CMU CS 15410  Lecture (43 pages)
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Lecture
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Lecture Notes
 Pages:
 43
 School:
 Carnegie Mellon University
 Course:
 Cs 15410  Operating System Design and Implementation
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Unformatted text preview:
Advanced disk scheduling Freeblock scheduling Eno Thereska slide contributions by Chris Lumb and Brandon Salmon PARALLEL DATA LABORATORY Carnegie Mellon University Outline Freeblock scheduling some theory Freeblock scheduling applied Some details Q A http www pdl cmu edu 2 Eno Thereska 15 410 Lecture April 2004 Some theory preview Next few slides will review show that disks are slow mechanical delays seek rotational latencies there is nothing we can do during a seek there is a lot we can do during a rotation rotational latencies are very large while rotation is happening go to nearby tracks and do useful work freeblock scheduling utilization of rotational latency gaps any idle time http www pdl cmu edu 3 Eno Thereska 15 410 Lecture April 2004 Are disks slow Are the xfer speeds that slow no xfer speeds of 200MB s are pretty good So what is slow workload often not sequential disk head has to move from place to place seek 4ms rotation 3ms Effective bandwidth can be very low 10 30MB s even when SPTF is used http www pdl cmu edu 4 Eno Thereska 15 410 Lecture April 2004 Surface organized into tracks http www pdl cmu edu 5 Eno Thereska 15 410 Lecture April 2004 Tracks broken up into sectors http www pdl cmu edu 6 Eno Thereska 15 410 Lecture April 2004 Disk head position http www pdl cmu edu 7 Eno Thereska 15 410 Lecture April 2004 Rotation is counter clockwise http www pdl cmu edu 8 Eno Thereska 15 410 Lecture April 2004 About to read blue sector http www pdl cmu edu 9 Eno Thereska 15 410 Lecture April 2004 After reading blue sector After BLUE read http www pdl cmu edu 10 Eno Thereska 15 410 Lecture April 2004 Red request scheduled next After BLUE read http www pdl cmu edu 11 Eno Thereska 15 410 Lecture April 2004 Seek to Red s track After BLUE read Seek for RED SEEK http www pdl cmu edu 12 Eno Thereska 15 410 Lecture April 2004 Wait for Red sector to reach head After BLUE read SEEK http www pdl cmu edu Seek for RED Rotational latency ROTATE 13 Eno Thereska 15 410 Lecture
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