Teaching Operating Systems Scheduling

Transcription

1 Prceedings f Infrming Science & IT Educatin Cnference (InSITE) 2010 Teaching Operating Systems Scheduling Shimn Chen MLA Academic Learning Center, Israel shamn51@gmail.cm Abstract The Operating System is a very cmplex prgram that runs n ur cmputer (prbably the mst cmplex ). It is difficult t cmprehend the diversity f its peratins, let alne teach it. Secnd/Third year students with beginner prgramming skills are verwhelmed by the OS size and its multiple tasks. This paper is fcused n hw-t better teach the peratin f the OS Scheduler that manages the user s prcesses. The scheduler tasks are: create the prcess, lad it int memry, allcate CPU time-slices fr its executin, handle keybard clicks and menu selectin, swap the prcess in and ut f memry, and instructs the devices t satisfy IO requests. The questin is: hw can we give the students hands-n experience n the scheduler peratin? Obviusly, a few slides and a state-machine diagram, is nt enugh. The answer cnsists f: explanatin (slides), simulatin and an exercise. The simulatin package includes tw simulatrs (tw levels f cmplexity) each running userdesigned scenaris. The 1 st simulatr (A) is a simple Rund-Rbin scheduler that is easy t fllw. The 2 nd simulatr (B) cmes much clser t a real OS scheduler (with all its cmplexity); as such it is very difficult fr the student t understand, unless they play with Simulatr-A first. The exercise gal is t write a scheduler that allcates the CPU time amng ten Prcesses each ding a lng task. The exercise can be accmplished by 2 nd -3 rd year students. Keywrds: Operating System, Simulatr, Teaching Intrductin Operating System The cmputer is inherently a parallel machine; it cnsists f several CPUs, Memry units, IO Cntrllers. Each cmpnent is in fact a basic-cmputer (my definitin) with a CPU, lgic (prgram), sme memry and IO cnnectrs. Fr example: a Disk-Cntrller is made f a CPU (called Cntrller), prgrammable lgic Material published as part f this publicatin, either n-line r in print, is cpyrighted by the Infrming Science Institute. Permissin t make digital r paper cpy f part r all f these wrks fr persnal r classrm use is granted withut fee prvided that the cpies are nt made r distributed fr prfit r cmmercial advantage AND that cpies 1) bear this ntice in full and 2) give the full citatin n the first page. It is permissible t abstract these wrks s lng as credit is given. T cpy in all ther cases r t republish r t pst n a server r t redistribute t lists requires specific permissin and payment f a fee. Cntact 0HPublisher@InfrmingScience.rg t request redistributin permissin. usually stred in EEPROM, memry fr data buffers and IO cnnectrs that enables it t cntrl the spinning disk, mve the read/write head and cmmunicate with ther devices r the mther bard. Tday these cmpnents are fabricated n ne chip (the s called cmputer n a chip ) On tp f the cmputer parallel/realtime hardware architecture, there are

2 Teaching Operating Systems Scheduling multiple sftware prcesses that cmpete n the use f the cmputer resurces (cmpnents). The gal f the Operating System (OS) scheduler is t divide/allcate/arbitrate the use f these resurces in the mst efficient pssible way. The CPU is the nly cmpnent that can execute prgrams; as such it must wear tw hats : (1) executing the multiple users prgrams, (2) managing the cmputer using the OS prgrams (Kernel). T d that, the CPU is cnstantly ding cntext switch amng all its bligatins. IO cntrllers are als required t d multiple tasks. Fr example: a Disk cntrller is instructed t lk fr several blcks, n different tracks, fr several prcesses. Or, the netwrk cntrller must send/receive different packets fr multiple brwser windws. Prblem / Questin / Answer The students have a prblem grasping the full cmplexity f the cmputer peratins. Being 2 nd /3 rd year students, they write simple prgrams (C, C++) that run, presumably alne, n the cmputer withut parallelism r threads. The students are nt fully aware f the cmputer realtime nature, and the BIG rll and many services prvided by the perating system. They struggle with the idea that the CPU is very fast but als very dumb, and that it needs very detailed instructins n hw and what t execute. As fr the lecturer: presenting slides, writing n the bard and being a better lecturer are the first necessary step, BUT usually the students quickly lse cntact with the material. There are t many new ideas / cncepts and they fail t see thrugh it. The questin is hw we can let them see inside the Scheduler and feel the peratin f the OS, CPU, and IO Cntrllers. The answer is: lecture (in tw steps), a pair f simulatrs (in increasing levels f cmplexity) and an exercise in writing a small scheduler. I suggest the fllwing time line: 1. Intrductin t OS, Interrupts, Prcesses States 2. Shw the 1 st simulatr 3. Explain System Calls, Timing and Scheduling 4. Shw the 2 nd simulatr 5. Present the Exercise Intrductin t the OS and the Scheduler This paper is nt abut Operating Systems; it is abut hw t teach OS (in particular the Scheduler). In rder t run (and understand ) the tw simulatrs, and d the exercise, the lecturer must cver the fllwing tpics: Cmputer architecture: CPU, Clck, BUS, Memry, IO cntrllers CPU: very *1 quick (speed f light) very * simple: fur arithmetic peratins Interrupts and System Calls Prcess structure, running, terminating, and Prcess Cntrl Blck (PCB) Resurce, IO devices, and Resurce Cntrl Blck (RCB) 1 Very, very very 610

3 Chen Prcess States (seven): new, ready, run, blcked, suspend ready/blcked, exit Scheduling, Timing, Rund-Rbin algrithm, Queues OS tasks: Prcess creatin (linker lad mdule ) Prcess lading (lader memry, DLLs) Prcess cntext-switch in/ut f the CPU Instructing IO devices Handling devices interrupts Suspending a Prcess ut f memry Relading suspended prcess int memry Prcess terminatin and cleanup Simulatrs (tw levels) The 1 st level simulatr is a simple scheduler that manages several prcesses. The prcesses may need the CPU r service frm ne f fur different IO devices. The simulatr shws hw the prcesses are kept in the READY and fur IO queues. The scheduler allcates the CPU time amng them using the Rund Rbin algrithm. Prcesses that need service frm IO devices are BLOCKED in the IO queues and are serviced accrding t the First-Cme-First-Serve methd. The 2 nd level simulatr is an elabrate scheduler that shws the details f the OS executin, hw it lads a prcess frm the disk t memry, perfrms cntext switching, schedules IO devices, cmmunicates with IO devices, handles interrupts, receives blcks, swaps prcess ut f memry (suspend state), swaps them in later and finally finish the prcess. The 2 nd simulatr shws hw the IO cntrller are wrking in parallel with the CPU. Exercise We define ten vectrs (length=1000), and fill them with ne thusand randm numbers. There are 10 prcesses that shuld srt the ten vectrs using Bubble srt. Bubble srt cnsists f repeated runds n the vectr; adjacent elements are cmpared and swapped if nt in rder. The algrithm cunts the number f swaps in each rund, and if the number is zer then it means that the vectr is srted. Using 1000 elements vectr with randm numbers results n the average with abut 120 runds (althugh in the wrst case scenari it might be 999) The gal f the prject is t write an OS that creates 10 prcesses (ding bubble srt) and schedules the CPU (executin) amng them. Using the Rund-Rbin algrithm, the OS allcates ne srt-rund per prcess t simulate time-slice. T simulate using IO we define the fllwing situatin: when the prcess makes mre than 500 swaps n the vectr it wants t print it status. In this case the prcess is taken ut f the CPUqueue and placed in the PRINT-queue. When a prcess is in the first place in the PRINT-queue then it can print but it takes 20 time-cycles. 611

4 Teaching Operating Systems Scheduling Simulatrs This sectin describes the tw simulatrs that run n tp f EXCEL. Simulatr A CPU IO1 IO2 IO3 IO4 P1 P2 P3 P4 P5 P6 P7 P8 P9 Clck 20 CYCLE RESET Wrk IDLE CPU IO FINISH Queues Simulatr A - by Shimn Chen P3 1 P6 2 P8 3 P1 4 CPU 3 CPU 2 CPU 2 CPU 3 CPU 4 CPU 2 CPU 6 CPU 2 CPU 3 P5 2 P4 2 IO3 7 IO3 6 IO1 7 IO1 2 IO1 2 IO1 8 IO2 9 IO2 8 IO4 8 P7 4 CPU 2 CPU 4 CPU 3 IO3 8 CPU 1 CPU 1 CPU 1 CPU 2 CPU 2 P9 1 IO2 7 IO2 7 IO2 7 IO2 7 IO2 7 IO2 7 IO1 7 IO4 7 IO2 7 P2 3 CPU 1 CPU 1 CPU 1 CPU 1 CPU 1 CPU 1 CPU 1 CPU 1 CPU 1 IO3 9 IO3 9 IO3 9 IO3 9 IO3 9 IO3 9 IO3 9 IO3 8 IO3 9 CPU 2 CPU 2 CPU 2 CPU 2 CPU 2 CPU 2 CPU 2 CPU 2 CPU 2 Line 1 header with names: ne CPU, fur IO and nine Prcesses Line 2 The clck (Yellw) and tw buttns: RESET t start (r restart) the simulatin CYCLE execute ne time-cycle The area belw the Simulatr A banner 1 st clumn Prcesses in the READY queue, each in need f CPU cycles, the first P3 needs 1 Cycle, the secnd P5 needs 2 mre cycles, Clumns 2-5 shw prcesses waiting fr IO service, P6 is serviced by IO1 needs 2 mre cycles t finish. Prcess P4 waits in the queue (needs 2 cycles).. Clumns 6-16 shw the scenaris f 1-9 prcesses CPU <n> - needs <n> cycles f CPU IO<a> <n> - needs IO<a> service fr <n> cycles Line CPU number f CPU cycles allcated t each prcess Line IO number f IO cycles allcated t each prcess Line IDLE number f idle cycles f each prcess Line FINISH clck-time when prcess finished its scenari Line WORK 1 st clumn CPU which prcess is serviced (Rund Rbin) 612

5 Chen 2-5 clumns 1 busy servicing 1 st prcess in queue, 0 idle 6-15 clumns number f item in the scenari (item clr is red) Hw t use Simulatr A Setup the prcesses scenaris yu may leave sme f the scenaris empty. Click n the RESET buttn Click the CYCLE buttn t execute ne cycle Repeat the previus step ( CYCLE click) until all prcesses reach the FINISH line. Hw t view the screen f Simulatr A In the beginning all prcesses need the CPU all in the CPU queue. Each prcess gets ne CPU-Cycle (Rund Rbin) When a prcess finishes using the CPU it needs IO service, the scheduler mves it t the apprpriate IO queue. In the IO queue nly the 1 st prcess is serviced, the ther wait in line. The lg (belw) is prduced autmatically by the simulatr 1 st clumn clck 2 nd clumn- time needed fr the task 3 rd clumn - Descriptin 613

6 Teaching Operating Systems Scheduling 17 7 P1 = >>> start IO P2 = $$$ T=4 K=5 put in Q=> CPU 17 0 P2 = <<< finish IO P4 = $$$ T=2 K=2 put in Q IO P4 = <<< finish CPU 16 7 P1 = $$$ T=7 K=2 put in Q IO P1 = <<< finish CPU 15 8 P8 = >>> start IO P8 = $$$ T=8 K=1 put in Q IO P8 = <<< finish CPU 14 8 P6 = >>> start IO P6 = $$$ T=8 K=1 put in Q IO P6 = <<< finish CPU 11 6 P2 = >>> start IO P2 = $$$ T=6 K=1 put in Q IO P2 = <<< finish CPU 0 3 P9 = $$$ T=3 K=9 put in Q=> CPU 0 2 P8 = $$$ T=2 K=8 put in Q=> CPU 0 6 P7 = $$$ T=6 K=7 put in Q=> CPU 0 2 P6 = $$$ T=2 K=6 put in Q=> CPU 0 4 P5 = $$$ T=4 K=5 put in Q=> CPU 0 3 P4 = $$$ T=3 K=4 put in Q=> CPU 0 2 P3 = $$$ T=2 K=3 put in Q=> CPU 0 2 P2 = $$$ T=2 K=2 put in Q=> CPU 0 3 P1 = $$$ T=3 K=1 put in Q=> CPU Simulatr A demnstrate? Hw the prcesses are waiting in the different queues. Hw ne CPU is multiplexing its time amng the prcesses using the Rund-Rbin algrithm. Hw each IO cntrller serves the 1st prcess in its queue. Hw prcesses are mved between the queues by the scheduler. NOTE: Operating System tasks (time) are nt cunted/simulated in Simulatr A Simulatr B is ding the elabrate jb f cunting and shwing the OS tasks. 614

7 Chen Simulatr B The simulatr screen is divided int the fllwing sectins: PCBs [tp-left] shws the status, service and ther parameters f the nine Prcesses Cntrl Blcks. CPU [tp-right] CPU status and the scenari used (see Data 7 ) The scenari itself is shwn belw RCBs [bttm-right] fur (4) Resurce Cntrl Blck, each RCB shws the status f an IO cntrller. Script [bttm-left] shws the scenaris used in the simulatin. Simulatr B main Screen CYCLE RESET P C B s C P U Data 7 ID Clck Status Run S-Blck Blck S-Blck S-Blck S-Ready S-Blck Blck S-Ready Status RUN Service Nne Wait IO Start IO Wait IO Wait IO Nne Wait IO Start IO Nne PCB 1 Swap Type Nne Nne Nne Nne Nne Nne Nne Nne Nne Memry 1000 Pririty Free 100 Memry Size Run % In Memry Yes N Yes N N N N Yes N Kernel % Q Number IDLE 10 3% Tp Script Request Request CPU IO IO IO IO CPU IO IO CPU Request IO Request Parameter RCB 1 RCB 2 Request Left Status WORK Status WORK Time Per Blck 50 Per Blck 60 Start Time PCB 5 PCB 2 End Time Ttal T 230 Ttal T 197 CPU Use Wait 23 Wait 21 IO Use Time 20 Time 43 KB Wait Cur Start RCB 3 RCB 4 Cur Time Status WORK Status WORK KB Time Per Blck 70 Per Blck 80 Live Time PCB 7 PCB 4 CPU % Ttal T 142 Ttal T 213 Script Wait 18 Wait 26 Previus O1 4 C 15 C 30 C 10 C 15 S 50 C 10 C 15 S 50 Time 68 Time 27 Current C 70 O2 4 O3 4 O4 4 O1 4 C 30 O3 4 O4 4 C 30 Next C 20 C 90 C 70 C 20 O2 4 C 70 C 20 K 200 Line ID PCB numbers Line Status RUN, READY, BLOCKED S-BLOCK (Suspend=Blck) Line Service Prcess is waiting fr OS service: Wait-IO, Start-IO Line Swap Type used when the prcesses is swap ut t a suspend state Line Memry Size Hw much memry is needed Line (4 lines) Request Parameters fr the current service request Line Time Cycles used in varius tasks Line Cur Start Clck when current task started Line Cur Time (in red) cycles needed t cmplete current task RCBs (fur gray squares n the right) RCB <number> Status: Wrk, IDLE 615

8 Teaching Operating Systems Scheduling Per Blck: Time needed t access ne blck n the disk PCB: wrking fr PCB Ttal T: ttal time wrking Wait: time waiting fr OS t handle interrupts TIME: Time remains t access the current blck Simulatr B Scenari The scenari instructins are: M <size> - size f prcess in memry P <n> - Prcess pririty S <cycles> - when t start the prcess C <cycles> - CPU cycles needed at this stage O<n> <cycles> - need <cycles> service frm IO number <n> $ P1 $ P2 $ P3 $ P4 $ P5 $ P6 $ P7 $ P8 $ P9 M 200 M 300 M 400 M 200 M 300 M 400 M 200 M 300 M 400 P 2 P 2 P 2 P 2 P 2 P 2 P 2 P 2 P 2 S 10 S 30 S 50 S 10 S 30 S 50 S 10 S 30 S 50 C 10 C 15 C 30 C 10 C 15 C 30 C 10 C 15 C 30 O1 4 O2 4 O3 4 O4 4 O1 4 O2 4 O3 4 O4 4 K 200 C 70 C 20 C 90 C 70 C 20 C 90 C 70 C 20 C 90 Hw t use Simulatr B Setup the scenari in a different sheet (in this example DATA7 ) Enter the number 7 in the blue cell (tp-right) Click the RESET buttn t initialize the simulatr Click the CYCLE Clck is running (ne r mre cycles) until the next event. Hw t view the screen f Simulatr A Watch what happens with the prcesses in the first three PCBs lines: Status, Service and Swap Type, and the hw time f current task CUR Time in red is changing. Watch the CPU cunters n the right Watch the RCBs statuses as they wrk fr different PCBs Simulatr B demnstrate? Hw the OS is using the CPU in rder t service the pcbs, there are varius services each need the sle attentin f the CPU. Hw the RCBs (IO cntrller) wrks in parallel t the CPU 616

9 Chen Simulatr B parameters The tables belw shw varius numbers used t cntrl the simulatin. Fr example: if the OS needs t start an IO peratin (line 7 Start IO ) it takes 2 simulatr cycles. Service Time Nne Nne 0 Create Create Prcess 10 Clse Clse Prcess 8 Start KB Start KB 1 Wait KB Suspend if KB > 70 INT KB Handle KB 1 Start IO Start IO Operatin 2 Wait IO Suspend if IO > 1 INT IO Handle IO Interrupt 4 Pririty Pririty Change 3 Memry Memry Change 5 Swap Nne Nne 0 Lad IN Lad Int Memry 6 L OUT Lad OUT f memry 9 S-IN Swap Int CPU 2 S-OUT Swap OUT f CPU 2 617

10 Teaching Operating Systems Scheduling Detailed Lg File: Simulatr B Lg File CPU ding P1 Status=Run T= P1 $ Status is = Run CPU ding P1 Status=Ready Swap=S-IN T= P1 * Swap Request is = S-IN IO3 handles a Blck fr P P7 = Service IO Next R3 Blcks=2 f CPU ding P7 Status=S-Blck Service=INT IO T= P1 $ Status is = Ready CPU ding P1 Status=Run Swap=S-OUT T= P1 * Swap Request is = S-OUT P7 = Service Request is = INT IO IO3 Interrupt fr P7!!! CPU ding P1 Status=Run T= P1 $ Status is = Run CPU ding P1 Status=Ready Swap=S-IN T= P1 * Swap Request is = S-IN P1 $ Status is = Ready CPU ding P1 Status=S-Ready Swap=Lad IN T= P1 * Swap Request is = Lad IN IO2 handles a Blck fr P P2 = Service IO Next R2 Blcks=1 f CPU ding P2 Status=S-Blck Service=INT IO T= P2 = Service Request is = INT IO IO2 Interrupt fr P2!!! CPU ding P5 Status=Blck Service=Wait IO Swap=L OUT T= P5 * Swap Request is = L OUT Cnclusin Undergraduate students find it difficult t cmprehend the cncepts f the perating systems. There are many new ideas, buzz-wrds, multiple tasks and strange algrithms. Using the prpsed methd (see belw) shws that the students experience and reactin t the curse was changed frm dull / t cmplicated t interesting and challenging. The use f the simulatrs cupled with the exercise resulted in much better understanding f the scheduler (as tested in the mid-term) 618

11 Chen 1. Intrductin t OS, Interrupts, Prcesses States 2. Shw the simulatr A 3. Explain System Calls, Timing and Scheduling 4. Shw the simulatr B 5. Exercise References Dietel, H. M. (1992). Operating systems (2nd ed.). Reading, MA: Addisn-Wesley. Finkel, R.A. (1988). An perating systems vade mecum", 2nd ed. Prentice-Hall, Englewd Cliffs, NJ,. Gscinski, A. (1991). Distributed perating systems: The lgical design. Hartley, S. J. (1994). Operating systems prgramming. Krakwiak, S. (1988). Principles f perating systems. MIT Press. Lane, M. G., & Mney, J. D. (1988). A practical apprach t perating systems. Byd and Fraser. Milenkvic, M. (1992). Operating systems: Cncept and design (2nd ed.). McGraw-Hill. Silberschatz, A., & Galvin, P.B. (1994). Operating system cncepts (4th ed.). Reading, MA: Addisn- Wesley. Tanenbaum, A. S. (1987). Operating systems: Design and implementatin. Tanenbaum, A. S. (1992). Mdern perating systems. Englewd Cliffs, NJ: Prentice-Hall. Tel, G. (2000). Intrductin t distributed algrithms. Bigraphy Shimn Chen has been wrking in the Israeli Hi-Tech Industry fr the last 30 years, including prjects in areas f: Databases, Parallel cmputing, Artificial Intelligence, E-Learning, Ge-Graphical infrmatin, Cellular Cmmunicatin. Teaching Operating Systems, Object Oriented Prgramming, Artificial Intelligence, and ther Cmputer Science curses. 619