Management of a Time Sharing System (1960s)

a college student in Mexico City uses a Lis program to develop techniques of language translation while a programmer in Santa Monica California debugs a graphic output formatting program and in the Pentagon a user retrieves information from a database by means of an online query system these users widely separated by geography and purpose have one thing in common they and more than $2 doz others are independently using the same computer at the same time each is linked by phone lines to a general purpose computer located at the system Development Corporation in Santa Monica this experimental general purpose time sharing system was developed by SDC under contract to the advanced research projects agency there are 450 independent authorized users but only 30 can have access to the system at any one time moreover as with all computer systems storage space is limited to the users this means problems in first gaining access to the system and then keeping their drum or disc space to to the system designers assuring enough storage space for all potential users is not merely a matter of building still larger machines an increase of system capacity would inevitably be surpassed by an increase in demand the basic problem is one of system management deciding on the best balance between system resources and user needs the man who must live with the day-to-day job of assigning or removing from the system any of its many users is the system operator a set of management tools has been developed to Aid him in this task these tools together with the operator's human judgment are a new and important part of the management of a time sharing system the operational nucleus of the time ing system is the research and Technology laboratory and its FS Q32 computer the Q32 data storage operating on a 48-bit word includes a 65,000 word core a 4 million word dis file 700,000 words on drums 16 tape drives complete the basic configuration a major peripheral component the pdp1 computer control contr realtime inputs and outputs the online input output units available to users within the lab are display consoles and telet typewriters remote users are linked by teletypewriter exchange and data phone managing this system is a complex task it requires scheduling time for each user implementing priorities deciding which users must be terminated to provide drum or dis space for other scheduled or higher priority users and alloc ating storage to the oncoming users to assist the operator in implementing these decisions he is provided with the capability to communicate with system users a guaranteed access schedule and a computer program the manager program which generates displays to help him determine up Toth minute system status the blanking effect seen here is due to the photography and is not normally apparent the operator's teletypewriter permits communication with all users through an ex executive system command known as dial for example the user can ask the operator for help on procedural problems or the operator can inform a user of termination time the operator can also communicate with all users at the same time giving them timely messages such as last minute schedule changes the system is heavily used and overloads are frequent to improve service a guaranteed access schedule was developed this schedule is based on requirements of various high priority online projects for drum space disc storage and computer time the schedule guarantees the user a channel and normal storage requirements for a certain period of time a project on channel 2 for example is guaranteed drum space of 47,000 words from 7:30 to 8:30 hours to promote maximum use of the system unscheduled users are allowed access when space is available this access is not guaranteed and the users generally have small programs to run the larger their program the greater the chance that their space will be usurped by a scheduled or high priority user in order to determine which non-scheduled or low priority users are to be terminated to provide the needed space the operator can call up a number of displays generated by the manager program the manager program processes system data in real time and displays it on the operator's console two types of displays can be generated a channel oriented display which is normally present and periodically updated and a set of dis storage displays called up by light pan action from the channel display in the channel display the listing for each currently active Channel gives the program name and size the assigned drum the user's job number preceded by a priority number the requested tape number and the operational status of the program the upper right portion of the display lists the tape drives for each channel the channel display also shows availability of dis tracks tapes drum space the time the number of users logged in at teletype stations and the number of users who have logged in and loaded programs an operator wanting a more detailed display for a channel can type the channel number to the manager program the program then places in the lower right corner of the display certain system parameter values which indicate the user's operational status his elapsed time since start his total compute time and the time since he last took action across the bottom of the display are two rows of alpha numeric characters by taking light pen action on any character the operator can call up a list of all files beginning with the letter or number selected from the beginning of the system available disk space steadily decreased originally programs or data stored on the disk and not used during a consecutive 2-day period were purged in time this period was reduced to 1 day and on the spot deletions by the operator Ator became necessary if an operator notices on the channel display that the number of available disc tracks is critically low or if a user asks for more space the operator can call up by light pen one of three disk storage displays to determine which dis files to delete or to unload onto tape backup the first disk display contains the disk inventory name along with the user's identification number the number of tracks he is using file type such as binary the file status either active or inactive the date last read or written and a numerical code indicating whether the system has automatically transferred the file to tape backup from this display the operator can delete files from the disk inventory or unload them onto backup tape by triggering the dot preceding the file name the file name is then displayed to make sure an Aiming error was not made with the light pen initially when final action is taken an updated version of the display is returned to the screen the second kind of disk display is used when the operator wants to inventory the disk storage in terms of file size it is similar to the first dis storage display except that it lists the 27 largest files in order of the number of tracks assigned to each file the third dis display enables the operator to scan large segments M ments of the dis rapidly it shows the file name and number of tracks for each file for as many as 54 files these then are the basic displays generated by the manager program the channel display which gives an overall picture of user activity and the three detailed disc storage displays which allow the operator to quickly scan the disk contents to delete files or to obtain additional data about the files in the data operation of the SDC time sharing system we will see how the manager program helps the operator resolve the problems of insufficient space for programs and data this user has been guaranteed computer time but cannot begin because there is not enough drum space for his 43,000 word program since this user has been guaranteed access the operator must now determine which non-scheduled users he must terminate he checks his channnel display because the program must be loaded onto a single drum he checks which of the drums has the most word space available drum 2 has space for 18,893 th000 Words will be needed from the users on drum 2 the operator picks out those users on drum 2 who are not scheduled the Pint program on channel 40 less than 20,000 words low priority 5 on channel 46 the program also is unscheduled using about 5900 words but it is a high priority user channel 38 however is low priority and occupies more than 7,000 words the totals of channels 40 and 38 plus the space already available on the drum are enough for the eligible user to load his program the users are notified and are given time to wrap up their work in some cases when extra time is needed the operator adds human judgment in implementing the scheduling process actually there is now a surplus of nearly 3,000 words available on drum 2 enabling some non scheduled user with a small program to enter the system in the second example the problem is to obtain disk space this user needs 120 dis tracks for a large database only three are unassigned but an additional 97 are available since they are on both dis and tape a total of 100 tracks can therefore be immediately assigned to the user the operator will have to reclaim an additional 20 tracks to meet the user's requirements in this display there are files that use a small number of tracks so it would be quicker and more efficient to look at a list of files with the largest number of tracks file name meta 01 has 12 tracks it is a program file it is inactive more than a day old and it is on disk only it is deleted from disk and unloaded onto backup tape the operator must now find and delete enough tracks of similar status to obtain the required total the new user is now assured of his needed disk space the manager program and its set of console displays are an approach to the solution of operational management problems in the SDC experimental time sharing system these and other techniques under development provide a basis for expanding the usefulness of time sharing and are Furnishing insights into the management of other computer-based systems