I/O Management and Disk Scheduling Chapter 11

Categories of I/O Devices Human readable Used to communicate with the user  Printers, Display terminals, Keyboard, Mouse Machine readable Used to communicate with electronic equipment  Disk and tape drives, Sensors, Controllers Communication Used to communicate with remote devices  Modems, Network interface cards

Differences in I/O Devices Data rate May be differences of several orders of magnitude between the data transfer rates Application (the use to which the device is put) Disk used to store files requires file-management software Disk used to store virtual memory pages depends on the hardware and software to support it Terminal used by system administrator may have a higher priority

Differences in I/O Devices Complexity of control Control of a disk may be more complex than a printer Unit of transfer Data may be transferred as a stream of bytes for a terminal or in larger blocks for a disk Data representation Encoding schemes (character code, parity) Error conditions The nature, reporting and responding to errors differ widely

Techniques for Performing I/O Programmed I/O Process issues I/O command and busy- waits for the operation to complete Interrupt-driven I/O I/O command is issued Processor continues executing instructions I/O module sends an interrupt when done

Techniques for Performing I/O Direct Memory Access (DMA) Processor requests DMA module to transfer a block of data DMA module controls exchange of data between main memory and the I/O device Processor interrupted only after entire block has been transferred

Evolution of the I/O Function Processor directly controls a peripheral device Controller or I/O module is added Processor uses programmed I/O without interrupts Processor does not need to handle details of external devices Controller or I/O module with interrupts Processor does not spend time waiting for an I/O operation to be performed

Evolution of the I/O Function Direct Memory Access I/O module given direct access to memory Processor involved at beginning and end only I/O module is a separate processor CPU directs I/O processor to execute an I/O program in main memory A sequence of I/O activities performed without interruption I/O processor has its own local memory It is a computer in its own right Minimal CPU involvement

Direct Memory Access DMA module transfers a block of data to/from memory one word at a time over the system bus DMA and CPU share the same system bus DMA unit steals bus cycles (from the CPU) to transfer data (cycle stealing) The CPU instruction cycle is suspended to allow the current data word transfer This is not an interrupt (no context saves) The CPU just pauses for one bus cycle

Operating System Design Issues Efficiency It is important because I/O is often the bottleneck for computation  Most I/O devices are extremely slow compared to processor and main memory  Use of multiprogramming allows for some processes to be waiting on I/O while another process executes  Swapping is used to bring in additional Ready processes, but is itself an I/O operation

Operating System Design Issues Generality Desirable to handle all I/O devices in a uniform manner Diversity of I/O devices makes this difficult Use hierarchical, modular approach Hide most of the details of device I/O in lower-level routines so that processes and upper levels see devices in general terms such as read, write, open, close, lock, unlock

I/O Buffering Reasons for buffering Processes must wait for I/O to complete before proceeding Certain pages must remain (locked) in main memory during I/O  Interferes with swapping decisions of the OS (the process cannot be swapped out) Use system buffers for I/O

Two Types of I/O Devices Block-oriented Information is stored in fixed sized blocks Transfers are made one block at a time Used for disks and tapes Stream-oriented Transfer information as a stream of bytes Used for terminals, printers, communication ports, mouse, and most other devices that are not secondary storage

Single Buffer Operating system assigns a buffer in main memory for an I/O request Block-oriented Input transfers made to system buffer Process moves block to user space when needed Another block is requested to be transferred into the buffer Called read ahead (or anticipated input): assume the block will be needed

I/O Buffering

Single Buffer Block-oriented User process can process one block of data while next block is read in Swapping can occur since input is taking place in system memory, not user memory Operating system keeps track of assignment of system buffers to user processes Similar considerations apply to block-oriented output also

Single Buffer Stream-oriented Can be line-at-a-time or byte-at-a-time Line-at-a-time  Input and output is one line at a time; buffer holds a single line Byte-at-a-time  Buffer is similar to that in producer/consumer model

I/O Buffering Double buffer Use two system buffers instead of one A process can transfer data to or from one buffer while the operating system empties or fills the other buffer Circular buffer More than two buffers are used Each individual buffer is one unit in a circular buffer Used when I/O operation must keep up with process

I/O Buffering

Utility of Buffering Smooths out peaks in I/O demand Increases OS efficiency Improves performance of processes However, no amount of buffering will allow I/O to keep pace with a process having a constant demand Eventually, the buffers will all fill up and the process will have to wait

Disk Performance Parameters To read or write, the disk head must be positioned at the desired track and at the beginning of the desired sector Seek time time it takes to position the head at the desired track Rotational delay or rotational latency time its takes for the beginning of the sector to reach the head

Timing of a Disk I/O Transfer

Disk Performance Parameters Access time Sum of seek time and rotational delay The time it takes to get in position to read or write Data transfer occurs as the sector moves under the head Typical average seek time = 5 to 10 ms Average rotational delay (10,000 rpm) = 3ms

Disk Scheduling Policies Reading a file from disk: Best performance when the file is stored in adjacent tracks, occupying all sectors (sequential access) Worst performance when file is stored in sectors distributed randomly over the disk (random access) When I/O requests to a disk from many processes are queued up, how to schedule these requests for optimal performance? Random scheduling gives worst performance

Disk Scheduling Policies First-in, first-out (FIFO) Process requests from queue in the order in which they arrived Fair to all processes Good if only a few processes with clustered accesses Approaches random scheduling in performance if there are many processes

Disk Scheduling Policies Priority Goal is not to optimize disk use but to meet other objectives Short batch jobs and interactive jobs may have higher priority Provide good interactive response time Long jobs may wait excessively long

Disk Scheduling Policies Last-in, first-out (LIFO) Most recent request is processed Good for transaction processing systems  The device is given to the most recent user  The user may be reading a sequential file (good locality), so there should be little arm movement Possibility of starvation since a job may never regain the head of the line

Disk Scheduling Policies Shortest Service Time First (SSTF) Select the disk I/O request that requires the least movement of the disk arm from its current position Always incurs the minimum seek time Better performance than FIFO

Disk Scheduling Policies SCAN Arm moves in one direction only, satisfying all outstanding requests until it reaches the last track or no more requests in that direction Direction is then reversed No starvation Biased against area most recently traversed: poor exploitation of locality

Disk Scheduling Policies C-SCAN (Circular SCAN) Restricts scanning to one direction only When the last track has been visited in one direction, the arm is returned to the opposite end of the disk and the scan begins again

Disk Scheduling Policies N-step-SCAN Segments the disk request queue into subqueues of length N Subqueues are processed one at a time, using SCAN New requests added to another queue when a queue is being processed FSCAN Uses two subqueues All requests are in one queue when scan begins New requests are put in other queue during scan

RAID Redundant Array of Independent Disks (RAID) A set (array) of physical disk drives viewed by the OS as a single logical device Data are distributed across the disks The disks operate independently and in parallel, improving performance  Multiple or even single I/O requests can be handled in parallel Improves reliability (guards against disk failure) by storing redundant parity information

RAID (cont’d) Use RAID instead of a single large disk Large disks are expensive Large disks are less reliable Consists of seven levels (0 to 6), each designating a different design

RAID 0 (non-redundant)

RAID 0 Data are striped across the disks The disk is divided into strips The disk strips are mapped to consecutive logical strips in round robin fashion The corresponding strips from each disk constitute a stripe If there are n disks, up to n strips can be transferred in parallel. Provides  High data transfer capacity (single request, large data)  High I/O request rate (multiple requests, small data)

RAID 1 (redundancy through mirroring) Data is duplicated (each logical strip mapped to two disks) Recovery from failure is simple

RAID 2 (redundancy through Hamming code) Every I/O request requires access to all disks (parallel access) Strips are very small (usually, byte or word) Hamming code calculated across corresponding bits on each data disk and stored on multiple parity disks

RAID 3 (bit-interleaved parity) Similar to RAID 2, but requires only a single parity disk Simple parity calculation (XOR)

RAID 4 (block-level parity) Separate I/O requests can be satisfied in parallel (independent access) Strips are relatively large

RAID 5 (block-level distributed parity) Similar to RAID 4, but distributes parity strips across all disks

RAID 6 (dual redundancy) Uses two different parity calculations