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1. 5 4 Actual Time Schedule The first check point on the time schedule concerned the final decisions on extensions They were delayed 6 weeks due to disagreements between the user groups The extensions chosen were vdu terminals card reader job controlled selection of print forms printer back up on magnetic tape According to the project specification this should delay the installation by three months of which 6 weeks were due to the delay of the decision The next check point was apparently reached on schedule with release of the report Boss 2 Internal Structure dated January 1971 However a close read ing of the report reveals many missing details in the interface between the coroutines The report contained a detailed version of Sections 2 3 and 4 of this paper In March and April 1971 the company changed their development policy claiming that they would neglect somehow the RC 4000 and try to find other areas for the development groups The Boss 2 group was heavily involved in these discussions and a delay of one month was accepted The new schedule looked like this May 1971 Debugged Central Logic All other parts punched September 1971 Prototype debugged November 1971 Installation completed The Central Logic was debugged on schedule but most other parts were still not written A lot of detailed revisions of the schedule were made but none of them reflected reality Several parts were completed and de bu2. as follows November 1970 Final decisions on extensions December 1970 Detailed specification of the internal structure and the interface between all parallel activities inside Boss February 1971 Debugged Central Logic All other parts punched May 1971 Prototype debugged July 1971 Installation of the prototype completed In view of the later facts that the prototype was completed April 1972 and the first public release was August 1972 this time schedule seems utterly ridiculous Let us see however how we arrived at it 385 The key point was the estimate of the total number of instructions Our basis was the list of facilities and strategies not horrible which we estimated as equiva lent to our Algol compiler First estimate Boss is 7 000 instructions From this the rest was deduced An old rule of thumb told that in a project from design to maintenance a programmer produced an average of 200 machine in structions a month Thus 35 man months were necessary Of these 6 were spent already with design 3 persons in 2 months so 29 were left for 4 persons This brought us to June 1971 We specified July to be cautious Another rule of thumb told that a good programmer made one error or design fault for each 20 instructions and that he could find one error for each test run With the debug technique we were accustomed to a good test run with succeeding careful inspection of the test output could reveal about 5 e
3. or change paper The list of incom pleted requests can be printed on demand The request display receives two kinds of requests One is insert which specifies an operator request to be printed and kept in the list This request is not an swered in Step 3 The other kind is delete which speci fies that a request should be deleted from the list pre sumably because the operator action is completed In this case Step 3 answers both the delete request and the earlier insert request The proof that all requests are eventually answered involves two rules First the operator is supposed to honor a request in a finite time and this must cause a delete to the request display The second rule imposes a limit on the number of insert requests which a coroutine may have pending This limit is one for each pS printer ps reader and ps job with addition of one for each tape station to be used either by the ps job or the remoter The limit is zero for all other corou 383 tines If these rules are followed by every coroutine we prevent Deadlock simply by making a sufficiently large pool of free request records As a more important example consider the coroutine Banker which allocates resources to the jobs When a coroutine wants to reserve or release resources it sends a private buffer Section 2 5 to the Banker stating the set of resources involved It gets the answer to a re lease request immediately
4. updates variables common to several processes or coroutines If a certain relation is to be maintained between the common variables one process must complete the critical region before another process can enter a critical region working on the same variables A sound solution is to use a queue with one record which contains the common variables When a process wants to update the variables it gets the record by means of waitg After the updating it returns the record to the same queue by means of sigq If other processes want to update the variables meanwhile they will be suspended when executing waitq Because only one record is involved its address may be known to all processes and a simple semaphore may replace the queue semaphore This was the solu tion described by Dijkstra and it is also used in Boss The corresponding locking semaphores are shown in Figure 3 as waved arrows The special coroutine scheduling of cpu time allows a simpler handling of critical regions in many cases the Central Logic can only pass control to another coroutine when a coroutine explicitly waits As a result critical regions without embedded waiting can be handled without semaphores This simplifies program ming in many cases for instance in the printer case of the preceding section 2 5 Private Buffers and Messages A special case of multi buffer communication is frequently used Each sender of a request has his own record and he wants the ans
5. Germany 1970 vol 1 pp 98 115 4 Courtois P J On the near complete decomposability of networks of queues and of stochastic models of multiprogramming computing systems Scientif Rep CMU CS 72 11 Carnegie Mellon U Nov 1971 5 Courtois P J Error analysis in nearly decomposable stochastic systems MBLE Rep R214 Mar 1973 To be published in Econometrica Mar 1975 6 Denning P J Thrashing its causes and prevention Proc AFIPS 1968 FJCC vol 33 AFIPS Press Montvale N J pp 915 922 7 Dijkstra E W The structure of the THE multiprogramming system Comm ACM 11 5 May 1968 341 346 8 Dijkstra E W Hierarchical ordering of sequential processes Acta Informatica 1 2 1971 115 138 9 Jackson J R Jobshop like queueing systems Man Sci 9 1 Oct 1963 131 142 10 Kleinrock L Certain analytic results for time shared processors Proc IFIP 68 North Holland Pub Co Amsterdam 1969 vol 2 pp 838 845 11 Little J D C A proof for the queueing formula L AW Oper Res 9 1961 383 387 12 Muntz R and Baskett F Open closed and mixed net works of queues with different classes of customers Tech Rep N 33 Digital Syst Lab Stanford U Aug 1972 13 Parnas D L and Darringer J A SODAS and a methodology for system design Proc AFIPS 1967 FJCC vol 31 AFIPS Press Montvale N J pp 449 474 14 Simon H A and Ando A Aggregation of variables in dynamic systems E
6. London 1972 8 Habermann A N Prevention of System Deadlocks Comm ACM 12 7 July 1969 373 377 385 9 Habermann A N On the harmonious cooperation of abstract machines Technische Hogeschool Eindhoven 1967 10 Horning J J and Randell B Process structuring Computing Surveys 5 1 Mar 1973 5 30 11 Lauesen S Job scheduling guaranteeing reasonable turn around times Acta Informatica 2 1973 1 11 12 Lauesen S Program control of operating systems BIT 13 3 1973 323 337 13 Lauesen S Forelgbig Specifikation af Operativsystemet Boss 2 in Danish RCSL No 55 D153 Regnecentralen Copenhagen 1971 14 Lauesen S Boss 2 user s manual operator s manual installation and maintenance RCSL No 31 D211 31 D230 and 31 D191 Regnecentralen Copenhagen 1972 15 Lauesen S Implementation of semaphores and parallel processes NBB Doc EC D4 Nordisk Brown Boveri Copenhagen 1973 16 Lindblad Andersen P Monitor 3 RCSL No 31 D109 Regnecentralen Copenhagen 1972 17 Naur P The design of the Gier Algol Compiler BIT 3 1963 124 140 and 145 166 18 Baker F T Chief programmer team management of production programming JBM Syst J 1 1972 56 73 Appendix A Survey of Coroutines in Boss The list below refers to Figure 3 and mentions all co routines in a row by row sequence Request Display Prints messages to the operator on the main console Messages demanding action from the opera
7. by the coroutine ccommandio As the two coroutines share the terminal they use a locking semaphc e to get exclusive terminal access during a conversation In Figure 2 the system is shown with only one corou tine to handle a terminal but then we have requests in both directions to the ps job As a result we would risk Deadlock because the terminal coroutine could wait for an answer from the ps job while the ps job waited for an answer from the terminal coroutine None of them would then be able to process the request from the other Also magnetic tape stations have a two way initia tive because Boss may try to rewind the tape the corou tine rewinder while the operator unloads it and mounts a new tape the coroutine remoter Because all communications are symmetrical pro ducer consumer algorithms the direction of the request may seem rather arbitrary However it is well defined Communications July 1975 of Volume 18 the ACM Number 7 from a semantic point of view For instance if the ps printer does not get a request it is because no printing is needed But if it does not get an answer printing is needed but not carried out In some cases we could imagine a system with a reversed request direction For instance we could try to construct a system where the tape reader issued a request whenever a paper tape was inserted We could then try to prove that all tapes were loaded sooner or later In the presen
8. responded by only allowing us run time in the night after saving the standard system on tape We responded by changing the autoload tapes to one which itself found out which of the two actions to perform After a while we regained our short periods of day time After reduction for the switch time we were left with 10 minutes of which we wanted to use four times 1 min ute for the planned translation and test of the four pro Communications July 1975 of Volume 18 the ACM Number 7 grammer s parts The 6 minutes left were spares for rush corrections In order to translate and generate a new version in half a minute all files had to be on disk and only a small text file had to be translated into binary form No linker loader exists in RC 4000 so we had to implement a special linker which loads the binary files into the virtual store in which Boss runs A simulation program was also devised to feed Boss with input lines from terminals etc in the 30 seconds left We used the well proven technique of permanent test output 17 from all parts of Boss so that the result of a test run was a list of test output for later inspection of what had happened This test output is still used in the normal production run where it is stored on disk or magnetic tape for analysis if the system breaks down 14 Chap 9 Installation and Maintenance A test out put record is generated by the Central Logic whenever a coroutine executes sig wait
9. were executing several pro grams at the same time The sequential execution of such a program is called a process Some of the proc esses are built in drivers external processes some of them are job processes executing a sequence of user defined programs job steps and some of them are operating systems Any two processes can communicate and synchro nize by means of messages Each process owns a set of message buffers in the protected Monitor and it has a message queue in which it can receive message buffers sent to it from other processes Figure 1 It can call a Monitor procedure asking to wait until a message buffer is in this queue and it can return a message buffer to the sender send answer Two other Monitor pro cedures allow it to send a message to another process 378 Fig 1 Process communication by means of messages In the ex ample B is the sender of a message to the receiver A Sender and receiver are identified by a process name of at most 11 characters Return parameters are underlined A WAIT MESSAGE SENDER MESSAGE BUFFER IDENT SEND ANSWER ANSWER BUFFER IDENT apee SEND MESSAGE RECEIVER MESSAGE BUFFER IDENT WAIT ANSWER ANSWER BUFFER IDENT Fig 2 Processes and coroutines Processes are implemented by the RC 4000 Monitor Coroutines are implemented inside the single Boss process The set of coroutines is idealized DRIVERS BOSS PROCESS JOB PROCESSES eS COROUTINES MESSAGE
10. COMMUNICATING COMMUNICATION BY SEMAPHORES MESSAGE COMMUNICATION Fig 3 All coroutines of Boss 2 and their communication Some peripheral devices are shown too REQUEST DISPLAY ete A eee s pacer t444 COMMUNICATIONS PRIVATE BUFFER lt QUEUE MULTI BUFFER LOCKING SEMAPHORE SIMPLE MULTI BUFFER Communications July 1975 of Volume 18 the ACM Number 7 and wait for an answer to be returned A further Monitor procedure essential for implementation of operating systems allows a process to wait for the first coming message or answer wait event A process owns a set of resources like working store and message buffers It can use a part of these resources to create another process a child running in parallel with the creator the parent Later the parent can remove the child and get the resources back A process which is an operating system creates job processes in this way The pragmatic rules for communication job operat ing system and job drivers were developed from 1969 to 1970 that is before any advanced operating system was planned In this early period a lot of compilers and utility programs were developed and for compatibility reasons Boss had to follow the old pragmatic rules During the period from 1969 to 1972 the computer was mostly operated with an extremely simple operat ing system which handled core resident job processes only The creation and removal of job pr
11. and the answer to a reserve request when the resources are available Again the Banker may skip the answer in Step 3 until more re sources have been released The proof in this case is somewhat complicated and assumes that the maximum set of resources needed by a job is stated at job start The allocation algorithm and details of the proof are given in 11 We should mention that the Banker is the only co routine not synchronized to a job or a peripheral device In fact the Banker is superfluous as a coroutine and it could be replaced by a reentrant procedure which is called by any coroutine wanting to reserve or release re sources for a job The solution would then become the private semaphore scheme described by Dijkstra 5 Later Dijkstra has proposed the private buffer scheme actually used in Boss the Secretary of 7 During the debugging we met some Deadlocks caused by trivial programming errors e g waiting for a wrong semaphore and one Deadlock caused by vio lation of the second rule for locking semaphores easily corrected by splitting the critical region in two 3 3 Hierarchical Structuring and Coroutine Structuring The basic proof of Deadlock absence is equivalent to that of Dijkstra and Habe nann 5 and 9 Especially the idle waiting in Step 1 corresponds to their homing position If we wish it we could regard the set of coroutines as structured in a hierarchy like Dijkstra s Each column o
12. ces Section 3 2 Finally Banker 3 handles the idle ps jobs and tells them when to start running a job and the kind of the job card job paper tape job internally generated job Pager Handles the virtual store as described in Sec tion 4 1 Communications July 1975 of Volume 18 the ACM Number 7
13. conometrica 29 2 Apr 1961 111 138 15 Smith J L Multiprogramming under a page on demand strategy Comm ACM 10 10 Oct 1967 636 646 16 Vantilborgh H On random partially preloaded page replace ment algorithms MBLE Rep R202 Sept 1972 17 Zurcher F W and Randell B Iterative multilevel modelling A methodology for computer system design Proc IFIP 68 Cong North Holland Pub Co Amsterdam 1969 vol 2 pp 867 871 377 R Stockton Gaines Editor A Large Semaphore Based Operating System S ren Lauesen Nordisk Brown Boveri Copenhagen Operating Systems The paper describes the internal structure of a large operating system as a set of cooperating sequential processes The processes synchronize by means of semaphores and extended semaphores queue sema phores The number of parallel processes is carefully justified and the various semaphore constructions are explained The system is proved to be free of deadly embrace deadlock The design principle is an alternative to Dijkstra s hierarchical structuring of operating systems The project management and the performance are discussed too The operating system is the first large one using the RC 4000 multipro gramming system Key Words and Phrases cooperating processes operating system semaphores semaphore applications queue semaphores deadlock deadly embrace hier archical structuring multiprogramming operating system structure as
14. decomposable systems This analysis is an example of how this distinction can be used to dissect models of computing systems into subsystems which can be i evaluated separately and ii represented by a few aggre gative variables whose interactions can be analyzed at a higher level of aggregation The degree of approxima tion necessitated by this approach remains known and is probably the price we have to pay to evaluate complex systems Acknowledgments I am indebted to Professors H A Simon and D L Parnas Carnegie Mellon University for their encouragement and many valuable suggestions to J Georges MBLE Research Laboratory because a great part of this research is the continuation of some earlier work 2 we did together and to R Vantilborgh MBLE Research Laboratory and to Professor P J Den ning Purdue University whose constructive criticisms contributed to the improvement of this paper Received May 1973 revised May 1974 References 1 Betourne C and Krakowiack S Simulation de l Allocation de Ressources dans un Syst me Conversationnel m moire virtuelle pagin e Proc Congr s AFCET Grenoble France Nov 1972 2 Buzen J P Computational algorithms for closed queueing networks with exponential servers Comm ACM 16 9 Sept 1973 527 531 3 Courtois P J and Georges J An evaluation of the stationary behavior of computations in multiprogramming computer systems Proc ACM Int Comput Symp Bonn
15. development of Boss 2 We would not change the existing software unless strictly necessary However we soon found it necessary to modify and extend the file handling procedures in the monitor and this project developed in parallel with Boss 2 16 Figure 2 shows the process Boss which executes the Boss 2 program The job processes are children of Boss Boss receives messages from the job processes and sends 379 the answer when the requested operation is completed Of course the jobs send all parent messages to Boss but some input output messages are also sent to Boss because Boss simulates some devices and behaves like a set of drivers toward the job The devices simulated are very slow devices requiring spooling and devices difficult to share low speed terminal paper tape reader printer The job sends input output messages directly to the drivers for fast devices like disk and magnetic tape Boss sends messages to various drivers either to complete the device simulation terminal reader printer or to execute a parent message reading a magnetic tape label in connection with the parent message mount magnetic tape Inside Boss a set of parallel activities is going on one activity synchronized to each job and one syn chronized to each peripheral device i e to the driver These activities will in principle communicate with each other as shown in Figure 2 Each activity could have been implemented as a process under t
16. e core store areas of the job processes considered special pages The synchronization to the surroundings replacing interrupts in a conven tional multiprogramming system is handled by means of the monitor procedure wait event which allows the Central Logic to wait for the first answer or message the first interrupt Below we will discuss the coroutines and show why Communications July 1975 of Volume 18 the ACM Number 7 the Deadlock problem changes the idealized picture of Figure 2 to the actual one in Figure 3 The design is shown to be governed by the parallel activities one synchronized to each job and one syn chronized to each peripheral device A hierarchical structure of the parallel activities is imposed afterward in order to prevent Deadlock 2 Coroutines and Semaphores 2 1 Basic Coroutine Scheme Each coroutine of Boss will perform an activity with a speed determined by a peripheral device or a job process Typically the following activities will be in progress simultaneously a Printing data from the disk to the line printer as fast as allowed by the printer performed by the corou tine ps printer b Reading data from card reader to the disk as fast as allowed by the reader and the operator coroutine ps card c Communicating with a user terminal about editing file listing etc coroutine commandio 1 d Communicating with a second user terminal co r
17. e statisti cal analysis on them Nearly all errors are simple pro gramming errors that occur also in uni programming Communications July 1975 of Volume 18 the ACM Number 7 loading a wrong register testing a wrong condition The most difficult bugs to find were associated with pag ing paging bugs see Section 5 3 and the correct re turn of borrowed resources see below A few bugs were typical multiprogramming bugs forgetting to reserve a resource which then happened to be used by another coroutine at the same time One case of Deadlock occurred see Section 3 2 but it was simple to correct The only important design faults were those associ ated with booking of resources on the backing store and parallel access to files I am afraid we still have only a superficial knowledge of these problems see 16 The successful detection and correction of errors after release of the system could not have been achieved without the permanent test output Section 5 2 If the system broke down the computer staff would send the test output file to the project group which then was able to find the error in most cases As normally only a short backing store file 80 000 characters is used for cyclical collection of test output some errors were still difficult to find because they occurred a long time before the symptoms For instance if a resource is borrowed and later only a part of it is returned it will take a long time before som
18. etc Some coroutines also produce private test output records showing intermedi ate results Obviously listing of source programs were the ex ception rather than the rule With a careful marking of all corrections in the latest listing we had no troubles finding our way in the latest version An important point was the use of an extremely careful programmer TIsa bella Carstensen for the clerical work of keeping track of paper tape corrections keeping track of on line rush corrections punching correction tapes according to pencil marks in the listings arranging safety copies etc This work was drastically underestimated in the planning stage Finally the system of project file directories and private file directories planned for the extended moni tor was utilized in the debug sessions to assure that one test file could be tested together with reasonably good versions of the other files 14 Section 4 6 3 User s Man ual The decision that a private file was reasonably good and could be made a project file had to be taken between debug sessions after careful inspection of the test out put 5 3 Choice of Programming Language Paging The author had previously been involved in the development of Boss 1 in Algol 5 for RC 4000 Although Algol 5 was sufficient for the purpose the necessary use of reentrant coroutines was very cumbersome to express in Algol Worse however was that the object code turned out to be 4 to 5 times
19. ething wrong becomes apparent The cause is then not available in the test output In the cases where we have had test output on magnetic tape one large reel every three hours we have always been able to locate the error possibly by means of an ad hoc pro gram for analyzing the tapes The prototype was very carelessly tested but after it had been put in operation nobody had time to make careful systematic test programs I believe that if the prototype had been delayed some months we could have found most errors via systematic test programs The question of proving an operating system is some times discussed We have proved some statements about the system for instance the absence of Deadlock How ever in order to prove the entire system we would have to formalize every statement in the manuals 120 pages and prove them Still I doubt whether a statement like absence of Deadlock would have emerged from the manuals It seems to me that the only systems which can realistically be proved are those where the entire manual is below say 10 pages Acknowledgments The design of Boss 2 is due to the exciting collaboration of Per Mondrup Klavs Land berg and the author The implementation is due to the hard work of the project group consisting of Isabella Carstensen Bo Jacoby Bo Tveden J rgensen Bj rn Mrding Thomsen and the three designers The author had the overall project responsibility until June 1972 when Klavs Landbe
20. eue semaphores The queue semaphores have no fixed relation to the processes and in principle any process may wait for or signal send into the queue When A or C has received a record they will later return it to a separate queue of free records not shown R can then get a new record from this queue A OR C B WAIT SEMAPHORE RECORD SIGQ SEMAPHORE RECORD LIS com SEMAPHORE message from the job process or a request from the operator s or user s terminal Steps 2 and 4 depend on the actual request and they may involve requests to a variety of coroutines and drivers In general a coroutine waits for a request in Step and for answers to requests in Steps 2 and 4 This is accomplished by calling the Central Logic which re turns to the coroutine in case the request or answer is ready If it is not ready the Central Logic returns to another coroutine which is ready to run or it calls the monitor function wait event Section 4 2 elaborates on this topic and on the use of reentrant code to imple ment identical activities like e and f The size of the coroutine algorithms varies con siderably rewinder is just 30 instructions ps job and commandio are several thousand Thus co routines are not a partition of the code into manageable pieces Rather they reflect the external requirements to parallel action Notice that if we want the basic scheme above and want to run all peripherals and a
21. f coroutines would then correspond to a level of the hierarchy But this is not the way things developed A hierarchical structuring is usually invented during the construction process This we have done inside each coroutine by arranging the program in nested parts and procedures in the usual manner Contrary to this the structuring into coroutines is mainly determined by the external requirements The general rule seems to be this Make one coroutine or process for each independent stream of external events In our case each job process and each peripheral device supplies an independent stream of events Devices with a two way initiative supply two independent streams of events and are consequently handled by two coroutines The only exception in Boss is the Banker as discussed in Section 3 2 The structuring into columns comes later and serves to guide the proof of Deadlock absence The author has used this structuring principle with equal success in later projects like message switching and remote process control One additional rule has turned Communications July 1975 of Volume 18 the ACM Number 7 up Assume that the processing of a critical event stream requires much cpu or disk time Then two coroutines with an intermediate multi buffer should be used in order to overlap event receiving and processing one coroutine which receives the external events and one which spends the cpu or disk time The latter coroutine may as well
22. g nificantly affected The rule of thumb saying 200 instructions per man month may now be checked for the actual schedule 26 000 instructions 115 man months 230 What might have helped us in the planning was a similar rule of thumb for the programming phase alone 387 6 Evaluation 6 1 Performance The facilities as originally specified were imple mented sometimes in an extended version We dropped one extension the vdu terminals because the hard ware was not developed Another extension printer back up on magnetic tape has not been put in opera tion partially because there seems to be no serious need for it The backing store and core store requirements are in good agreement with the specification However the system overhead was somewhat underestimated When Boss runs in the specified core area 15 000 characters the overhead is many times more than estimated because of thrashing When Boss uses 30 000 characters in the core store the overhead is 1 5 times the estimate actu ally 3 seconds overhead to execute a job The larger core demand is related to the code being 3 times longer than estimated The factor 1 5 could easily have been anticipated if a more detailed analysis was carried out in the specification phase 6 2 Reliability Bugs Proof When we started the Boss 2 design we knew that the RC 4000 software was extremely reliable In a uni versity environment the system typically ran under the simp
23. gged and around July 1971 about half of the system was in the test phase A general reluctance to face the facts caused me as a project manager to discard all planning and just let the group implement as fast as possible Around August 1971 the group was augmented with Bo Jacoby and Bo Tveden J rgensen and the responsibilities of the group Communications July 1975 of Volume 18 the ACM Number 7 were extended to also cover some related software proj ects excluded in the project specification In January 1972 everything was in the test phase but some parts were in a preliminary version This point most likely corresponded to the check point All Parts Punched but the delay was 8 months In January 1972 we finally detected the real reason for the delay We had programmed 20 000 instructions instead of the estimated 7 000 When the project entered the maintenance stage in August 1972 we had pro grammed 26 000 instructions including a few further extensions The prototype was put in operation in April 1972 Further extensions for conversational jobs time shar ing and process control were implemented until August 1972 when the first public release took place At that time 115 man months had been used instead of the latest estimate of 55 man months 5 5 Conclusion The main fault in the estimates was that the number of instructions were underestimated by a factor of 3 excluding contributions from later extensions A p
24. gies the user catalog the account and statistics files One section estimated the storage demands and the total system overhead A ten page section described 21 major extensions of the basic project to be decided before implementation started They included things like conversational jobs the basic version proposed only remote job entry com bined with fast on line editing vdu terminals card reader remote batch terminals and simple facilities for process control experiments These parts of the report were published The un published part five pages defined the extent of the project the preconditions the time schedule the use of man months and computer time the cost for extensions of the basic project and the necessary implementation group The extent of the project fixed the relations to other parts of the software accounting programs writing of manuals transitions to maintenance and of course the facilities as described above Some facilities which were not considered but which might be important were pointed out The preconditions stated that certain other software parts should be finished at certain times that 76 hours of computer time should be available for debugging that the extensions had to be decided and the specifica tion accepted within a month A total of four persons the three designers and Bj rn rding Thomsen were assumed as the implemen tation group with a time schedule built on check points
25. he monitor but in an unsuccessful project Boss 1 we had learned that processes and messages were completely unsuited for the purpose The major problem is that each process has its own message queue For instance it is not possible to let a message queue represent a pool of free records com mon to several processes because only one proce 3 can get messages from the queue For the same reason Dijkstra s semaphore construction for critical regions cannot be made directly with messages It would be possible to solve these problems by introducing an administrator process but its logic would be compli cated Other problems are that RC 4000 processes are very difficult to make reentrant and they cannot readily share code or data tables especially when swapping or paging is used Finally only 23 processes plus drivers can be created in RC 4000 and we needed more than a hundred parallel activities inside Boss So in all cases we would have to simulate more parallel activities inside one process The solution we chose was another level of multi programming running inside the single Boss process The monitor of this multiprogramming system is called the Boss 2 Central Logic the parallel activities are called coroutines and the communication and synchronization is done by means of simple semaphores and queue semaphores as explained in the sequel The coroutines run in a virtual memory simulated by the Central Logic and with th
26. her to a semaphore when the coroutine waits for it to the pager queue when the coroutine waits for page transfers to the active queue when it waits for cpu time or to the answer queue when it waits for a driver answer One word is used for identifying the driver answer waited for or for working during operations on queue semaphores Five words contain virtual addresses which represent pages to be held in the core store while the coroutine uses the cpu A bit in each of these page description words shows whether the page is modified by the corou tine so that it should be written back The first page description always represents the code page in which the current coroutine algorithm is found The last word represents the return address to the coroutine The ad dress is relative to the beginning of the code page When a coroutine runs the first five words of its cur rent code page contain the absolute core addresses of the five pages of its coroutine description In this way the 384 code can easily access data in the five pages Note that the page allocation can only change when the coroutine explicitly calls the Central Logic The coroutine gets ac cess to other pages by changing the coroutine descrip tion and then calling the Central Logic In this case other coroutines may run during the page transfers Reentrant coroutines are handled by allocating all the variables in pages other than the code page Thus two reentrant c
27. ivers which by definition complete their operation in a finite time at least if they are handled properly by the operator Now it follows that coroutines in column 2 from the left terminate their Steps 2 and 4 in a finite time and hence they produce the answer Step 3 in a finite time If the request queue is implemented as First In First Out or any other kind of fair scheduling we have then proved that all co routines in column 2 answer a request in a finite time We can now proceed to column 3 and so on until absence of Deadlock has been proved for every co routine Note that data may flow in the same direction as the request e g for a printer or in the opposite direc tion e g requests for paper tape input In the next section we will tackle the Deadlock prob lems originating from locking semaphores and other deviations from the basic coroutine scheme The requirement that all requests go from right to left is a very strong design criterion In some cases it has caused a coroutine synchronized to a single periph eral device to be split up in two This has happened with terminals which have a two way initiative The job may request input or output from the terminal and this is handled by the coroutine termout The user may also request actions from the system while his job is running For instance he may want a print out of the job state or the job queue or he may ask the system to kill the job This is handled
28. ives messages from the job proc ess all parent messages and some input output mes sages Some messages are checked and then passed on to other coroutines e g print a backing store file write special operator request In the same queue the ps job receives certain buffers from commandio kill job answer to special operator request from remoters tape ready and from timer time exceeded Messages which will cause a longer waiting time cause the ps job to ask the Banker to swap out the job meanwhile When the job is finished the ps job cleans temporary files asks for rewind of tapes produces account records etc Job The job process In a given moment only some of the ps jobs have associated job processes Unknown Sender Receives messages from unlogged terminals and processes other than Boss jobs May either pass the login request to a free commandio or enroll an internal job via Banker and a free ps job or reject the message Rewinder One rewinder to each magnetic tape sta tion Rewinds the tape when it is released Unloads the tape when the station is to be used for something else A short queue of such requests may exist because the unload cannot be ordered until the tape is rewound Remoter One remoter to each magnetic tape station Reads the tape label when the operator sets the station in the remote state Tells request display if the label is unreadable or if the tape is not needed now Obeys op erato
29. le operating system for three months without crashes Although some errors existed in compilers and utility programs they affected at most one job The crashes present were possibly due to transient hardware errors Errors in the peripherals were more frequent but did not wreck the rest of the system From the beginning we aimed at a similar stability for Boss six months after release So we did not imple ment any restart facilities except that the permanent files on disk were preserved by a simple strategy We be lieve that this decision is a major reason for the relatively simple and stable operating system In fact Boss 2 passed a 3 week delivery test in Sep tember 1972 one month after the first release In these 3 weeks the contract allowed at most four crashes be cause of hardware or software Actually four crashes occurred three of them caused by bugs in Boss In the period the system was heavily loaded with unrestricted user jobs program debugging process control jobs and conversational jobs From September 1972 to March 1973 dozens of errors were reported as the users explored the corners of the system In April 1973 all reported errors except one were located and corrected Today the system seems to be error free The corrections nearly always converged in the sense that one correction did not cause errors somewhere else We have collected all corrections during the devel opment and maintenance but we have not don
30. ll jobs in parallel we need at least one coroutine for each periph eral and each job 2 2 Queue Semaphores The communication and synchronization between coroutines is done by means of queue semaphores and simple semaphores A queue semaphore is an abstraction which represents a queue of records or a set of co routines waiting for records to be put in the queue Figure 4 Two procedures of the Central Logic handle the queue semaphores waitg semaphore record The procedure wait queue removes the first record from the queue represented by the semaphore A pointer to the record is returned to the calling co routine If the queue is empty the calling coroutine is suspended waiting until a record is available in the queue sigq semaphore record The procedure signal queue inserts a record specified by a pointer into the queue of the sema Communications July 1975 of Volume 18 the ACM Number 7 phore If coroutines are waiting for records in the queue one of them will be activated and it will run later The main difference between queue semaphores and messages is that a queue semaphore does not be long to a single process or coroutine In principle any coroutine may wait for or signal any queue sema phore Another difference is that the records may have any length while the message buffers are restricted to eight words When coroutines communicate two semaphores are involved One semaphore holds
31. longer than equivalent handwritten assembly language code With the same overhead in page transfers this would need 4 to 5 times more core store It should be noted that the Algol 5 compiler gener ates fairly good code For instance it is claimed that Algol W on IBM 360 generates very efficient code but in vestigations have shown 1 that it is just as long as Algol 5 code As the long object code was prohibitive for the goals of Boss 2 assembly language was chosen 386 From the beginning it was assumed that the code and variables of Boss were to be in a virtual store controlled by a software paging system RC 4000 has no hardware for paging so software page references must be inserted where needed We had experience with this kind of soft ware paging from several Algol compilers The main problem was the hand written standard procedures which had to fit into a few of the fixed size pages Whenever changes were needed it caused great troubles to fit the new version into the fixed size pages Another problem was a lot of difficult paging bugs caused by code which had brought a page to core and later referenced it with an absolute address when it was possibly not in core any more We solved the first problem by designing the paging system for variable length pages We partially solved the second problem paging bugs by allowing a coroutine to specify that an entire set of pages had to reside in core simultaneously when it was running
32. m of 50 terminals various types of backing stores and magnetic tapes printers readers punch plotter and various process control devices The resources are available for all types of service Boss has a dynamic priority system based on swapping and updates an estimate of the job completion times taking into account all resources demanded by the jobs This estimate is available from the terminals All the facilities can be used with a core store from 32 k words of 24 bits and a disk of 2 M words Performance measurements have been made on a service center configuration with 20 terminals 64 k words of core store and a small added drum The jobs are production runs and debugging of medium and large data processing programs During the six busiest hours the cpu utilization is 40 50 pct used by jobs 10 20 pct by the operating system and the monitor The average operating system overhead per job is 3 sec including nonoverlapping 1 o transfers in the operating system The response time to simple on line commands like editing is negligible less than 0 5 sec During the first year of operation the system typically ran for weeks without crashes Today it seems to be error free 1 2 The RC 4000 System Without Boss 2 Boss 2 runs under an extended version of the Monitor Nucleus described in 2 3 and 16 The principles of the Monitor may be outlined as follows The Monitor is a set of procedures which make the computer appear as if it
33. nication principle corresponds to the beautiful symmetrical producer consumer algorithm of Dijkstra the ps job consumes free records and produces requests The ps printer consumes requests and produces free records The principle can also be thought of as a generalization of the conventional double buffer scheme to multi buffers A record cor responds to a buffer In Figure 3 4 double arrows show such multi buffer communication based on queue semaphores The arrow shows the direction of the re quest 381 2 3 Simple Semaphores Simple semaphores were introduced by Dijkstra 5 6 They resemble queue semaphores but the queue is not represented explicitly Only a count of the records in an abstract queue is kept track of Thus simple semaphores can be used to implement queues where the records are linked in a user defined manner or where the record does not contain essential information Simple semaphores are handled by the following two procedures of the Central Logic wait semaphore sig semaphore It is assumed that coroutines follow the discipline of handling a semaphore either with waitq sigg exclusively or with wait sig As an example consider the handling of output from a job to the terminal Each request consists of one word to be printed 3 characters and an ordinary queue would be too cumbersome Instead the ps job and the termout agree to use a backing store area in a cyclical manner Two simple sema
34. ocesses was always ordered manually and executed immediately or rejected i e no job queue existed in the computer A job process could send two kinds of messages input output messages to the drivers and parent messages to the operating system The input output messages asked for operations very close to the hardware like transfer of a data block or position of a magnetic tape Input output strategies and error recovery were mostly handled by the user programs and the library procedures The parent messages asked for a variety of functions like mount a magnetic tape mount a paper tape terminate the job abort the job step in at most x seconds print an operator message The simple operating system simply printed all parent messages on a console without trying to under stand them Although it was quite convenient to handle tape mounting and paper tapes in this way the method was unsuited for handling files on disk open file create file etc A major fault in the early design was that file handling was not communicated as parent messages with automatic handling in the simple operating system Instead a special set of monitor procedures was supplied which did not even use the message mecha nism As a result later operating systems could not catch the file handling requests and disk allocation strategies became limited by the monitor 1 3 The Place of Boss 2 in RC 4000 These were the conditions upon which we started the
35. oroutines may run with the same code page but different variable pages 4 3 Semaphores Each semaphore is represented by 3 core resident words One word is a count of the number of records or when negative the number of coroutines waiting on the semaphore Two words point to the beginning and the end of the record queue or the queue of coroutine descriptions waiting on the semaphore This semaphore representation is rather clumsy and could be replaced by a one word representation as shown in 15 The records of a queue are pages in the virtual store linked together through their first word 4 4 Paging Method As outlined above demand paging is used Of par ticular interest is the fact that several pages may be needed simultaneously and any page size really section size may be used The core resident pager coroutine transfers pages upon request from other coroutines It will complete all page transfers for one coroutine before it handles the next coroutine in its request queue It awaits the disk and drum transfers as all other coroutines by means of an entry to the Central Logic thereby allowing the exe cution of other coroutines with all their current pages in the core store The pager coroutine and the Central Logic maintain a priority for each page in the core store The priority represents a least recently used strategy but with regard to pages presently used as 1 0 buffers or job processes The latter type of pages
36. os sible cause for this pointed out by Peter Naur is that we initially compared the project to an Algol compiler which used a very advanced table technique to bring down the program length 17 A similar technique was not used in Boss possibly because we concentrated too much on the structuring into coroutines and forgot to design the sequential algorithms inside each coroutine in the same careful manner Another cause may be that each facility and strategy were conceived and imple mented individually whereas an Algol compiler bene fits from the extremely homogeneous structure of Algol 60 A secondary reason for the underestimate was the interface to the Monitor The Monitor was extended in the same period with several facilities needed by Boss Especially the interface concerning drum and disks was much more complicated to utilize than anticipated This problem was felt severely during the development and caused major revisions of Boss and the Monitor A minor contribution to the underestimate is the late inclusion of further extensions and related projects user catalog updating accounting process control etc A main fault in the project management the author primarily was the sticking to a time schedule instead of revising the more fundamental estimate of the number of instructions It is typical that the main influence of the underestimate is in the programming phase The debug phase and the installation phase were not so si
37. outine commandio 2 e Performing a user job i e handle the messages sent from the job process to Boss coroutine ps job 3 f Performing a second user job coroutine ps job 4 The coroutines may be thought of as parallel processes the only essential difference being the rigid scheduling of cpu time The coroutines use only a little cpu time and disk time and as a result they need not delay each other Of course an activity like a above may run out of data to be printed and then it will have to await the arrival of new data from activities like e and f In general the algorithm executed by a coroutine follows this basic scheme Step 1 Wait for a request to do some work Step 2 Send a finite number of requests to other co routines or drivers Step 3 Answer the request of Step 1 Step 4 Send a finite number of requests to other co routines or drivers Step 5 Go to Step 1 For a ps printer activity a Step 1 waits for a request to print some data The requests are sent from other coroutines by means of semaphores and in busy periods several requests may be queued up Step 2 sends requests to the printer driver in the form of messages and awaits the answers When there are trou bles with the printer Step 2 may also send requests to the operator Step 4 is blind For a ps job activity e and f Step 1 waits for a 380 Fig 4 Process or coroutine communication by means of qu
38. phores represent the number of full and free words and the algorithms look exactly like those originally proposed by Dijkstra Multi buffers implemented in this way appear as dotted arrows in Figure 3 As another example we will elaborate on the parallel printers of Section 2 2 In practice we want to sort the files according to paper type one copy two copies special forms etc in order to minimize paper chang ing So we use one queue for each paper type and a common abstract queue representing the total number of print requests The abstract queue is implemented by a simple semaphore common print Now the ps job sends a print request in this way Step 2 waitg print free record store print request in record sigg paper type queue record sig common print The ps printer proceeds like this Step 1 wait common print if queue of current paper type is empty then begin select a non empty paper type queue at least one exists according to some strategy current paper type queue selected end waitq current paper type record Step 2 Note that selection of current paper type is a critical region which should be executed by at most one ps printer at a time Otherwise two ps printers could decide to wait for the same paper type queue which happened to contain only one request Communications July 1975 of Volume 18 the ACM Number 7 2 4 Critical Regions A critical region is a part of an algorithm which
39. r commands telling the name of the tape or asking for a label to be written When the tape has been identified in one way or an other a possible ps job waiting for the tape is activated Watch dog Detects whether a station has been set remote and activates the corresponding remoter This coroutine is only needed because the Monitor has a special driver process responding to any station set in remote Ps reader Loads paper tapes either via a multi 389 buffer as job controlled input or via a double buffer to a backing store file Tells the Banker when the reader becomes empty and available for other jobs Ps card Loads card files just like the ps reader Tells the Banker when a job separation card is met this sig nals the presence of a new job in the reader Kit Changers One kit changer to each disk drive with exchangeable kits Clears up on the old disk pack when the operator requests a kit change waits until the new kit is ready reads the kit label and adjusts the file catalog Timer Supervises the run time of the jobs in the core store and tells the ps jobs when time has expired Banker The Banker receives records and handles them in one out of three rather different parts of the code with different status in the hierarchy of coroutines Banker 1 receives requests for swap in and swap out of jobs and performs the swapping on a priority basis Banker 2 receives requests for reservation or relase of temporary resour
40. rg took over the job I would like to thank Peter Lindblad Andersen and Hans Rischel for their patient development and modi fications of the Monitor and the utility programs An independent project run by Ole Caprani Lise 388 Lauesen and Flemming Sejergird Olsen extended the system to serve also as a remote batch terminal for CDC 6400 and Univac 1106 Finally we are very indebted to Christian Gram our manager for his patience and encouragement especially during the exhausting winter 1971 72 Received October 1973 revised October 1974 References 1 Andersen J Mgller T Ravn A P and Stamp S Rapport over Effektivt Kgrende Algol System in Danish Projekt 71 9 7 Datalogisk Institut U of Copenhagen 1972 2 Brinch Hansen P The nucleus of a multiprogramming sys tem Comm ACM 13 4 Apr 1970 238 250 3 Brinch Hansen P RC 4000 Software Multiprogramming System RCSL No 55 D140 Regnecentralen Copenhagen 1971 4 Denning P J Third generation computer systems Computing Surveys 3 4 Dec 1971 175 216 5 Dijkstra E W The structure of the THE multiprogram ming system Comm ACM 11 5 May 1968 341 346 6 Dijkstra E W Cooperating sequential processes In Program ming Languages F Genuys Ed Academic Press New York 1968 pp 43 112 7 Dijkstra E W Hierarchical ordering of sequential processes In Operating Systems Techniques C A R Hoare and R M Perroth Eds Academic Press
41. rrors To be careful because of the multiprogramming we assumed that only half an error could be found and corrected for each test run So the 7 000 instructions should give 350 errors and 700 test runs The modular design of the system was estimated to be sufficiently good so that the four pro grammers could work nearly independently of each other reducing the debug demand to 200 sessions at the computer We planned the debug sessions to 15 minutes each i e a total of 50 hours and asked for 76 The main debug period should be February March April 80 days so two periods of 15 minutes a day should be sufficient In order to utilize periods of 15 minutes we planned the debug sessions carefully as explained in the next section 5 2 Planning a Debug Session A new and extended version of the Monitor was to be used together with Boss so that a total exchange of the software had to take place during each debug ses sion We devised a method for a fast exchange of the system based on exchange of disk packs and swap of drum contents to the disk A special autoload paper tape was used to switch at the beginning of each session and another tape was used to switch back to the end of the session This took a total of 5 minutes When we had used this system for a few weeks we happened to load the start tape at the end of the session and the entire standard system disappeared and had to be loaded from magnetic tape The computer staff
42. serve several coroutines of the first kind 4 Implementation Principles 4 1 Virtual Store The total space occupied by coroutine algorithms records and other variables is much too large for the core store Instead we implemented a virtual store based on software paging Each word in the virtual store is identified by a 22 bit virtual address The lower range of addresses corre sponds to resident parts of the virtual store with the virtual address being the hardware core address The middle range of addresses corresponds to virtual store parts on drum which are transferred to the core store upon demand The high range of addresses is similar but corresponds to virtual store parts on a disk The virtual store is divided in sections where each section is a full number of physical blocks on the device block size equals 256 words of 24 bits Whenever a word of the virtual store is needed and it is not in the core store the entire section containing the word is transferred In practice we consider the virtual store as divided in pages of consecutive words A page is always allocated inside a section to allow fast addressing of the entire page after an initial page access Several small pages can be allocated in a one block section whereas large pages occupy a section each 4 2 Coroutines Each coroutine is represented by an 8 word corou tine description which is resident in the core store One of the words is used as a link eit
43. t system we can only prove that a job request ing a tape will be able to progress sooner or later 3 2 Absence of Deadlock Special Cases The basic coroutine scheme does not explicitly men tion the use of locking semaphores to control critical regions So we will have to prove separately that no coroutine waits forever to enter a critical region When a coroutine has two or more nested critical regions all other coroutines must use the same sequence of nesting Then the entering into an inner critical region cannot be delayed endlessly by another coroutine being in the same critical region If a coroutine awaits an answer inside a critical re gion the producer of the answer may not require en trance to the same critical region This rule propagates recursively as the producer of the answer may not even await an answer from another coroutine using the same critical region Thus the locking semaphores create two rules to be followed and proved in each coroutine Next we will discuss a more serious deviation from the basic coroutine scheme In some cases a coroutine does not produce the answer in Step 3 until it has gotten other requests in Step 1 The proof methods used here vary from case to case As an example consider the coroutine request dis play which prints operator requests and maintains a list of operator requests which have not yet been com pleted Operator requests are messages like mount magnetic tape
44. the requests the other the answers As an example consider the communication be tween a ps job and a ps printer The requests are queued by the semaphore print queue and a request record specifies that a certain file should be printed The free records are queued by the semaphore print free Now the ps job uses the basic scheme in this way Step 1 Wait for a request from job operator or user Step 2 if request is print a file then begin waitq print free record store print request in record sigq print queue record end Step 3 The ps printer uses the basic scheme in this way Step 1 waitg print queue record Step 2 print the file using send message and wait answer Step 3 sigg print free record Step 5 goto step1 Note that these algorithms cause waiting in the proper way When no free records are available the ps job will wait on print free in Step 2 When no requests are in the queue the ps printer will wait on print queue in Step 1 If several printers exist each of them is served by its own ps printer Any printer may print a file This is obtained automatically if print queue is common to all ps printers As long as print queue is nonempty all printers will be busy This simple implementation of parallel request processing is difficult to obtain with most other communication methods For instance messages are completely unsuited for that purpose The commu
45. tor are kept until the action is completed Section 3 2 Commandio One commandio to each on line termi nal and to the operator s console Performs all conver sation with the on line user and the operator including editing and listing of files Some commands are passed on to other coroutines commands like run job kill job start printer Termout One termout to each on line terminal Communications July 1975 of Volume 18 the ACM Number 7 Prints job output on the terminal from a multi buffer on disk For nonconversational jobs the progress of the job is not delayed by the slow terminal Ps printer One ps printer to each physical printer Prints completed files from the backing stores or job controlled output from a multi buffer Attempts to mini mize change of paper type see Section 2 3 Communi cates with request display about change of paper etc Tape Printer May copy print files from the backing store to a magnetic tape When the backing store is about to run full of print files the tape printer asks the opera tor for a tape and copies the files Ps job One ps job to each on line terminal plus spares for execution of batch jobs and internally gen erated jobs A ps job interprets the job specification the first job control command loads possible data files from card or paper tape reserves most of the tem porary resources for the job and creates and starts the job process Now the ps job rece
46. wer to be returned through his private semaphore which may be simple The private semaphore is specified in the request The sender uses the basic scheme in this way Step 2 or 4 sigq request queue private record wait private answer semaphore Because all the algorithms are loops you may think of the scheme in a rotated form with wait preceding sigq Then it looks again like the producer consumer al gorithm The receiver uses the basic scheme in this way Step 1 waitq request queue record Step 3 sig record private answer This private buffer communication corresponds exactly to the message answer communication with the drivers In Figure 3 they are all represented by normal arrows 3 Deadlock and Hierarchical Structuring 3 1 Absence of Deadlock Basic Proof We can prove the absence of Deadlock in Boss by proving that no coroutine waits forever when it has 382 useful work to do In the basic scheme of Section 2 1 we can distinguish two kinds of waiting Waiting in Step 1 for a request is idle waiting If the coroutine waits forever here it does no harm as it then has no work to do Waiting in Step 2 or 4 is answer waiting If the coroutine waits forever here we have a Deadlock as it will not be able to process the requests In Figure 3 all requests go from right to left and we can then prove by induction that all requests are processed in a finite time All coroutines to the extreme left are dr
47. will have a very high priority until the answer arrives or the job process is swapped out by the Banker The pager tries with a simple strategy to allocate all demanded pages in low priority parts of the core store 5 Project Plan and Time Schedule 5 1 First Project Plan The design of Boss 2 was started at Regnecentralen Copenhagen in August 1970 by Klavs Landberg Per Mondrup and the author In October 1970 we com pleted the project specification as an internal report That part of the report which specified the facilities of the first version and the possible later extensions was published in March 1971 13 In this section I will give a summary of the project specification and explain the Communications July 1975 of Volume 18 the ACM Number 7 key estimates In Section 5 4 I will compare it to the ac tual progress of the project The project specification was 25 closely written pages The facilities were described as follows For each type of peripheral device there was described the strat egy to be used by Boss for handling it and the actions to be taken by the operator The devices considered were typewriter like terminals paper tape reader printers several types of magnetic tape stations several kinds of drums and disks The on line commands the implementation and conventions for the on line editor were outlined Also described were the resource allocation on backing stores the job scheduling and swapping strate
48. ynchronous structuring buffering parallel processes synchronizing primitives reentrant code RC 4000 project management time schedule debugging project planning project scheduling relia bility program proving coroutines correctness program maintenance software paging CR Categories 4 30 4 31 4 32 4 42 4 43 5 24 a eee Copyright 1975 Association for Computing Machinery Inc General permission to republish but not for profit all or part of this material is granted provided that ACM s copyright notice is given and that reference is made to the publication to its date of issue and to the fact that reprinting privileges were granted by permission of the Association for Computing Machinery Author s address until October 1975 UNDP P O Box 1423 Accra Ghana Permanent address Nordisk Brown Boveri Vester Farimagsgade 7 DK 1606 Copenhagen V Denmark Communications July 1975 of Volume 18 the ACM Number 7 1 Introduction 1 1 Facilities of Boss 2 The operating system Boss 2 was developed for RC 4000 in the period 1970 to 1972 Boss 2 is a general purpose operating system offering the following types of service simultaneously batch jobs remote job entry time sharing conversational jobs jobs generated in ternally by other jobs process control jobs The system can at the same time be part of a computer network allowing jobs to transmit files to cpc 6400 and Univac 1106 1108 Boss 2 handles a maximu
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