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Patent Title: Scheduling input/output operations in multitasking systems

Assignee: IBM
Patent Number: US5220653
Issue Date: 06-15-1993
Application Number:
File Date:10-26-1990

Abstract: In a multitasking data processing system, I/O requests to a disk drive are staged in holding queues from which they are transferred to a service queue. Requests in the latter queue are directly serviced on a FIFO basis by a device driver module running on the system. The system maintains a set of holding queues and an associated service queue separately for each physical drive in the system. Holding queues in each set are prioritized in accordance with base priorities of tasks, and I/O requests to disk drives are entered into associated holding queues having priorities corresponding to those of task threads for which such requests are originated. Prioritization of the holding queues, and a starvation advancement process performed to advance "oldest" enqueued requests to higher priority holding queues, causes the requests to be presented to the disk drive in a sequence based in part on respective task priorities and in part on "fairness" servicing of "service starved" requests. A selection operation in respect to certain transfers from the holding queuest to the service queues orders selection of transferred requests on the basis of relative proximity of disk addresses in the queued requests to current positions of the read/write head assembly in the associated disk drive.


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IBM Pledge dated 1/11/2005