C-LOOK  LOOK a version of SCAN, C-LOOK a 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  Total number of cylinders? Operating System Concepts Essentials – 8 th Edition 11.25 Silberschatz, Galvin and Gagne ©2011
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C-LOOK  LOOK a version of SCAN, C-LOOK a 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  Total number of cylinders? Operating System Concepts Essentials – 2nd Edition 9.26 Silberschatz, Galvin and Gagne ©2013
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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 Operating System Concepts with Java – 8th Edition 12.16 Silberschatz, Galvin and Gagne ©2009
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C:\UMBC\331\java> java.ext.dirs=C:\JDK1.2\JRE\lib\ext java.io.tmpdir=C:\WINDOWS\TEMP\ os.name=Windows 95 java.vendor=Sun Microsystems Inc. java.awt.printerjob=sun.awt.windows.WPrinterJob java.library.path=C:\JDK1.2\BIN;.;C:\WINDOWS\SYSTEM;C:\... java.vm.specification.vendor=Sun Microsystems Inc. sun.io.unicode.encoding=UnicodeLittle file.encoding=Cp1252 java.specification.vendor=Sun Microsystems Inc. user.language=en user.name=nicholas java.vendor.url.bug=http://java.sun.com/cgi-bin/bugreport... java.vm.name=Classic VM java.class.version=46.0 java.vm.specification.name=Java Virtual Machine Specification sun.boot.library.path=C:\JDK1.2\JRE\bin os.version=4.10 java.vm.version=1.2 java.vm.info=build JDK-1.2-V, native threads, symcjit java.compiler=symcjit path.separator=; file.separator=\ user.dir=C:\UMBC\331\java sun.boot.class.path=C:\JDK1.2\JRE\lib\rt.jar;C:\JDK1.2\JR... user.name=nicholas user.home=C:\WINDOWS C:\UMBC\331\java>java envSnoop -- listing properties -java.specification.name=Java Platform API Specification awt.toolkit=sun.awt.windows.WToolkit java.version=1.2 java.awt.graphicsenv=sun.awt.Win32GraphicsEnvironment user.timezone=America/New_York java.specification.version=1.2 java.vm.vendor=Sun Microsystems Inc. user.home=C:\WINDOWS java.vm.specification.version=1.0 os.arch=x86 java.awt.fonts= java.vendor.url=http://java.sun.com/ user.region=US file.encoding.pkg=sun.io java.home=C:\JDK1.2\JRE java.class.path=C:\Program Files\PhotoDeluxe 2.0\Adob... line.separator=
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The New Stack – Process Is the Next Platform Client Punch Card or Terminal Custom (10’s of users) Application Data Management Applicatio n OS and Databas e Mainframe 4 Request Form Request Form Request Form Request Form Request Form Request Form Request Form Request Form Request Form Request Form Request Form Request Form Request Form Request Form Request Form Request Form Request Form Request Form Request Form Request Form Request Form Request Form Request Form Request Form Any Device (Millions of users) PC or Internet (1000’s of users) Composites Applicatio n Applicatio n Applicatio n OS OS OS DB OS Client Server OS OSApplicatio ns OS OS DB OS DB OS DB OS OS SOA Magal and Word ! Essentials of Business Processes and Information Systems | © 2009 Composites OS OSApplicatio ns OS OS DB OS DB OS DB OS OS
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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 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 Operating System Concepts with Java – 8th Edition 12.8 Silberschatz, Galvin and Gagne ©2009
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Disk Scheduling (Cont.)  There are many sources of disk I/O request  OS  System processes  Users processes  I/O request includes input or output mode, disk address, memory address, number of sectors to transfer  OS maintains queue of requests, per disk or device  Idle disk can immediately work on I/O request, busy disk means work must queue  Optimization algorithms only make sense when a queue exists  Note that drive controllers have small buffers and can manage a queue of I/O requests (of varying “depth”)  Several algorithms exist to schedule the servicing of disk I/O requests  The analysis is true for one or many platters  We illustrate scheduling algorithms with a request queue (0-199) 98, 183, 37, 122, 14, 124, 65, 67 Head pointer 53 Operating System Concepts Essentials – 8 th Edition 11.17 Silberschatz, Galvin and Gagne ©2011
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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 Operating System Concepts with Java – 8th Edition 12.14 Silberschatz, Galvin and Gagne ©2009
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