a lookahead heuristic for the minimization of open stacks problem
DESCRIPTION
A Lookahead Heuristic for the Minimization of Open Stacks Problem. Marco A. M. Carvalho mamc@ iceb.ufop.br Federal University of Ouro Preto - Brazil Nei Y. Soma soma@ ita.br Technological Institute of Aeronautics - Brazil OR54 Annual Conference – Edinburgh, UK 04-06 September 2012. - PowerPoint PPT PresentationTRANSCRIPT
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A Lookahead Heuristic for the Minimization of Open Stacks Problem
Marco A. M. [email protected] University of Ouro Preto - Brazil
Nei Y. [email protected] Institute of Aeronautics - Brazil
OR54 Annual Conference – Edinburgh, UK04-06 September 2012
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INTRODUCTION
Problem DescriptionMotivationExample
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Introduction
• A factory manufactures different types of products in batches;
• Customers place orders for different products– The contents of each order are placed in a separated
stack during manufacturing;– When the stack receives the first product, it is opened;– When the stack receives the last product , it is closed• The products are delivered;• The space is freed.
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Introduction
• There is a limitation on the physical space used in the production environment– There is not enough space for all customer’s stacks to be
opened simutaneously;– If the number of open stacks increases beyond the available
space, stacks must be removed in order to give space to the new stacks.
• The sequence in which the products are manufactured can reduce the maximum number of simultaneously open stacks– This is the aim of the Minimization of Open Stacks Problem
(MOSP).OR54 Annual Conference, 04-06 September 2012 – Edinburgh, UK
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Motivation
• The problem is NP-Hard and has a variety of equivalent problems:– Cutting stock
• Cutting Patterns Sequencing.– VLSI design
• Gate Matrix Layout Problem;• PLA Folding.
– Graph Problems• Pathwidth;• Interval Thickness;• Node Search Game;• Narrowness;• Split bandwidth;• Edge and Vertex Separation.
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Example #1
• Six customers;• Six product types.
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Example #1
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• Manufacturing Sequence:• Open Stacks: 0
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Example #1
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• Manufacturing Sequence:• Open Stacks: 3
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Example #1
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• Manufacturing Sequence:• Open Stacks: 4
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Example #1
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• Manufacturing Sequence:• Open Stacks: 5
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Example #1
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• Manufacturing Sequence:• Open Stacks: 4
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Example #1
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• Manufacturing Sequence:• Open Stacks: 4
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Example #1
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• Manufacturing Sequence:• Open Stacks: 2
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Introduction
• Formally, given a boolean matrix M:– Rows correspond to customer’s orders;– Columns correspond to products;– mij = 1 iff order i contains product j;
– mij = 0 otherwise;– Stacks are associated to rows
• First product is manufactured: stack opened;• Last product is manufactured: stack closed;
• The objective is to find a permutation of columns such that the maximum number of open stacks is minimized.
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Example #1 Revisited
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p1 p2 p3 p4 p5 p6c1 1 0 0 1 1 0c2 1 1 0 0 0 0c3 0 0 1 1 0 0c4 1 1 1 0 1 0c5 0 1 0 1 1 1c6 0 1 0 0 0 1
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Example #1 Revisited
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p6 p2 p1 p3 p4 p5c1 1 -- 1 1c2 1 1c3 1 1 c4 1 1 1 -- 1c5 1 1 -- -- 1 1c6 1 1
p2 p4 p5 p1 p3 p6c1 1 1 1 c2 1 -- -- 1c3 1 -- -- 1 c4 1 -- 1 1 1c5 1 1 1 -- -- 1c6 1 -- -- -- -- 1
Manufacturing Sequence
Stac
ks
Max Open Stacks: 6
Manufacturing Sequence
Stac
ks
Max Open Stacks: 4
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A LOOKAHEAD HEURISTIC
RepresentationPreprocessingBreadth-First SearchProducts SequencingImprovement Rules
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Representation
• In MOSP graphs, nodes correspond to customer’s orders– Edges connect customers that ordered at least one
product in common;– Multiple edges and loops are not considered;– Each product produces a clique in the graph;– There are polynomial algorithms for some special
topologies.
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MOSP Graph
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MOSP Graph
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MOSP Graph
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MOSP Graph
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MOSP Graph
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MOSP Graph
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MOSP Graph
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Preprocessing #1
• If the MOSP graph is disconnected, the problem is decomposable.
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Preprocessing #2
• Let c(pi) determine the set of customers that ordered product pi
– If c(pj) ⊆ c(pi), then pi and pj can be considered as one product.
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p1 p2 p3 p4 p5 p6c1 1 0 0 1 1 0c2 1 1 0 0 0 0c3 0 0 1 1 0 0c4 1 1 1 0 1 0c5 0 1 0 1 1 1c6 0 1 0 0 0 1
p1 p2p6 p3 p4 p5
c1 1 0 0 1 1c2 1 1 0 0 0c3 0 0 1 1 0c4 1 1 1 0 1c5 0 1 0 1 1c6 0 1 0 0 0
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Breadth-First Search
• The MOSP resembles the Matrix Bandwidth Minimization Problem (MBM)– The MBM problem aims to find a permutation of rows
and columns which keeps the nonzero elements of a matrix as close as possible to the main diagonal.
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Breadth-First Search
• The Cuthill-Mckee (1969) heuristic for MBM explores a corresponding graph by Breadth-First Search (BFS):– Choice of lower degree nodes• Ties are broken in favor of the lower index node;
– The sequence of the search determines the permutation of rows and columns.
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Breadth-First Search
• The BFS has never been applied to the MOSP solution– MOSP instances may not be sparse, symmetric or
square, as MBM matrices; – The band structure is not a required condition.
• However, when applied to the MOSP, it generates good results.
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Breadth-First Search
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Not examinedExaminedAll neighbors examined
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Products Sequencing
• After sequencing the nodes (orders), we obtain the products permutation:– The orders are analyzed using LIFO policy;– Each ordered product is inserted in the solution using
LIFO policy.
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Products Sequencing
• Q={3, 1, 4, 5, 2, 6}
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p1 p2p6 p3 p4 p5c1 1 0 0 1 1c2 1 1 0 0 0c3 0 0 1 1 0c4 1 1 1 0 1c5 0 1 0 1 1c6 0 1 0 0 0
p2 p6c1 0 0c2 1 0c3 0 0c4 1 0c5 1 1c6 1 1
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Products Sequencing
• Q={3, 1, 4, 5, 2, 6}
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p1 p2 p6c1 1 0 0c2 1 1 0c3 0 0 0c4 1 1 0c5 0 1 1c6 0 1 1
p1 p2p6 p3 p4 p5c1 1 0 0 1 1c2 1 1 0 0 0c3 0 0 1 1 0c4 1 1 1 0 1c5 0 1 0 1 1c6 0 1 0 0 0
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Products Sequencing
• Q={3, 1, 4, 5, 2, 6}
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p4 p5 p1 p2 p6c1 1 1 1 0 0c2 0 0 1 1 0c3 1 0 0 0 0c4 0 1 1 1 0c5 1 1 0 1 1c6 0 0 0 1 1
p1 p2p6 p3 p4 p5c1 1 0 0 1 1c2 1 1 0 0 0c3 0 0 1 1 0c4 1 1 1 0 1c5 0 1 0 1 1c6 0 1 0 0 0
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Products Sequencing
• Q={3, 1, 4, 5, 2, 6}
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p3 p4 p5 p1 p2 p6c1 0 1 1 1 0 0c2 0 0 0 1 1 0c3 1 1 0 0 0 0c4 1 0 1 1 1 0c5 0 1 1 0 1 1c6 0 0 0 0 1 1
p1 p2p6 p3 p4 p5c1 1 0 0 1 1c2 1 1 0 0 0c3 0 0 1 1 0c4 1 1 1 0 1c5 0 1 0 1 1c6 0 1 0 0 0
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Products Sequencing
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p3 p4 p5 p1 p2 p6c1 1 1 1c2 1 1c3 1 1c4 1 -- 1 1 1c5 1 1 -- 1 1c6 1 1
Max Open Stacks: 4
Manufacturing Sequence
Stac
ks
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Breadth-First Search
• Breadth-First Search features:– Low degree nodes are not the problem’s bottleneck• Sequenced first.
– Clique’s and high degree nodes tend to be sequenced contiguously;
– Preprocessing #1 is inherent;– Computational complexity;– Ease of implementation.
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Improvement Rules
• Special topologies of the MOSP graph cause BFS to generate errors:– Cliques loosely connected;– A dominant clique with a few nodes in its
neighborhood.
• Improvement rules:1. Close inactive open stacks, by anticipating its
product's manufacturing;2. Delay the opening of new stacks, by postponing its
product’s manufacturing.
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Improvement Rule #1
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p4 p5 p2p6 p1 p3c1 1 1 -- 1 c2 1 1c3 1 -- -- -- 1c4 1 1 1 1c5 1 1 1c6 1
Manufacturing Sequence
Stac
ks
Max Open Stacks: 6
p4 p5 p1 p2p6 p3
c1 1 1 1
c2 1 1
c3 1 -- -- -- 1
c4 1 1 1 1
c5 1 1 -- 1
c6 1
Manufacturing Sequence
Stac
ksMax Open Stacks: 5
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Improvement Rule #2
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p5 p3 p1 p2p6 p4c1 1 -- 1 -- 1c2 1 1 c3 1 -- -- 1c4 1 1 1 1c5 1 -- -- 1 1c6 1
Manufacturing Sequence
Stac
ks
Max Open Stacks: 6
p5 p1 p2p6 p3 p4
c1 1 1 -- -- 1
c2 1 1
c3 1 1
c4 1 1 1 1
c5 1 -- 1 -- 1
c6 1
Manufacturing Sequence
Stac
ksMax Open Stacks: 5
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COMPUTATIONAL EXPERIMENTS
Data setsComputational Environment
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Datasets
• First Constraint Modeling Challenge (2005)– 5,806 smaller instances;– Decomposable instances;– Polynomial topologies of MOSP graphs.
• Harder Instances (2009)– 200 larger instances;– No decomposable instances;– No polynomial topologies of MOSP graphs.
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Computational Experiments
• Intel i5 Quad Core 3.2 GHz processor;• 16 GB RAM;• Ubuntu 12.4.1; • No optimization options;• Chu and Stuckey (2009) original code, compiled and
run as recommended– MOSP state-of-the-art exact method.
• Implementation of Becceneri, Yanasse and Soma (2004) as originally described– MOSP state-of-the-art heuristic.
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Dataset #1
• Running times (ms)
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Method Min Mean Max
Chu and Stuckey 0.00 1.65 6,865.00
Lookahead 0.00 29.72 1,424.00
Becceneri et al. 0.00 0.02 24.00
Method Lookahead Becceneri et al.
Best solutions 832 (14%) 41 (0.71%)
Optimal solutions 5,644 (97.21%) 4,889 (84.21%)
Max error from optimal 2 stacks 8 stacks
Gap from Optimal 0.18% 1.32%
• Solutions
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Dataset #1
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1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45 47 49 51 53 55 57 59 610%
5%
10%
15%
20%
25%
Average Gap from Optimal
Lookahead
Becceneri et al
collections of instances
gap
1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45 47 49 51 53 55 57 59 610.000.501.001.502.002.503.003.504.004.50
Average Error
Becceneri et al
Lookahead
collections of instances
# of
stac
ks
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Dataset #2
• Running times (ms)
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Method Lookahead Becceneri et al.
Best solutions 144 (72%) 4 (2%)
Optimal solutions 110 (55%) 44 (22%)
Max error from optimal 4 stacks 15 stacks
Gap from Optimal 1.41% 7.27%
• Solutions
Method Min Mean Max
Chu and Stuckey 0.00 12,851.00 945,151.00
Lookahead 0.00 1,587.00 15,220.00
Becceneri et al. 0.00 5.34 20.00
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Dataset #2
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1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106 113 120 127 134 141 148 155 162 169 176 183 190 19702468
10121416
Error
Becceneri et al
Lookahead
Instances
# of
stac
ks
1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106 113 120 127 134 141 148 155 162 169 176 183 190 1970%
20%
40%
60%
80%
100%
120%
140%
Gap from Optimal
Becceneri et alLookahead
Instances
gap
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SUMMARY
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Summary
• A novel approach to MOSP;• O(p3) heuristic, where p denotes the number of products
– Outperforms the state-of-the art heuristic in solution quality• Smaller gaps from optimal;• Robust - smaller errors;• Higher index of optimal solutions.
– Fast.• Can be used to generate good upper bounds;• Can be used directly to solve MOSP and equivalent
problems.OR54 Annual Conference, 04-06 September 2012 – Edinburgh, UK
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Acknowledgements
• Prof. Geoffrey Chu (University of Melbourne);• This work was funded by the State of São Paulo
Research Foundation - FAPESP, process 2009/51831-9 (first author).
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THANK YOUQuestions?
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