CS152 Computer Architecture and Engineering Lecture 24 I O Systems II November 24 1999 John Kubiatowicz http cs berkeley edu kubitron lecture slides http www inst eecs berkeley edu cs152 11 24 99 UCB Fall 1999 CS152 Kubiatowicz The Big Picture Where are We Now Today s Topic I O Systems Network Bus Processor Processor Input Input Control Control Memory Datapath 11 24 99 Memory Output Output UCB Fall 1999 Datapath CS152 Kubiatowicz Outline of Today s Lecture Basic behavior of disks Queueing theory Interfacing between processor and I O devices RAID disk arrays Summary 11 24 99 UCB Fall 1999 CS152 Kubiatowicz Organization of a Hard Magnetic Disk Platters Track Sector Typical numbers depending on the disk size 500 to 2 000 tracks per surface 32 to 128 sectors per track A sector is the smallest unit that can be read or written Traditionally all tracks have the same number of sectors Constant bit density record more sectors on the outer tracks Recently relaxed constant bit size speed varies with track location CS152 Kubiatowicz 11 24 99 UCB Fall 1999 Magnetic Disk Characteristic Track Sector Cylinder all the tacks under the head at a given point on all surface Read write data is a three stage process Seek time position the arm over the proper track Cylinder Head Platter Rotational latency wait for the desired sector to rotate under the read write head Transfer time transfer a block of bits sector under the read write head Average seek time as reported by the industry Typically in the range of 8 ms to 12 ms Sum of the time for all possible seek total of possible seeks Due to locality of disk reference actual average seek time may Only be 25 to 33 of the advertised number 11 24 99 UCB Fall 1999 CS152 Kubiatowicz Typical Numbers of a Magnetic Disk Track Sector Rotational Latency Most disks rotate at 3 600 to 7200 RPM Approximately 16 ms to 8 ms per revolution respectively An average latency to the desired information is halfway around the disk 8 ms at 3600 RPM 4 ms at 7200 RPM Cylinder Head Platter Transfer Time is a function of Transfer size usually a sector 1 KB sector Rotation speed 3600 RPM to 10000 RPM Recording density bits per inch on a track Diameter typical diameter ranges from 2 5 to 5 25 in Typical values 2 to 40 MB per second 11 24 99 UCB Fall 1999 CS152 Kubiatowicz Disk I O Performance Request Rate Service Rate Processor Queue Queue Disk Controller Disk Disk Controller Disk Disk Access Time Seek time Rotational Latency Transfer time Controller Time Queueing Delay Estimating Queue Length Utilization U Request Rate Service Rate Mean Queue Length U 1 U As Request Rate Service Rate 11 24 99 Mean Queue Length Infinity UCB Fall 1999 CS152 Kubiatowicz Review Disk Device Terminology Disk Latency Queueing Time Controller time Seek Time Rotation Time Xfer Time Order of magnitude times for 4K byte transfers Average Seek 8 ms or less Rotate 4 2 ms 7200 rpm Xfer 1 ms 7200 rpm 11 24 99 UCB Fall 1999 CS152 Kubiatowicz Example 512 byte sector rotate at 5400 RPM advertised seeks is 12 ms transfer rate is 4 MB sec controller overhead is 1 ms queue idle so no service time Disk Access Time Seek time Rotational Latency Transfer time Controller Time Queueing Delay Disk Access Time 12 ms 0 5 5400 RPM 0 5 KB 4 MB s 1 ms 0 Disk Access Time 12 ms 0 5 90 RPS 0 125 1024 s 1 ms 0 Disk Access Time 12 ms 5 5 ms 0 1 ms 1 ms 0 ms Disk Access Time 18 6 ms If real seeks are 1 3 advertised seeks then its 10 6 ms with rotation delay at 50 of the time 11 24 99 UCB Fall 1999 CS152 Kubiatowicz Reliability and Availability Two terms that are often confused Reliability Is anything broken Availability Is the system still available to the user Availability can be improved by adding hardware Example adding ECC on memory Reliability can only be improved by Better environmental conditions Building more reliable components Building with fewer components 11 24 99 Improve availability may come at the cost of lower reliability UCB Fall 1999 CS152 Kubiatowicz Simple Producer Server Model Queue Producer Server Throughput The number of tasks completed by the server in unit time In order to get the highest possible throughput The server should never be idle The queue should never be empty Response time Begins when a task is placed in the queue Ends when it is completed by the server In order to minimize the response time 11 24 99 The queue should be empty The server will be idle UCB Fall 1999 CS152 Kubiatowicz Disk I O Performance 300 Metrics Response Time Throughput Response Time ms 200 100 0 100 0 Throughput total BW Queue Proc IOC Device Response time Queue Device Service time 11 24 99 UCB Fall 1999 CS152 Kubiatowicz Response Time vs Productivity Interactive environments Each interaction or transaction has 3 parts Entry Time time for user to enter command System Response Time time between user entry system replies Think Time Time from response until user begins next command 1st transaction 2nd transaction What happens to transaction time as shrink system response time from 1 0 sec to 0 3 sec With Keyboard 4 0 sec entry 9 4 sec think time With Graphics 0 25 sec entry 1 6 sec think time 11 24 99 UCB Fall 1999 CS152 Kubiatowicz Response Time Productivity conventional 0 3s conventional 1 0s graphics 0 3s entry graphics 1 0s 0 00 5 00 resp 10 00 think 15 00 Time 0 7sec off response saves 4 9 sec 34 and 2 0 sec 70 total time per transaction greater productivity Another study everyone gets more done with faster response but novice with fast response expert with slow 11 24 99 UCB Fall 1999 CS152 Kubiatowicz Administrivi a Get your project entered on web page I will remove the ability to enter new projects soon won t be able to submit final report Please take time to enter reasonable title no trademarked names Should have two or three sentence description Pending schedule Monday 11 29 no class work on projects Monday 11 29 Update on design due to TAs Wednesday 12 1 Last class wrap up evaluations etc Monday 12 6 Oral reports 10 12am and 2 4pm Signup sheet will be on my office door next week Project reports must be submitted via web by 5pm on 12 6 Friday 12 10 Grades ready System for examining grades is up Victor posted description of how to use it on the newsgroup Solutions to Midterm II not up yet sorry 11 24 99 UCB Fall 1999 CS152 Kubiatowicz Computers in the News Electronic Ink Electronic Ink Little capsules with charged balls that are half black half white Placing an electronic charge of one polarity makes dot black and the other
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