An Operating Systems Vade Mecum

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32 Time Management Chapter 2Unfortunately, no policy is truly fair. <strong>An</strong>y improvements in performance for oneclass of processes is at the expense of degraded performance for some other class. Wewill therefore examine how policies treat a wide range of classes. For each policy, wewill first show its behavior for a simple set of processes:Process name Arrival time Service requiredA 0 3 B 1 5 C 3 2 D 9 5 E 12 5This set depicted in Figure 2.3.3C5161217BE03914AD05101520Figure 2.3 Processes requiring serviceThe time units are not important; if we like, we can imagine they are seconds. Weassume that the arrival time and service time required are whole numbers of whateverunit we have picked.Second, we will compare scheduling policies by simulating them for a largenumber of arrivals. Figure 2.4 shows the penalty ratio for many of the policies we willstudy as a function of time required. This figure is based on a simulation of 50,000processes. Service times were randomly drawn from an exponential distribution withβ=1.0, and arrival rates were similarly drawn from an exponential distribution withα=0.8. The saturation level was therefore ρ=0.8. Statistics were gathered on each processexcept for the first 100 to finish in order to measure the steady state, which is thebehavior of a system once initial fluctuations have disappeared. Processes were categorizedinto 100 service-time percentiles, each of which had about 500 processes. The averagepenalty ratio for each percentile is graphed in the figure. The lines in the graph havebeen smoothed slightly (simulation results are somewhat bumpy).The average service time needed by processes in each percentile is shown in Figure2.5. Figure 2.6 shows the missed time for each percentile under the various methods.
100FCFSPenalty ratio 10HPRNFBRRRRPSPNSPNSPNHPRNFBFCFSPSPN10 10 20 30 40 50 60 70 80 90 100Percentile of time requiredFigure 2.4 Penalty ratios of short-term scheduling policies654Time required32100 10 20 30 40 50 60 70 80 90 100Percentile of time requiredFigure 2.5 Average service time for each percentile
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186 File Structures Chapter 6distin
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chapter 7THE USER INTERFACEWe have
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chapter 8CONCURRENCYAs operating sy
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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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An Operating Systems Vade Mecum

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