Chapter 13: I/O Systems Hung Q. Ngo KyungHee University Spring Note: Some slides and/or pictures in the following are adapted from slides ©2005 Silberschatz, Galvin, and Gagne. Many slides generated from my lecture notes by Kubiatowicz.

13.2 Hung Q. Ngo Spring 2009 Operating System The Requirements of I/O So far in this course: –We have learned how to manage CPU, memory What about I/O? –Without I/O, computers are useless (disembodied brains?) –But… thousands of devices, each slightly different »How can we standardize the interfaces to these devices? –Devices unreliable: media failures and transmission errors »How can we make them reliable??? –Devices unpredictable and/or slow »How can we manage them if we don’t know what they will do or how they will perform? Some operational parameters: –Byte/Block »Some devices provide single byte at a time (e.g. keyboard) »Others provide whole blocks (e.g. disks, networks, etc) –Sequential/Random »Some devices must be accessed sequentially (e.g. tape) »Others can be accessed randomly (e.g. disk, cd, etc.) –Polling/Interrupts »Some devices require continual monitoring »Others generate interrupts when they need service

13.3 Hung Q. Ngo Spring 2009 Operating System Modern I/O Systems

13.4 Hung Q. Ngo Spring 2009 Operating System Example Device-Transfer Rates (Sun Enterprise 6000) Device Rates vary over many orders of magnitude –System better be able to handle this wide range –Better not have high overhead/byte for fast devices! –Better not waste time waiting for slow devices

13.5 Hung Q. Ngo Spring 2009 Operating System The Goal of the I/O Subsystem Provide Uniform Interfaces, Despite Wide Range of Different Devices –This code works on many different devices: FILE fd = fopen(“/dev/something”,”rw”); for (int i = 0; i < 10; i++) { fprintf(fd,”Count %d\n”,i); } close(fd); –Why? Because code that controls devices (“device driver”) implements standard interface. We will try to get a flavor for what is involved in actually controlling devices in rest of lecture –Can only scratch surface!

13.6 Hung Q. Ngo Spring 2009 Operating System Want Standard Interfaces to Devices Block Devices: e.g. disk drives, tape drives, DVD-ROM –Access blocks of data –Commands include open(), read(), write(), seek() –Raw I/O or file-system access –Memory-mapped file access possible Character Devices: e.g. keyboards, mice, serial ports, some USB devices –Single characters at a time –Commands include get(), put() –Libraries layered on top allow line editing Network Devices: e.g. Ethernet, Wireless, Bluetooth –Different enough from block/character to have own interface –Unix and Windows include socket interface »Separates network protocol from network operation »Includes select() functionality –Usage: pipes, FIFOs, streams, queues, mailboxes

13.7 Hung Q. Ngo Spring 2009 Operating System How Does User Deal with Timing? Blocking Interface: “Wait” –When request data (e.g. read() system call), put process to sleep until data is ready –When write data (e.g. write() system call), put process to sleep until device is ready for data Non-blocking Interface: “Don’t Wait” –Returns quickly from read or write request with count of bytes successfully transferred –Read may return nothing, write may write nothing Asynchronous Interface: “Tell Me Later” –When request data, take pointer to user’s buffer, return immediately; later kernel fills buffer and notifies user –When send data, take pointer to user’s buffer, return immediately; later kernel takes data and notifies user

13.8 Hung Q. Ngo Spring 2009 Operating System Main components of Intel Chipset: Pentium 4 Northbridge: –Handles memory –Graphics Southbridge: I/O –PCI bus –Disk controllers –USB controllers –Audio –Serial I/O –Interrupt controller –Timers

13.9 Hung Q. Ngo Spring 2009 Operating System Device Controller read write control status Addressable Memory and/or Queues Registers (port 0x20) Hardware Controller Memory Mapped Region: 0x8f Bus Interface How does the processor actually talk to the device? CPU interacts with a Controller –Contains a set of registers that can be read and written –May contain memory for request queues or bit-mapped images Regardless of the complexity of the connections and buses, processor accesses registers in two ways: –I/O instructions: in/out instructions »Example from the Intel architecture: out 0x21,AL –Memory mapped I/O: load/store instructions »Registers/memory appear in physical address space »I/O accomplished with load and store instructions Address+ Data Interrupt Request Processor Memory Bus CPU Regular Memory Interrupt Controller Bus Adaptor Bus Adaptor Other Devices or Buses

13.10 Hung Q. Ngo Spring 2009 Operating System Device I/O Port Locations on PCs (partial)

13.11 Hung Q. Ngo Spring 2009 Operating System Example: Memory-Mapped Display Controller Memory-Mapped: –Hardware maps control registers and display memory into physical address space »Addresses set by hardware jumpers or programming at boot time –Simply writing to display memory (also called the “frame buffer”) changes image on screen »Addr: 0x8000F000—0x8000FFFF –Writing graphics description to command-queue area »Say enter a set of triangles that describe some scene »Addr: 0x —0x8001FFFF –Writing to the command register may cause on-board graphics hardware to do something »Say render the above scene »Addr: 0x0007F004 Can protect with page tables Display Memory 0x8000F000 0x Physical Address Space Status 0x0007F000 Command 0x0007F004 Graphics Command Queue 0x

13.12 Hung Q. Ngo Spring 2009 Operating System Transfering Data To/From Controller Programmed I/O: –Each byte transferred via processor in/out or load/store –Pro: Simple hardware, easy to program –Con: Consumes processor cycles proportional to data size Direct Memory Access: –Give controller access to memory bus –Bypasses CPU to transfer data directly between I/O device and memory

13.13 Hung Q. Ngo Spring 2009 Operating System DMA

13.14 Hung Q. Ngo Spring 2009 Operating System A Kernel I/O Structure

13.15 Hung Q. Ngo Spring 2009 Operating System I/O Systems Layers

13.16 Hung Q. Ngo Spring 2009 Operating System Device Drivers Device Driver: Device-specific code in the kernel that interacts directly with the device hardware –Supports a standard, internal interface –Same kernel I/O system can interact easily with different device drivers –Special device-specific configuration supported with the ioctl() system call Device Drivers typically divided into two pieces: –Top half: accessed in call path from system calls »Implements a set of standard, cross-device calls like open(), close(), read(), write(), ioctl(), strategy() »This is the kernel’s interface to the device driver »Top half will start I/O to device, may put thread to sleep until finished –Bottom half: run as interrupt routine »Gets input or transfers next block of output »May wake sleeping threads if I/O now complete

13.17 Hung Q. Ngo Spring 2009 Operating System Life Cycle of An I/O Request Device Driver Top Half Device Driver Bottom Half Device Hardware Kernel I/O Subsystem User Program

13.18 Hung Q. Ngo Spring 2009 Operating System I/O Device Notifying the OS The OS needs to know when: –The I/O device has completed an operation –The I/O operation has encountered an error I/O Interrupt: –Device generates an interrupt whenever it needs service –Handled in bottom half of device driver »Often run on special kernel-level stack –Pro: handles unpredictable events well –Con: interrupts relatively high overhead Polling: –OS periodically checks a device-specific status register »I/O device puts completion information in status register »Could use timer to invoke lower half of drivers occasionally –Pro: low overhead –Con: may waste many cycles on polling if infrequent or unpredictable I/O operations Actual devices combine both polling and interrupts –For instance: High-bandwidth network device: »Interrupt for first incoming packet »Poll for following packets until hardware empty

13.19 Hung Q. Ngo Spring 2009 Operating System Interrupts CPU Interrupt-request lines triggered by I/O device Interrupt handler receives interrupts Maskable to ignore or delay some interrupts Interrupt vector to dispatch interrupt to correct handler –Based on priority –Some nonmaskable Interrupt mechanism also used for exceptions

13.20 Hung Q. Ngo Spring 2009 Operating System Interrupt-Driven I/O Cycle (Interrupt-request lines) Page fault Interrupt suspends current process & jumps to the page- fault handler in the kernel Handler saves process state, moves it to wait queue, performs page- cache management, schedules an I/O op. to fetch the page Handler schedules another process to resume execution, then return from interrupt

13.21 Hung Q. Ngo Spring 2009 Operating System Intel Pentium Processor Event-Vector Table

13.22 Hung Q. Ngo Spring 2009 Operating System Summary I/O Devices Types: –Many different speeds (0.1 bytes/sec to GBytes/sec) –Different Access Patterns: »Block Devices, Character Devices, Network Devices –Different Access Timing: »Blocking, Non-blocking, Asynchronous I/O Controllers: Hardware that controls actual device –Processor Accesses through I/O instructions, load/store to special physical memory –Report their results through either interrupts or a status register that processor looks at occasionally (polling) Device Driver: Device-specific code in kernel

13.23 Hung Q. Ngo Spring 2009 Operating System Disk I/O Operations

13.24 Hung Q. Ngo Spring 2009 Operating System Disk Structure Disk drives are addressed as large 1-dimensional arrays of logical blocks, where the logical block is the smallest unit of transfer. The 1-dimensional array of logical blocks is mapped into the sectors of the disk sequentially. –Sector 0 is the first sector of the first track on the outermost cylinder. –Mapping proceeds in order through that track, then the rest of the tracks in that cylinder, and then through the rest of the cylinders from outermost to innermost.

13.25 Hung Q. Ngo Spring 2009 Operating System Disk Scheduling The operating system is responsible for using hardware efficiently — for the disk drives, this means having a fast access time and disk bandwidth. Access time has two major components –Seek time is the time for the disk are to move the heads to the cylinder containing the desired sector. –Rotational latency is the additional time waiting for the disk to rotate the desired sector to the disk head. Minimize seek time Seek time  seek distance Disk bandwidth is the total number of bytes transferred, divided by the total time between the first request for service and the completion of the last transfer.

13.26 Hung Q. Ngo Spring 2009 Operating System Disk Scheduling (Cont.) Several algorithms exist to schedule the servicing of disk I/O requests. We illustrate them with a request queue (0-199). 98, 183, 37, 122, 14, 124, 65, 67 Head pointer 53

13.27 Hung Q. Ngo Spring 2009 Operating System FCFS Illustration shows total head movement of 640 cylinders.

13.28 Hung Q. Ngo Spring 2009 Operating System SSTF Selects the request with the minimum seek time from the current head position. SSTF scheduling is a form of SJF scheduling; may cause starvation of some requests. Illustration shows total head movement of 236 cylinders.

13.29 Hung Q. Ngo Spring 2009 Operating System SCAN The disk arm starts at one end of the disk, and moves toward the other end, servicing requests until it gets to the other end of the disk, where the head movement is reversed and servicing continues. Sometimes called the elevator algorithm. Illustration shows total head movement of 208 cylinders.

13.30 Hung Q. Ngo Spring 2009 Operating System C-SCAN Provides a more uniform wait time than SCAN. The head moves from one end of the disk to the other. servicing requests as it goes. When it reaches the other end, however, it immediately returns to the beginning of the disk, without servicing any requests on the return trip. Treats the cylinders as a circular list that wraps around from the last cylinder to the first one.

13.31 Hung Q. Ngo Spring 2009 Operating System C-LOOK Version of C-SCAN Arm only goes as far as the last request in each direction, then reverses direction immediately, without first going all the way to the end of the disk.

13.32 Hung Q. Ngo Spring 2009 Operating System Selecting a Disk-Scheduling Algorithm SSTF is common and has a natural appeal SCAN and C-SCAN perform better for systems that place a heavy load on the disk. Performance depends on the number and types of requests. Requests for disk service can be influenced by the file-allocation method. The disk-scheduling algorithm should be written as a separate module of the operating system, allowing it to be replaced with a different algorithm if necessary. Either SSTF or LOOK is a reasonable choice for the default algorithm.