COSC 2610 Final Study Guide

Movable-Head Magnetic Drives

-Access time = seek time + search time + transfer time -Search time and transfer time calculation: Same as fixed-head DASD -Blocking: good way to minimize access time

File Naming Conventions

-Components: Relative filename and extension -Complete file name includes all path information -Relative filename is name without path information. Appears in directory listings, folders. Provides filename differentiation within directory

Responsibilities of the File Manager

-File storage tracking -Policy implementation: Determine where and how files are stored -File allocation if user access cleared -File deallocation

Communication Among Devices

-I/O subsystem units finish independently of others -CPU processes data while I/O performed -Success requires device completion knowledge: Hardware flag tested by CPU, Flag tested with polling and interrupts

Fixed-Head Magnetic Drives

-Record access requires Track number and record number -Total access time = search time + transfer time -DASDs rotate continuously -DASD has little access variance -Blocking minimizes access time

Optical Disk Storage

-Sustained data-transfer rate: Speed to read massive data amounts from disc -Average access time: Average time to move head to specific disc location in milliseconds (ms) -Cache size (hardware): Buffer to transfer data blocks from disc

Process Synchronization

1) the need for algorithms to resolve conflicts between processors in a multiprocessing environment or 2) the need to ensure that events occur in the proper order even if they are carried out by several processes

Producers and consumers

A classic problem in which a process produces data that will be consumed such as a file or databases

I/O Subsystem

A collection of modules within the OS that controls all I/O requests

Mutex

A condition that specifies that only one process may update (modify) a shared resource at a time to ensure correct operation and results

Virtual Devices

A dedicated device that has been transformed into a device through the use of spooling techniques. Some printers can become shared devices by using a program that reroutes all print requests to a storage space on a disk

Subdirectories

A directory created by a user within the the boundaries of an existing directory. Some OSs call this a folder.

Master File Directory (MFD)

A file stored immediately after the volume descriptor. It lists the names and characteristics of every file contained in that volume, as well as subdirectories and remainder of volume.

Field

A group of related bytes that can be identified by the user with a name, type and size. A record is made up of fields.

Record

A group of related fields treated as a unit. A file is a group of related records.

Database

A group of related files that are interconnected at various levels to give uses flexibility of access to the data stored.

File

A group of related records that contains information to be used by specific application programs to generate reports

WAIT and SIGNAL

A modification of the test-and-set synchronization mechanism that's designed to remove busy waiting. WAIT - Activated when process encounters busy condition code SIGNAL - Activated when process exits critical region and condition code set to "free"

Loosely coupled

A multiprocessing configuration in which each processor has a copy of the OS and controls its own resources

Symmetric

A multiprocessing configuration in which processor scheduling is decentralized and each processor is of the same type

Critical Region

A part of a program that must complete execution before other processes can begin. Processes within critical region cannot be divided

Seek Strategy

A predetermined policy used by the I/O device handler to optimize seek times

Readers and writers

A problem that arises when two types of processes need to access a shared resource such as a file of database

Fixed-Length Record

A record format that always contains the same number of characters. Easy direct access & record size critical.

Variable-Length Record

A record format that isn't of uniform length, doesn't leave empty storage space, and doesn't truncate any characters. Difficult direct access. File descriptor stores record format. Used with files accessed sequentially.

LOOK

A scheduling strategy for DASDs that's used to optimize seek time. Sometimes known as the elevator algorithm

SCAN

A scheduling strategy for DASDs that's used to optimize seek time. The most common variations are N-step SCAN and C-SCAN

SSTF

A scheduling strategy for DASDs that's used to optimize seek time. The track requests are ordered so the one closest to the currently active track is satisfied first and the ones farthest away are made to wait Shortest Seek Time First

I/O Channel

A specialized programmable unit placed between the CPU and the control units, which synchronizes the fast speed of the CPU with the slow speed of the I/O device and vice versa, making it possible to overlap I/O operations with CPU operations

Directories

A storage area in a secondary storage volume (disk, disk pack, etc) containing information about files stored in that volume.

Blocking

A storage saving and I/O-saving technique that groups individual records into a block that's stored and retrieved as a unit

Semaphore

A type of shared data item that may contain either binary or non negative integer values and is used to provide mutual exclusion.

RAID

Acronym for Redundant Array of Independent Disks - a group of hard disks controlled in such a way that they speed read access of data on secondary storage devices and aid data recovery. -Preferable over few large-capacity disk drives -Improved I/O performance -Improved data recovery: Disk failure event -Introduces redundancy -Increases hardware costs

Direct Memory Access (DMA)

An I/O technique that allows a control unit to access main memory directly and transfer data without the intervention of the CPU

Rotational Ordering

An algorithm used to reorder record requests within tracks to optimize search time .Reduces time wasted due to rotational delay

Master/slave

An asymmetric multiprocessing configuration consisting of a single processor system connected to "slave" processors each of which is managed by the primary "master" processor, which provides the scheduling functions and jobs

Test-and-set

An indivisible machine instruction, which is executed in a single machine cycle to determine whether the processor is available. Uses a key (single bit in storage location) to test if processor is available - zero (free) or one (busy). Works well for small # of processes, but could cause starvation and busy waiting.

Interblock gap (IBG)

An unused space (always .5 inch) between blocks of records on a magnetic tape, facilitates the tape's start/stop operations. More efficient than individual records and IRG.

Interrecord gap (IRG)

An unused space (always .5 inch) between records on a magnetic tape. It facilities the tape's start/stop operations.

Concurrent Programming

Another type of multiprocessing in which one job uses several processors by executing sets of instructions in parallel. Requires programming language and computer system support

Sequential Access Media

Any medium that allows records to be accessed only in a sequential manner - one after the other. Early computer systems would record everything with serials on magnetic tape

Direct Access Storage Devices (DASD)

Any secondary storage device that can directly read or write to a specific place. Sometimes called a random access storage device. Access time varies and is directly affected by physical location on magnetic tape.

Volume

Any secondary storage unit, such as hard disks, disk packs, CDs, DVDs, removable disks, flash memory or tapes.

Benefits & Challenges of Multiprocessing

B: Increased reliability (in case one processor fails), faster processing C: Connecting processors into configurations, orchestrating processor interaction

COEND

Command used with COBEGIN to indicate to a multiprocessing compiler the end of section where instructions can be processed concurrently

COBEGIN

Command used with COEND to indicate to a multiprocessing compiler the beginning of a section where instructions can be processed concurrently

Data File

Contain data

Program files

Contain instructions

Multi-file volume

Contains many files

Q: ____ peripheral devices are assigned to only one job at a time. A. Dedicated B. Shared C. Virtual D. Static

Dedicated

Q: The number of characters that can be recorded per inch on a magnetic tape is determined by the ____ of the tape. A. Width B. Length C. Density D. Parity

Density

Dedicated Devices

Devices that are assigned to only one job at a time, serve the job the entire time its active or until it releases them. Devices like monitors, mouses and printers require this bc it would be awkward to let several users share them

Shared Devices

Devices that can be allocated to several processes at the same time. A disk or other direct access storage device can be shared by several processes by interleaving their requests by the device manager

Multi-volume files

Extremely large files spread across several volumes

Q: A compressed image file can be reconstructed if a lossy compression algorithm is used because the compression process is reversible. T/F

False

Q: A field is a group of related records that can be identified by the user with a name, type, and size. T/F

False

Q: Device management principles are changing rapidly to accommodate cloud computing. T/F

False

Q: The Shortest Seek Time First device-scheduling algorithm moves the arm methodically from the inner to the outer track, servicing every request in its path. T/F

False

Q: The job of the I/O control unit is to keep up with the I/O requests from the CPU and pass them down the line to the appropriate control unit. T/F

False

Q: The two operations that can be performed on a semaphore variable are select and increment (P and V). T/F

False

Q: Within a file, variable-length records are easier to access directly than fixed-length records. T/F

False

Transfer Time

Fastest access time factor. The time required for data to be transferred between secondary storage and main memory.

File descriptor

Information kept in the directory to describe a file or file extent. Contains: -Filename: each must be unique -File type: organization and usage -File size: for convenience -File location: Identifies first physical block (or all blocks) -Date and time of creation -Owner -Protection information: access restrictions -Record size: fixed size, maximum size

Job Level

Level of multiprocessing where each job has its own processor and all processes and threads are run by that processor. No explicit synchronization is required after jobs are assigned to a processor

Thread Level

Level of multiprocessing where threads, regardless of job and process, can be assigned to available processors. High degree of synchronization required to track each process and each thread.

Process Level

Level of multiprocessing where unrelated processes, regardless of the job, can be assigned to available processors. Moderate amount of synchronization required

Process cooperation

Occasions when several processes work directly together to complete a common task. Two famous examples are the problems of producers & consumers and of readers & writers. Both requires mutual exclusion and synchronization and implementing by using semaphores

I/O Scheduler

One of the modules of the I/O subsystem that allocated the input/output devices, control units, and channels

I/O Traffic Controller

One of the modules of the I/O subsystem that monitors the status of every device, control unit and channel

Purpose of P and V operations

Operations happen in response to calls issued by any process named a semaphore as a parameter. If s = 0, that means the critical region is busy and the processing calling on the test operation must wait until the operation can be executed.

Lock-and-key synchronization

Process determines key availability Process obtains key Puts key in lock Makes it unavailable to other processes

Physical File Organization

Record arrangement and medium characteristics on a magnetic disk. Considerations: -Data volatility -File activity -File size -Response time

Q: ____ uses the same underlying philosophy as shortest job next, where the shortest jobs are processed first and longer jobs are made to wait. A. LOOK B. FCFS C. SSTF D. SCAN

SSTF

V - increment

Semaphone operation that increments the semaphone variable. Necessitates a fetch, increment and store sequence. V(s): s: = s + 1

P - test

Semaphone operation that tests the semaphone variable, and if its not 0, to decrement it by 1. Involves a test, fetch, decrement and store sequence. P(s) = If s > 0 then s: = s - 1

Multi-core processing

Single chip with multiple processor cores in same space instead of several processors next to each other to reduce leakage and heat

Seek Time

Slowest access time factor. Time required to position the read/write head on the proper track from the time the I/O request is received on the device. Does not apply to fixed read/write head devices.

Management of I/O Requests

The I/O Traffic Controller watches status of devices, control units, channels and -Determine if path available -If more than one path available, determine which one to select -If paths all busy, determine when one is available. Also maintains database containing each unit's status and connections

I/O Device Handler

The module within the OS that processes the I/O interrupts, handles error conditions, and provides detailed scheduling algorithms that are extremely device dependent

Parallel Processing

The process of operating two or more CPUs executing instructions simultaneously

FCFS

The simplest scheduling algorithm for DASD that satisfies track requests in the order in which they are received First Come, First Served

Search Time (Rotational Delay)

The time it takes to rotate the disk until the requested record is under the read/write head.

Access Time

The total time required to access data in secondary storage

Q: ____ semaphores are used in the producer and consumer problem. A. One B. Two C. Three D. Four

Three

Q: Although noncontiguous allocation schemes eliminate external storage fragmentation and the need for compaction, they don't support direct access because there's no easy way to determine the exact location of a specific record. T/F

True

Q: Buffers are used to synchronize the movement of data between the relatively slow I/O devices and the very fast CPU. T/F

True

Q: Directories are special files that contain listings of filenames and their attributes. T/F

True

Q: Nested RAID levels, also called hybrid levels, are complex RAID configurations created by combining multiple standard levels. T/F

True

Q: Parallel processing is a situation in which two or more processors operate in one system at the same time and may or may not work on related activities. T/F

True

Multiprocessing

When two or more processes share system resources that may include some or all of the following: the same main memory, I/O devices and control program routines

Q: The primary disadvantage of contiguous storage is that ____. A. the file system is difficult to create, implement, and manage B. a file can be spread in small pieces throughout the storage area C. a file can't be expanded unless there is empty space available immediately following it D. the file system cannot support direct access

a file can't be expanded unless there is empty space available immediately following it

Q. The computer system ____ a file by activating the appropriate secondary storage device and loading the file into memory while updating its records of who is using that file. A. creates B. formats C. allocates D. configures

allocates

Device categories (types)

dedicated, shared, and virtual

Q: Many computer users and some operating systems call subdirectories ____. A. folders B. files C. volumes D. databases

folders

Q: In a direct record organization scheme, the program used to store the data follows a set of instructions, called a ____ algorithm, that transforms each key into a number, the record's logical address. A. hashing B. grouping C. translation D. lookup

hashing

Q: When data is stored in fixed length fields, data that extends beyond the fixed size ____. A. generates an operating system error B. is truncated C. is broken up and stored in more than one field D. combines multiple fields to accommodate the data

is truncated

Q: Most current operating systems support the implementation of threads, or ____, which have become part of numerous application packages. A. parallel processes B. lightweight processes C. heavyweight processes D. semaphores

lightweight processes

Q. On an optical disc, all sectors are ____. A. of varying sizes depending upon where you are on the disc B. larger as you move to the edge C. smaller as you move to the edge D. of the same size throughout the disc

of the same size throughout the disc

Q: Of the three components of access time in a movable-head DASD, ____ is the slowest. A. seek time B. search time C. transfer time D. delay time

seek time

Q: The ____ multiprocessing configuration is the most difficult to implement because the processes must be well synchronized to avoid races and deadlocks. A. master/slave B. loosely coupled C. symmetric D. shared load

symmetric