An Operating Systems Vade Mecum

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166 Transput Chapter 5on the way out.To illustrate these disk-scheduling algorithms, let us consider a disk with 200cylinders (a typical number). Let’s assume that at a given instant, the read/write head issitting at cylinder 100. Transfers to or from the following cylinders have been requested.The requests are listed in the order in which they arrived. It is irrelevant whether they areread or write requests.23 89 132 42 187 165 21 34 101 102 34 2 167 3 34 199 20We will assume that no new requests arrive while we service this set. (By the way, it isquite unusual for more than three or four requests to be outstanding in a well-tunedoperating system.) The algorithms we have seen have the following behavior.Requests 23 89 132 42 187 165 21 34 101 102 34 2 167 3 34 199 20 motion FCFS 23 89 132 42 187 165 21 34 101 102 34 2 167 3 34 199 20 1472 Pickup 89 42 34 34 34 23 101 102 132 165 167 187 21 20 3 2 199 623 SSF 101 102 89 132 165 167 187 199 42 34 34 34 23 21 20 3 2 322 Look 101 102 132 165 167 187 199 89 42 34 34 34 23 21 20 3 2 296 C-Look 101 102 132 165 167 187 199 2 3 20 21 23 34 34 34 42 89 383 The SSF and Look methods have similar results, but if requests had continued to arriveduring the time others were serviced, it is likely that SSF would have turned back to pickup some new requests close to its current position. Look turns back only when there isno further work in the current direction. As expected, FCFS has the worst behavior.To compare these methods further, we turn to simulation results. The simulateddisk has 200 tracks. Seek latency is (0.5 + 0.4 . Distance ) milliseconds if the distance isat least one track and zero otherwise. (The 0.5 term represents time needed just to startand stop the head.) There are 20 sectors per track. The disk spins at 3600 revolutionsper minute. Rotational latency depends on when the head arrives at the track and the sectorthat is needed. The simulation compares the behavior of the five policies on a workloadof 1000 requests, with interarrival time selected from an exponential distribution.The average arrival rate is allowed to vary between 5 and 100 arrivals per second. Boththe track and the sector needed by each request are selected from a uniform distribution.(Very similar results were obtained from using a normal distribution for tracks, withaverage value 100 and standard deviation 100.)Figure 5.9 shows the average time for a disk request to be serviced for the differentmethods across the given workloads. The FCFS method is clearly terrible. The othermethods are not particularly easy to distinguish. Surprisingly, C-Look did not perform aswell as Look, even with high arrival rate. (It did perform slightly better than Look withrequested tracks selected from a normal distribution.) Figure 5.10 shows the throughput— that is, the number of requests served per second — for the five methods. This figureshows even more clearly that FCFS is by far the worst but that all the other methods areessentially indistinguishable. If the arrival rate is less than 22 requests per second, FCFSmanages just as well as the others with respect to throughput, although its response timestarts falling behind the others at about 10 requests per second.
Device drivers 16716.008.00FCFS4.002.001.000.500.250.12C-LookSSFPickupLook0.060.03010 20 30 40 50 60 70 80 90arrival rateFigure 5.9 Average response time10012864LookSSFPickupC-Look32FCFS168401020 30 40 50 60 70 80arrival rateFigure 5.10 Throughput90100
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ContentsvWhich page should be swapp
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Contentsvii1.3 Interacting with pro
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ContentsixBalance is a registered t
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12 Prefaceareas of current activity
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2 Introduction Chapter 1We will giv
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4 Introduction Chapter 1Another way
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6 Introduction Chapter 1executing j
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8 Introduction Chapter 1The running
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10 Introduction Chapter 1waiting fo
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12 Introduction Chapter 1During log
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16 Introduction Chapter 1processesA
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18 Introduction Chapter 1Various ev
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20 Introduction Chapter 1decomposit
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24 Introduction Chapter 1(The trap
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26 Introduction Chapter 111. How ca
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28 Time Management Chapter 2runread
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30 Time Management Chapter 2the use
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32 Time Management Chapter 2Unfortu
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34 Time Management Chapter 2109FBRR
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54 Time Management Chapter 25 EXERC
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58 Space Management Chapter 3Proces
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60 Space Management Chapter 38 bits
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descriptor base registervirtual add
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Page 134 and 135: 122 Resource Deadlock Chapter 4file
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Page 160 and 161: 148 Transput Chapter 5comes from sp
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Page 164 and 165: 152 Transput Chapter 5Many operatin
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Page 168 and 169: 156 Transput Chapter 52 THE DEVICE
Page 170 and 171: 158 Transput Chapter 5processeskern
Page 172 and 173: 160 Transput Chapter 5language. The
Page 174 and 175: 162 Transput Chapter 5page onto bac
Page 176 and 177: 164 Transput Chapter 5error. After
Page 180 and 181: 168 Transput Chapter 54 DATA MODIFI
Page 182 and 183: 170 Transput Chapter 5If we want B
Page 184 and 185: 172 Transput Chapter 5within a reas
Page 186 and 187: 174 Transput Chapter 5transfer, in
Page 188 and 189: 176 Transput Chapter 5Input buffers
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Page 194 and 195: 182 Transput Chapter 5Security: Enc
Page 196 and 197: 184 Transput Chapter 513. The text
Page 198 and 199: 186 File Structures Chapter 6distin
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Page 202 and 203: 190 File Structures Chapter 6Master
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218 File Structures Chapter 6descri
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222 File Structures Chapter 6Figure
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224 File Structures Chapter 6159234
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226 File Structures Chapter 6machin
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228 File Structures Chapter 6disk i
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230 File Structures Chapter 610 typ
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232 File Structures Chapter 61. Imp
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chapter 7THE USER INTERFACEWe have
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236 The User Interface Chapter 7ins
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238 The User Interface Chapter 7Use
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chapter 8CONCURRENCYAs operating sy
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258 Concurrency Chapter 8process Ap
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260 Concurrency Chapter 8ABCDEFGHFi
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262 Concurrency Chapter 82.2 Busy w
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264 Concurrency Chapter 8to arbitra
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266 Concurrency Chapter 8The Intere
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268 Concurrency Chapter 8It is libe
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270 Concurrency Chapter 8share the
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272 Concurrency Chapter 8Variables
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274 Concurrency Chapter 8scope (a b
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280 Concurrency Chapter 8time, a re
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286 Concurrency Chapter 8To represe
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288 Concurrency Chapter 822 procedu
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290 Concurrency Chapter 8The parame
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292 Concurrency Chapter 86. Prove t
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chapter 9CO-OPERATING PROCESSESIn C
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296 Co-operating Processes Chapter
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REFERENCESJ. E. ALLCHIN, M.S.MCKEND
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324 ReferencesR. FINKEL, M.SOLOMON,
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326 ReferencesG. L. PETERSON, ‘
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GLOSSARYMany standard English words
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330 GlossaryBacking store. The larg
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332 GlossaryCiphertext. The result
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334 GlossaryDatabase. A collection
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336 GlossaryEncryption. A data tran
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338 GlossaryHint. Usually but not n
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340 GlossaryLocal. (1) A local page
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342 GlossaryOpen shop. A style for
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344 GlossaryProcess. The execution
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346 GlossaryRotational latency. The
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348 GlossaryStarvation. The situati
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350 GlossaryTrap-door encryption. A
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