Rockwell Automation FactoryTalk Scheduler Users Guide - Manual (Page 58)

F ACTORY T ALK S CHEDULER U SER ’ S G UIDE
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work (rather than number of operations) to be done. The basic premise is that the
remaining slack time itself is not as important in determining the most critical job as it
relates to the remaining work to be done. If we have a lot of remaining work, then we need
a larger slack time to ensure that we will not be late.
Minimum Job Flow Times
In some environments, the critical issue is to schedule the facility efficiently to minimize
the average time that a job spends in the facility. The rules in the Minimum Job Flow
Times category ignore the job due date and are focused on reducing the time that the job
spends in the system. The rules in this category include Smallest Processing Time, Fewest
Remaining Operations, and Smallest Work Remaining.
All of the rules in this category are based on the concept that by processing the “shortest”
job first, the average job flow time for all jobs is reduced. To illustrate this concept,
consider the processing of two jobs, A and B, on a single machine. Let us assume that Job
A requires 4 hours to process, and Job B requires 10 hours to process. If we process A and
then B, Job A starts immediately and completes at Time 4, yielding a flow time of four.
Job B starts at Time 4, and ends at Time 14, yielding a flow time of 14. The average flow
time in this case is 9. If we reverse the order, the flow time for B is 10, and the flow time
for A is 14, producing a higher average flow time of 12. Although this is a very simple
example, the basic concept generalizes to complex scheduling applications involving
many machines and many different jobs. The rules in this category are all built on this
same concept of processing the shortest job first and only differ in how they determine the
shortest job.
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The Smallest Processing Time rule is a static rule that selects the operation with the
shortest operation time.
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In contrast, the Smallest Work Remaining rule is a dynamic rule that selects the
operation whose parent job has the smallest remaining work. The remaining work is
defined as the sum of operation times for all remaining operations for this job.
The Smallest Processing Time rule examines the processing time for a single operation,
whereas the Smallest Work Remaining rule examines the processing times for all
remaining operations.
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The Fewest Remaining Operations rule is similar to the Smallest Remaining Work
rule, except that this rule is based on the number of remaining operations rather than
the sum of the processing times.
Maximum Facility Capacity
In some make-to-stock environments, the critical issue is to schedule the facility
efficiently to maximize the overall production capacity of the facility. The rules in the