I/O Management and Disk Scheduling Chapter 8 Advanced Operating System

Test In your opinion, what multimedia applications or tasks, if any, might be candidates for inclusion in a near-future operating system

Categories of I/O Devices Human readable Used to communicate with the user Printers Video display terminals Display Keyboard Mouse

Categories of I/O Devices Machine readable Used to communicate with electronic equipment Disk and tape drives Sensors Controllers Actuators

Categories of I/O Devices Communication Used to communicate with remote devices Digital line drivers Modems

Differences in I/O Devices Data rate May be differences of several orders of magnitude between the data transfer rates

Differences in I/O Devices Application Disk used to store files requires file management software Disk used to store virtual memory pages needs special hardware and software to support it Terminal used by system administrator may have a higher priority

Differences in I/O Devices Complexity of control 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 Error conditions Devices respond to errors differently

Performing I/O Programmed I/O Interrupt-driven I/O Process is busy-waiting 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

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

Programmed I/O No interrupts occur Processor is kept busy checking status procedure readString (var s); repeat Send I/O command “go read a word” Read I/O status until I/O done Read word from I/O module Write word into memory until finished reading …… I/O are much slower than the CPU. It is very inefficient for the CPU to wait for I/O completion in a tight loop. (busy waiting).

Interrupt-Driven I/O No busy waiting. Processor can proceed to do other things when I/O is in progress When I/O is done, the CPU is interrupted Still consumes a lot of processor time because every word read or written passes through the processor

Interrupt-Driven I/O procedure readString (var s); repeat Send I/O command “go read a word” … now, the CPU will do something else don’t bother checking I/O status until finished reading …… When the I/O module finished reading the word, it interrupts the CPU. The CPU will execute an interrupt handler to move the word to memory. /* interrupt handler */ Read word from I/O module Write word into memory return Afterwards, control is returned to the program which continues to read the next word. Interrupt-driven I/O is still inefficient in data transfer of large amount because the CPU has to transfer the data word by word between I/O module and memory.

Direct Memory Access Processor grants I/O module authority to read from or write to memory DMA module controls exchange of data between main memory and the I/O device processor interrupted only after entire block has been transferred The processor is only involved at the beginning and end of the transfer

DMA procedure readString (var s); Request the DMA module The DMA module starts reading each word of the data and save them in the memory. procedure readString (var s); Request the DMA module to read some data … now, the CPU will do something else don’t bother checking I/O status DMA I/O is more efficient in large data transfer because the interaction with the I/O module and data transfer between I/O module and memory are performed by the DMA module. After the DMA has transferred all the data requested to the memory, it notifies that it has finished the I/O by sending the CPU an interrupt

Direct Memory Access Processor delegates I/O operation to the DMA module DMA module transfers data directly to or form memory When complete DMA module sends an interrupt signal to the processor

DMA

DMA Configurations

DMA Configurations

Operating System Design Issues Efficiency Most I/O devices extremely slow compared to main memory Use of multiprogramming allows for some processes to be waiting on I/O while another process executes I/O cannot keep up with processor speed Swapping is used to bring in additional Ready processes which is an I/O operation

Operating System Design Issues Generality Desirable to handle all I/O devices in a uniform manner 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 in main memory during I/O

I/O Buffering Block-oriented Stream-oriented Information is stored in fixed sized blocks Transfers are made a 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 other pointing devices, 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 buffer Block moved to user space when needed Another block is moved into the buffer Read ahead

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

Single Buffer Stream-oriented Used a line at time User input from a terminal is one line at a time with carriage return signaling the end of the line Output to the terminal is one line at a time

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

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

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

Disk Scheduling Policies Seek time is the reason for differences in performance For a single disk there will be a number of I/O requests If requests are selected randomly, we will poor performance

OS has to read these tracks: 2,3,4,5,8. Disk Scheduling At runtime, I/O requests for disk tracks come from the processes OS has to choose an order to serve the requests Process A reads tracks 2, 5 OS has to read these tracks: 2,3,4,5,8. Process B reads tracks 3, 5 Process C reads tracks 8, 4

Disk Scheduling The order that the read/write head is moved to satisfy several I/O requests determines the total seek time affects performance the OS cannot change the rotational delay or transfer time, but it can try to find a ‘good’ order that spends less time in seek time. If requests are selected randomly, we will get the worst possible performance...

Disk Scheduling Policy FIFO: fair, but near random scheduling SSTF: possible starvation SCAN: favor requests for tracks near the ends C-SCAN FSCAN: avoid “arm stickiness” in SSTF, SCAN and C-SCAN

First-in-first-out, FIFO process requests in the order that the requests are made fair to all processes approaches random scheduling in performance if there are many processes Read write head starting at track 8, in the direction of increasing track number 8 1 2 7 10 Pending read/write request for a track

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 choose the minimum seek time new requests may be chosen before an existing request 8 1 2 7 10

SCAN arm moves in one direction only, satisfying all outstanding requests until there is no more requests in that direction. The service direction is then reversed. favor requests for tracks near the ends 8 1 2 7 10

C-SCAN restrict 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. 8 1 2 7 10

FSCAN “Arm stickiness” in SSTF, SCAN, C-SCAN in case of repetitive requests to one track FSCAN uses two queues. When a scan begins, all of the requests are in one of the queues, with the other empty. During the scan, all new requests are put into the other queue. Service of new requests is deferred until all of the old requests have been processed.

Disk Cache Buffer in main memory for disk sectors Contains a copy of some of the sectors on the disk

Disk Cache, Hit and Miss When an I/O request is made for a particular sector, the OS checks whether the sector is in the disk cache. If so, (cache hit), the request is satisfied via the cache. If not (cache miss), the requested sector is read into the disk cache from the disk.

Least Recently Used The block that has been in the cache the longest with no reference to it is replaced The cache consists of a stack of blocks Most recently referenced block is on the top of the stack When a block is referenced or brought into the cache, it is placed on the top of the stack

Least Recently Used The block on the bottom of the stack is removed when a new block is brought in Blocks don’t actually move around in main memory A stack of pointers is used

Least Frequently Used The block that has experienced the fewest references is replaced A counter is associated with each block Counter is incremented each time block accessed Block with smallest count is selected for replacement Some blocks may be referenced many times in a short period of time and the reference count is misleading

Hybrid and Open source

Windows Supports a wide range of computer hardware Large library software Unstable: blue screen of death (Except windows XP) Requires a system reboot when facing errors Secure operating system Target of Viruses and spyware

Windows Microsoft’ s reaction Windows TCO and Security Windows Genuine Advantage Notifications Internet Explorer has lost market share to Firefox and Opera Internet Explorer: Windows Firefox: Windows, Linux Opera: Windows, Linux

Windows Total cost of ownership (TCO) Cost of computer and software Maintenance Training Technical support Hardware and software upgrade Windows has a lower TCO: “Get the Facts”

Windows Security Windows Vista (Longhorn) Closed-source is invisible for crackers Windows Vista (Longhorn) High security Easy to management Detecting hardware feature problem Faster start up time

Linux Cost less More secure: Open source OS for networking Not viruses target

Technical Comparison Kernel space: The central module of an operating system. It is the part of the operating system that loads first, and it remains in main memory. Because it stays in memory, it is important for the kernel to be as small as possible while still providing all the essential services required by other parts of the operating system and applications. Typically, the kernel is responsible for memory management, process and task management, and disk management. Windows: file system, Internet explorer, windows media player, GUI Linux: only file system Memory management Windows: Swap file Linux: avoiding swapping and allocating memory

Technical Comparison Swapping To replace pages or segments of data in memory. Swapping is a useful technique that enables a computer to execute programs and manipulate data files larger than main memory. The operating system copies as much data as possible into main memory, and leaves the rest on the disk. When the operating system needs data from the disk, it exchanges a portion of data (called a page or segment ) in main memory with a portion of data on the disk.

Technical Comparison Stability Device Driver Windows: blue screen of death (instability) Linux: More stability Common source of instability is due to bugs in various device drivers Device Driver Windows: run in kernel space Linux: run in user space