An Analysis of Central Processor Scheduling in Multiprogrammed Computer Systems

Transcription

1 STAN-CS I SU-SE An Analyss of Central Processor Schedulng n Multprogrammed Computer Systems (Dgest Edton) by Thomas G. Prce October 1972 Techncal Report No. 57 Reproducton n whole or n part s permtted for any purpose of the Unted States Government. Ths document has been approved for publc release and sale; ts dstrbuton s unlmted. Ths work was supported n part by the Jont Servces Electroncs Program: U.S. Army, U.S. Navy, and U.S. Ar Force under Contract N A and by Mr. Prce s fellowshp from the Natonal Scence Foundaton. I STRflFORD EBECTROllltS ABORPTORIES

2 STAN-CS SE AN ANAYSIS OF CENTRA PROCESSOR SCHEDUING IN MUTIPROGRAMMED COMPUTER SYSTEMS (Dgest edton) - I. Thomas G. Prce October 1972 Techncal Report No. 57 t Reproducton n whole or n part s permtted for any purpose of the Unted States Government. c Ths document has been approved for publc release and sale; ts dstrbuton s unlmted. b DIGITA SYSTEMS ABORATORY Dept. of Electrcal Engneerng Dept. of Computer Scence Stanford Unversty Stanford, Calforna Ths work was supported n part by the Jont Servces Electroncs Program: U.S. Army, U.S. Navy, and U. S. Ar Force under contract ~-0001~-6~-A ~k and by Mr. Prce's fellowshp from the Natonal Scence Foundaton.

3 r ;. - ABSTRACT A smple fnte source model s used to gan nsght nto the effect of central processor schedulng n multprogrammed computer systems. CPU utlzaton s chosen as the measure of performance and ths decson s dscussed. A relaton between CPU utlzaton and flow tme s developed. It s shown that the shortest-remanngprocessng-tme dscplne maxmzes both CPU utlzaton and I/O utlzaton for the queueng model M/G/l/N. An exact analyss of processor utlzaton usng shortest-remanng-processng-tme schedulng for systems wth two jobs s gven and t s observed that the processor utlzaton s ndependent of the form of the processng 1. tme dstrbuton. The effect of the CPU processng tme dstrbuton on performance s dscussed. For frst-come-frst-served schedulng, t s shown that dstrbutons wth the same mean and varance can yeld sgnfcantly dfferent processor utlzatons and that utlzaton may or may not sgnfcantly decrease wth ncreasng varance. The results are used to compare several schedulng dscplnes of practcal nterest. An approxmate expresson for CPU utlzaton usng shortest-remanngprocessng-tme schedulng n systems wth N jobs s gven.

4 DISCUSSION I major areas. Schedulng n a multprogrammng system can be dvded nto three Sze of the man memory lmts the number of jobs whch - can be resdent smultaneously. Snce there wll frequently be more jobs to execute than wll ft n memory, the system must have a rule for selectng a set of jobs from the jobs watng to be executed to resde n man memory. Ths rule wll be referred to as the job selecton rule or job schedulng. The job selecton rule may try to optmze the job mx to gve good turnaround tme to short jobs, make effcent utlzaton of the hardware, etc. The jobs that are currently resdent n man storage wll alternate between I/O and CPU operatons. If the number of jobs multprogrammed exceeds the number of CPUs, a watng lne or queue for the CPUs may develop. We wll refer to the rule used to manage ths queue as the CPU 1 schedulng algorthm. Smlarly, a queue may form n front of the I/O channels and devces and I/O schedulng must be consdered. The purpose of ths paper s to present an analyss of CPU schedulng algorthms. I Several authors have gven heurstc approaches to CPU schedulng [Stevens, 19681, [Marshall, 19691, [Ryder, 19701, [Wulf, 19691, [HASP, Most of the heurstcs are based on the dea that some jobs are "compute bound" and other jobs are "I/O bound." The I/O bound jobs should be gven prorty for the CPU over CPU bound jobs n order to 4 prevent "hoggng" of the CPU by a compute bound job and hence poor CPU-I/O overlap. All of the above references descrbe mplementatons of heurstc schedulng algorthms on specfc systems. Stevens [1968], Marshall [1969], Ryder [1970], and Wulf [1969]have all observed sgnfcant t

5 f mprovement n CPU utlzaton and throughput usng ther new schedulng algorthms. In [Stevens, and [Wulf, other parts of the system were changed at the same tme the schedulng algorthm was changed, so t s dffcult to determne exactly what the effect of changng the schedulng algorthm was. Ryder's [1970] heurstc s complcated, and t s dffcult to determne whch features are mportant. Fnally, the relaton between the job mx and the CPU schedulng algorthm s dffcult to see because an adequate descrpton of the system loads s not avalable. Ryder [1970] makes an effort to deal wth ths problem. Sherman [1971] has used a trace drven smulaton model to systematcally compare several CPU schedulng algorthms and has overcome most of the problems mentoned above. Theoretcal work on computer system schedulng has been performed by ị. I, usng a probablty model to represent the computer system and then performng an analyss of partcular schedulng algorthms for the chosen model. Probablty models of computer systems can be dvded nto two major classes, fnte source models and nfnte source models [McKnney, Most of the work on schedulng has used nfnte source models. Ths s due prmarly to the fact that nfnte source models are easer to analyze than fnte source models. Although nfnte source models can gve some nsght nto the operaton of computer systems, they appear to be poor models of multprogrammed computer systems snce only a few jobs can ft n man storage smultaneously. Fnte source models have been analyzed for frst-come-frst-served (FCFS) schedulng [Gaver, 19671, [Jaswal, 19681, processor sharng schedulng [Baskett, 19711, and preemptve resume last-come-frst-served (CFS) schedulng [Chandy, Baskett [1971] has noted that processor sharng can gve consderable r, mprovement over FCFS when the compute tmes are hghly varable. remarks apply to preemptve resume CFS. 2 Smlar

6 , r In ths paper a smple fnte source model s used to study the effect of the CPU schedulng algorthm on system performance. utlzaton s used as the measure of system performance and the reason CPU for ths s dscussed n secton 2. The basc fnte source model s. shown n Fg 1. When a job arrves at the CPU the processng tme for ts next CPU operaton, whch wll be called a "CPU tme," s selected ndependently from a CPU processng tme dstrbuton whch s dentcal for all jobs. At the completon of ts CPU servce, the job goes to an,. I/O processor (or channel). The servce tme at the I/O processor, whch wll be called an "I/O tme," s selected from an I/O processng tme dstrbuton, agan dentcal for all jobs. Each job s ether dong I/O or usng the CPU or watng and no overlap s allowed. Throughout ths paper the assumptons necessary for each result are stated explctly wth the result. A few general comments about the assumptons wll be gven here. Most of the results n the paper assume that the CPU processng tme dstrbuton s general and the I/O processng tme dstrbuton s exponental. It s usually assumed I that no queung for I/O occurs. Wth these assumptons the model s equvalent to the M/G/l/N queueng process. Ths notaton s due to I Kendall and s descrbed n [Jaswal, Ths model s sometmes referred to as the machne nterference model [Cox, The machne nterference model has been suggested as a model of multprogrammed t computer systems [Gaver, 19671, [Baskett, and of tme sharng systems [Scherr, 19671, [McKnney, Many of these assumptons are not strctly true n real systems. c t - I/O tmes are usually not exponentally dstrbuted. Successve CPU

7 I/O Processors E Fgure 1. The Fnte Source Model c 4

8 f I- tmes and I/O tmes may not be ndependent. Dfferent jobs have dfferent resource requrements. Many systems do allow for overlap of I/O and computng for a sngle job. Baskett [1972] has dscussed some of the defcences of ths type of model. Despte all of the problems,. models of ths type have been shown to be a far approxmaton to real systems [Baskett, Thus, whle the model does not exactly reflect the structure and behavor of real systems, t retans many of the - sgnfcant aspects of multprogrammng and s close enough to real systems to gve us nsght nto the effect of schedulng. REFERENCES Baskett, F. and Palacos, F. G. Processor sharng n a central server queung model of multprogrammng wth applcatons. Proceedngs of the Sxth Annual Prnceton Conference on Informaton Scence and Systems, Prnceton Unversty (March 1972). Baskett, F. Mathematcal models of multprogrammed computer systems. TSN-17, Computaton Center, Unversty of Texas, Austn, Texas (January 1971). Chandy, K. M. The analyss and solutons for general queung networks. Proceedngs of the Sxth Annual Prnceton Conference on Informaton Scences and Systems, Prnceton Unversty (March 1972). Gaver, D. P. Probablty models for multprogrammng computer systems. J. ACM 14, 3 (July 1967), HASP. Houston Automatc Spoolng Prorty System - II (Verson 2), IBM Type III Program Documentaton, Order No. 360D , IBM Corporaton, Hawthorne, New York (1969). Jaswal, N. K. Prorty Queues. Academc Press, New York, (1968). Marshall, B. S. Dynamc calculaton of dspatchng prortes under OS/360 MVT. Datamaton (August 1969), McKnney, J. M. A survey of analytcal tme-sharng models. Computng Surveys 1, 2 (June 1969),

9 Ryder, K. D. A heurstc approach to task dspatchng. IBM Systems Journal 8, 3 (1970), Scherr, A.. An Analyss of Tme-Shared Computer Systems. MIT Press, Cambrdge, Massachusetts (1967). Schrage,. A proof of the optmalty of the shortest remanng processng tme dscplne. Operatons Research 16, 3 (May 1968), Sherman, S., Baskett, F., and Brown, J. C. Trace-drven modelng and analyss of CPU schedulng n a mult-programmed system, Proc. ACM SIGOPS Workshop on System Performance Evaluaton, Harvard Unversty (Aprl 1971), Stevens, D. F. On overcomng hgh-prorty paralyss n multprogrammng systems: A case hstory. C. ACM 11, 8 (August 1968), Wulf, W. A. Performance montors for mult-programmng systems. Proceedngs of the Second Symposum on Operatng Systems Prncples (October, 1969),