disks and network lines) and more abstract resources like files • 4 main methods: • open() • close() • read() • write() • Semantics depend on the type of the device (block, char, net) • These methods are system calls because they are the methods the OS provides to all processes. • A Java program can’t access these methods directly, they are used by the JVM. C and C++ programs can call these directly. CS 416: Operating Systems

  6. Unix I/O Methods • fileHandle = open(pathName, flags, mode) • a file handle is a small integer, valid only within a single process, to operate on the device or file • pathname: a name in the file system. In unix, devices are put under /dev. E.g. /dev/ttya is the first serial port, /dev/sda the first SCSI drive • flags: blocking or non-blocking … • mode: read only, read/write, append … • errorCode = close(fileHandle) • Kernel will free the data structures associated with the device CS 416: Operating Systems

  7. Unix I/O Methods • byteCount = read(fileHandle, byte [] buf, count) • read at most count bytes from the device and put them in the byte buffer buf. Bytes placed from 0th byte. • Kernel can give the process less bytes, user process must check the byteCount to see how many were actually returned. • A negative byteCount signals an error (value is the error type) • byteCount = write(fileHandle, byte [] buf, count) • write at most count bytes from the buffer buf • actual number written returned in byteCount • a Negative byteCount signals an error CS 416: Operating Systems

  8. Unix I/O Example • What’s the correct way to write a 1000 bytes? • calling: • ignoreMe = write(fileH, buffer, 1000); • works most of the time. What happens if: • can’t accept 1000 bytes right now? • disk is full? • someone just deleted the file? • Let’s work this out on the board CS 416: Operating Systems

  9. I/O semantics • From this basic interface, three different dimension to how I/O is processed: • blocking vs. non-blocking • buffered vs. unbuffered • synchronous vs. asynchronous • The O/S tries to support as many of these dimensions as possible for each device • The semantics are specified during the open() system call CS 416: Operating Systems

  10. Blocking vs. Non-Blocking I/O • Blocking –process is suspended until all bytes in the count field are read or written • E.g., for a network device, if the user wrote 1000 bytes, then the operating system would write the bytes to the device one at a time until the write() completed. • + Easy to use and understand • - if the device just can’t perform the operation (e.g. you unplug the cable), what to do? Give up an return the successful number of bytes. • Nonblocking – the OS only reads or writes as many bytes as is possible without suspending the process • + Returns quickly • - more work for the programmer (but really for robust programs)? CS 416: Operating Systems

  11. Buffered vs. Unbuffered I/O • Sometime we want the ease of programming of blocked I/O without the long waits if the buffers on the device are small. • buffered I/O allows the kernel to make a copy of the data • write() side: allows the process to write() many bytes and continue processing • read() side: As device signals data is ready, kernel places data in the buffer. When the process calls read(), the kernel just copies the buffer. • Why not use buffered I/O? • - Extra copy overhead • - Delays sending data CS 416: Operating Systems

  12. Synchronous vs. Asynchronous I/O • Synchronous I/O: the user process does not run at the same time the I/O does --- it is suspended during I/O • So far, all the methods presented have been synchronous. • Asynchronous I/O: The read() or write() call returns a small object instead of a count. • separate set of methods in unix: aio_read(), aio_write() • The user can call methods on the returned object to check “how much” of the I/O has completed • The user can also allow a signal handler to run when the the I/O has completed. CS 416: Operating Systems

  13. Handling Multiple I/O streams • If we use blocking I/O, how do we handle > 1 device at a time? • Example : a small program that reads from serial line and outputs to a tape and is also reading from the tape and outputting to a serial line • Structure the code like this? • while (TRUE) { • read(tape); // block until tape is ready • write(serial line);// send data to serial line • read(serial line); • write(tape); • } • Could use non-blocking I/O, but huge waste of cycles if data is not ready. CS 416: Operating Systems

  14. Solution: select system call • totalFds = select(nfds, readfds, writefds, errorfds, timeout); • nfds: the range (0.. nfds) of file descriptors to check • readfds: bit map of fileHandles. user sets the bit X to ask the kernel to check if fileHandle X ready for reading. Kernel returns a 1 if data can be read on the fileHandle. • writefds: bit map if fileHandle Y for writing. Operates same as read bitmap. • errorfds: bit map to check for errors. • timeout: how long to wait before un-suspending the process • totalFds = number of set bits, negative number is an error CS 416: Operating Systems

  15. Three Device Types • Most operating system have three device types: • Character devices • Used for serial-line types of devices (e.g. USB port) • Network devices • Used for network interfaces (E.g. Ethernet card) • Block devices: • Used for mass-storage (E.g. disks and CDROM) • What you can expect from the read/write methods changes with each device type CS 416: Operating Systems

  16. Character Devices • Device is represented by the OS as an ordered stream of bytes • bytes sent out the device by the write() system call • bytes read from the device by the read() system call • Byte stream has no “start”, just open and start/reading writing • The user has no control of the read/write ratio, the sender process might write() 1 time of 1000 bytes, and the receiver may have to call 1000 read calls each receiving 1 byte. CS 416: Operating Systems

  17. Network Devices • Like I/O devices, but each write() call either sends the entire block (packet), up to some maximum fixed size, or none. • On the receiver, the read() call returns all the bytes in the block, or none. CS 416: Operating Systems

  18. Block Devices • OS presents device as a large array of blocks • Each block has a fixed size (1KB - 8KB is typical) • User can read/write only in fixed sized blocks • Unlike other devices, block devices support random access • We can read or write anywhere in the device without having to ‘read all the bytes first’ • But how to specify the nth block with current interface? CS 416: Operating Systems

  19. The file pointer • O/S adds a concept call the file pointer • A file pointer is associated with each open file, if the device is a block device • the next read or write operates at the position in the device pointed to by the file pointer • The file pointer is measured in bytes, not blocks CS 416: Operating Systems

  20. Seeking in a device • To set the file pointer: • absoluteOffset = lseek(fileHandle, offset, whence); • whence specifies if the offset is absolute, from byte 0, or relative to the current file pointer position • the absolute offset is returned; negative numbers signal error codes • For devices, the offset should be a integral number of bytes relative to the size of a block. • How could you tell what the current position of the file pointer is without changing it? CS 416: Operating Systems

  21. Block Device Example • You want to read the 10th block of a disk • each disk block is 2048 bytes long • fh = open(/dev/sda, , , ); • pos = lseek(fh, size*count, ); • if (pos < 0 ) error; • bytesRead = read(fh, buf, blockSize); • if (bytesRead < 0) error; • … CS 416: Operating Systems

  22. Getting and setting device specific information • Unix has an IO ConTroL system call: • ErrorCode = ioctl(fileHandle, int request, object); • request is a numeric command to the device • can also pass an optional, arbitrary object to a device • the meaning of the command and the type of the object are device specific CS 416: Operating Systems

  23. Programmed I/O vs. DMA • Programmed I/O is ok for sending commands, receiving status, and communication of a small amount of data • Inefficient for large amount of data • Keeps CPU busy during the transfer • Programmed I/O  memory operations  slow • Direct Memory Access • Device read/write directly from/to memory • Memory  device typically initiated from CPU • Device  memory can be initiated by either the device or the CPU CS 416: Operating Systems

  24. Disk Disk Disk Programmed I/O vs. DMA CPU Memory CPU Memory CPU Memory Interconnect Interconnect Interconnect Programmed I/O DMA DMA Device  Memory Problems? CS 416: Operating Systems

  25. Direct Memory Access • 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 CS 416: Operating Systems

  26. Six step process to perform DMA transfer CS 416: Operating Systems

  27. Life Cycle of an I/O Request CS 416: Operating Systems

  28. 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 CS 416: Operating Systems

  29. Intercomputer communications CS 416: Operating Systems

  30. 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 CS 416: Operating Systems

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