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Scheduling to an Academic. Use small examples to: Understand why scheduling is hard Develop possible solutions Eventually, evaluate candidate approaches based on performance on real problems. Why Scheduling is Like Tetris. Scheduling and Tetris.
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Scheduling to an Academic • Use small examples to: • Understand why scheduling is hard • Develop possible solutions • Eventually, evaluate candidate approaches based on performance on real problems
Why Scheduling is Like Tetris
Scheduling and Tetris • The idea is to squish a lot of stuff into a small space • It helps to know what’s coming • I am terrible at both
Fact: Computer scientists tend to work on problems at which they are personally lousy. Consequence: Beware of people who say that they are working on “general intelligence.”
Summary • The idea is to squish a lot of stuff into a small space • It helps to know what’s coming • I am terrible at both • … but Tetris is much more forgiving than scheduling is
A Scheduling Problem • The capacity of the red resource is one. • The capacity of the blue resource is infinite. • The capacity of the purple resource is two.
How Scheduling Algorithms Work • Select the highest-priority unscheduled task whose predecessors have all been scheduled. • Schedule it at its current early start. • Repeat.
How Scheduling Algorithms Work • Select the highest-priority unscheduled task whose predecessors have all been scheduled. • Schedule it at its current early start. • Repeat.
Order Matters • Select the highest-priority unscheduled task whose predecessors have all been scheduled. • Schedule it at its current early start. • Repeat.
Order Matters • The purple resource has capacity two. • The bottom task should not be scheduled last.
Order Matters • The purple resource has capacity two. • The bottom task should not be scheduled last.
No Obvious Order Works longest, least float
No Obvious Order Works longest, least float
The Example in Primavera • Use a silly order • Prioritize by hand longest, least float
Least Float First least float
Most Float First? least float
The Order That Works • Prioritize by hand? least float
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Prioritization by Hand • Unlikely to work in practice • What can you do? • Being good at Tetris will • not save you
Outline • If you want a good schedule, prioritization matters • Not obvious how to prioritize automatically • Prioritization by hand is impractical • Schedule packing • Squeaky wheel optimization • How it works • Strengths, weaknesses • Access
Schedule Packing WARNING: Contents may have settled during shipping
Schedule Packing • Slide tasks as far to right as possible • Reschedule using start time as priority 3 4 1 2
Outline • If you want a good schedule, prioritization matters • Not obvious how to prioritize automatically • Prioritization by hand is impractical • Schedule packing • Squeaky wheel optimization • How it works • Strengths, weaknesses • Access
Squeaky Wheel Optimization • Too many priority orders to figure out the best one • Use search
Primavera • schedule packing • squeaky wheel P=NP? think search
Solving Problems Using Search • The key is to control the search
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Solving Problems Using Search • The key is to control the search • A machine can look at many candidate schedules, but it still needs to focus on schedules that are likely to be good • Squeaky wheel: Analyze the current schedule before generating the next one
Squeaky Wheel Optimization • C-A-P loop • Construct • Analyze • Prioritize
Squeaky Wheel Optimization • C-A-P loop • Construct a schedule from current priorities • Analyze • Prioritize
Squeaky Wheel Optimization • C-A-P loop • Construct a schedule from current priorities • Analyze to evaluate performance by task • Prioritize
Squeaky Wheel Optimization • C-A-P loop • Construct a schedule from current priorities • Analyze to evaluate performance by task • Prioritize so that poorly scheduled tasks become more important • The squeaky wheel gets the grease!
Squeaky Wheel Where no Order Worked 3 1 2 1 3 2
Squeaky Wheel Where no Order Worked 3 • Search improves your ability to find good solutions, even if you guess wrong initially 1 2
Enough Already with the Toy Problems! • Turnaround data from existing Primavera customer • 12,248 tasks, analyzed in 4.5 minutes
Schedule Packing Simple Fast (because no search) Patented by Boeing Squeaky Wheel Not so simple Potentially slower because of search Better coverage Patented by U Oregon Available as an addon for Primavera 5.0 Pros and Cons
One Last Example sand and prepare -1 mix and paint
One Last Example sand and prepare • No predecessor-based algorithms works! • Squeaky wheel need not follow “network” order • Schedule packing does -1 mix and paint
Schedule Packing Simple Fast (because no search) Patented by Boeing Add search? Squeaky Wheel Not so simple Potentially slower because of search Still Patented by U Oregon Available as an addon for Primavera 5.0 Pros and Cons Better coverage