GB1345950A - Digital electric data processing system - Google Patents

Abstract

1345950 Digital computers; time sharing INTERNATIONAL STANDARD ELECTRIC CORP 16 Dec 1971 [22 Dec 1970] 58428/71 Heading G4A In a time sharing digital electric data processing system tasks are organized into queues for execution. A number of remote terminals 10 communicate with a time shared computer 11. A scheduling unit 16 (which may form part of unit 12 or 13) schedules tasks which are solely interrupt driven and those generated by a terminal user. Some of the tasks have predictable durations while others are unpredictable. Other tasks require a resource allocation and their duration depends on the availability of the resource. Scheduling is controlled by unit 16 using programmed timer interrupts. In this embodiment there are 18 queues, P1-P18, having different priorities, each queue containing descriptions of the tasks therein and each queue being arranged on a first in first out basis. The first three queues have highest priority and their tasks are executed in sequence, all the P1 tasks being executed first. The tasks in P1-P3 are urgent tasks with a short predictable duration and all of these are executed before queues P4-P18 are dealt with. Queues P4-P16 are of unpredictable duration and the level of queue for normal cases depends upon the amount of CPU time accumulated. Jobs start at P4 and, as they are given increments of CPU time, they are stepped down to lower queues. Each queue level contains a specific quantity of CPU time (a quantum) which increases as the queue level increases. The size of the quantum depends on the swapping media used, e.g. drum or disc. Scanning means determine whether a task is to continue being executed or another task is to replace it, depending on the time available. Queues P17 and P18 are for internal user queues and are serviced in a similar manner to queues P4-P16. A complete scheduling programme is also described with reference to Fig. 4 (not shown).