vehicle routing & job shop scheduling: whats the difference? icaps03, june 13, 2003 j....
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Vehicle Routing & Vehicle Routing & Job Shop Scheduling: Job Shop Scheduling: What’s the Difference?What’s the Difference?
ICAPS’03, June 13, 2003
J. Christopher Beck, Patrick Prosser, & Evgeny SelenskyJ. Christopher Beck, Patrick Prosser, & Evgeny Selensky
Dept. of Computing ScienceDept. of Computing Science
University of Glasgow University of Glasgow
{pat,evgeny}@dcs.gla.ac.uk{pat,evgeny}@dcs.gla.ac.uk
Cork Constraint Computation CentreCork Constraint Computation Centre
University College Cork University College Cork
c.beck@4c.ucc.iec.beck@4c.ucc.ie
2
Old Solutions for New ProblemsOld Solutions for New Problems
We have strong techniques to solve hard problems
Use them! – use existing problem models and solution
techniques to solve a new problemCommon approach in research and in
practice– SAT, IP, CP, etc
If you have a hammer, …
3
A Nice Idea, ButA Nice Idea, But
New problems don’t fit exactly the old models
New problems look “strange”– Scheduling with 0 duration activities– Routing with 0 travel time
How will solution techniques work?
… is the problem really a nail?
4
Get the Picture?Get the Picture?
ExistingProblemModels
Real-WorldProblem
+ ?
?
5
This PaperThis Paper
Basic Question– How does existing solution technology cope with
changed characteristics?Basic Approach
– Create problems “between” JSP & VRP– Compare the relative performance of routing and
scheduling solution techniques– What problem characteristics are important to the
solution techniques?More in talk than in the paper
6
Vehicle Routing ProblemVehicle Routing Problem
T1
T2T3
Make a set of deliveries (visits) with a set of vehicles– Vehicles have limited
capacity– Visits have time
windows– Minimize total
distance traveled
7
R1
R0
R1
R0 R2
R2 R0
R1 R2
R1
R2
R0
R2
R1 R1
R0 R0 R0
R1
R2R2
makespan
Job Shop Scheduling Problem (JSP)Job Shop Scheduling Problem (JSP)
8
Off-the-Shelf Solution TechnologyOff-the-Shelf Solution Technology
VRP: ILOG Dispatcher– First Solution: Savings Heuristic– Improvement: Guided Local Search
JSP: ILOG Scheduler– Constructive CP tree-search
• Slack-based heuristics
– Strong constraint propagation• Edge-finding, precedence graph
9
Evaluating the TechnologyEvaluating the Technology
Cx: cost of solution found by technology x with fixed time limit (10 minutes)
> 1: routing technology is better < 1: scheduling technology is better
rout
sched
C
C
10
JSP JSP VRP Transformation VRP Transformation
[Beck et al. 2002] We can transform JSPs to VRPs
and vice versaScheduling technology is poor
on reformulated VRPsRouting technology is poor on
reformulated JSPs– Can’t find first solutions due to
precedence constraints!
12
CharacteristicsCharacteristics
What are the problem characteristics that lead to this difference?Ideas:
– Alternative resources– Optimization criteria– Precedence constraints– (3 more not really discussed here)
13
From VRPFrom VRP
VRP
?
?
?
?
14
From JSPFrom JSP
JSP
+
?
?
?
?
15
Alternative ResourcesAlternative Resources
VRP: many (e.g., 25)JSP: few (1, 4, 8)
Savings can’t solve ~70% of problems with 2 alternatives– Only problems solved by
both are included
16
Alternative Resources: VRPAlternative Resources: VRP
17
Alternative Resources: JSPAlternative Resources: JSP
18
Optimization CriteriaOptimization Criteria
VRP: total travelJSP: makespan
19
Optimisation Criteria: VRPOptimisation Criteria: VRP
20
Optimisation Criteria: JSPOptimisation Criteria: JSP
21
Precedence ConstraintsPrecedence Constraints
VRP: noneJSP: paths of totally
ordered activities
Savings can’t find first solution– Start with scheduling
solution
22
Precedence Constraints: VRPPrecedence Constraints: VRP
23
Precedence Constraints: JSPPrecedence Constraints: JSP
24
Experimental SummaryExperimental Summary
VRP JSP
Alt Res Alt Res
Precedence Cts Precedence Cts
Opt. Makespan Opt. Total Travel
scheduling performance
routing performance
25
Other CharacteristicsOther Characteristics
Smaller impactTemporal Slack
slack = scheduling performanceVehicle Resource Capacity
– Like alternative resourcesActivity duration to transition time ratio
– VRP: ratio = routing performance– JSP: ratio = scheduling performance
26
ConclusionsConclusions
Try scheduling technology on VRP with– makespan minimization (strong propagation?)– complex temporal constraints
Try routing technology on JSP with– total time minimization (weak propagation?)– few temporal constraints (open shop?)
27
ConclusionsConclusions
Even isolated changes in problem characteristics change the best choice of off-the-shelf problem model
Understanding this is important to extending the scope of optimisation techniques to– new problems – new people
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Alternative ResourcesAlternative Resources
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