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# Priority Scheduling Algorithm

IP.com Disclosure Number: IPCOM000034988D
Original Publication Date: 1989-May-01
Included in the Prior Art Database: 2005-Jan-28
Document File: 2 page(s) / 42K

IBM

## Related People

Christenson, DA: AUTHOR

## Abstract

This scheduling algorithm is based on priority level and arrival time. Higher priority requests are serviced before lower priority requests which have not yet aged sufficiently. When a lower priority request has aged sufficiently, it will be serviced and cannot be preempted by a higher priority request. Requests of equal priority are serviced FIFO. The operation of the scheduling algorithm, as illustrated in the figure, is described below. The following variables are defined: PRIORITY_NUMBER (PN) Specifies the priority level of a request. The smaller the number, the higher the priority level. (PN = 0,1,2,...n) ARRIVAL_NUMBER (AN) Gauges the arrival time of a request.

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Priority Scheduling Algorithm

This scheduling algorithm is based on priority level and arrival time. Higher priority requests are serviced before lower priority requests which have not yet aged sufficiently. When a lower priority request has aged sufficiently, it will be serviced and cannot be preempted by a higher priority request. Requests of equal priority are serviced FIFO. The operation of the scheduling algorithm, as illustrated in the figure, is described below. The following variables are defined: PRIORITY_NUMBER (PN) Specifies the priority level of a request.

The smaller the number, the higher

the

priority level. (PN = 0,1,2,...n)

ARRIVAL_NUMBER (AN) Gauges the arrival time of a request. AN is

incremented each time a request is

process. Its initial value is

zero. (AN = 0,1,2,...n) SERVICE_NUMBER (SN)

Specifies the actual order in which

requests

are serviced. SN is assigned to

each request and set to the sum of

PN and AN. The request with the

smallest SN is processed first.

Requests are enqueued in ascending

order by SN. (SN = 0,1,2,...n)

BUSY_INDICATOR (BI) Indicates whether the Servicing Routine is

busy (ON) or not busy (OFF). BI

ensures that

only one request is outstanding to

the

Servicing Routine at a time. Its

initial

value is OFF. When a scheduling request is received from a User Job, an SN is assigned, taking the sum of PN and AN. AN is incremented by one to continue the aging process. The request in enqueued in ascen...