# Scheduling Shop Scheduling. Tim Nieberg

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1 Scheduling Shop Scheduling Tim Nieberg

2 Shop models: General Introduction Remark: Consider non preemptive problems with regular objectives Notation Shop Problems: m machines, n jobs 1,..., n operations O = {(i, j) j = 1,..., n; i M j M := {1,..., m}} with processing times p ij M j is the set of machines where job j has to be processed on PREC specifies the precedence constraints on the operations

3 Shop models: General Introduction Notation Shop Problems: m machines, n jobs 1,..., n operations O = {(i, j) j = 1,..., n; i M j M := {1,..., m}} with processing times p ij M j is the set of machines where job j has to be processed on PREC specifies the precedence constraints on the operations Flow shop: M j = M and PREC = {(i, j) (i + 1, j) i = 1,..., m 1; j = 1,..., n}

4 Shop models: General Introduction Notation Shop Problems: m machines, n jobs 1,..., n operations O = {(i, j) j = 1,..., n; i M j M := {1,..., m}} with processing times p ij M j is the set of machines where job j has to be processed on PREC specifies the precedence constraints on the operations Flow shop: M j = M and PREC = {(i, j) (i + 1, j) i = 1,..., m 1; j = 1,..., n} Open shop: M j = M and PREC = Job shop: PREC contain a chain (i 1, j)..., (i M j, j) for each j

5 Shop models: General Introduction Disjunctive Formulation of the constraints C ij denotes completion time of operation (i, j) PREC have to be respected:

6 Shop models: General Introduction Disjunctive Formulation of the constraints C ij denotes completion time of operation (i, j) PREC have to be respected: C ij p ij C kl for all (k, l) (i, j) PREC no two operations of the same job are processed at the same time:

7 Shop models: General Introduction Disjunctive Formulation of the constraints C ij denotes completion time of operation (i, j) PREC have to be respected: C ij p ij C kl for all (k, l) (i, j) PREC no two operations of the same job are processed at the same time: C ij p ij C kj or C kj p kj C ij for all i, k M j ; i k no two operations are processed jointly on the same machine:

8 Shop models: General Introduction Disjunctive Formulation of the constraints PREC have to be respected: C ij p ij C kl for all (k, l) (i, j) PREC no two operations of the same job are processed at the same time: C ij p ij C kj or C kj p kj C ij for all i, k M j ; i k no two operations are processed jointly on the same machine: C ij p ij C il or C il p il C ij for all (i, j), (i, l) O; j l C ij p ij 0 the or constraints are called disjunctive constraints some of the disjunctive constraints are overruled by the PREC constraints

9 Shop models: General Introduction Disjunctive Formulation - makes pan objective min C max s.t. C max C ij C ij p ij C kl C ij p ij C kj or C kj p kj C ij C ij p ij C il or C il p il C ij C ij p ij 0 (i, j) O (k, l) (i, j) PREC i, k M j ; i k (i, j), (i, l) O; j l (i, j) O

10 Shop models: General Introduction Disjunctive Formulation - sum objective min w j L j s.t. L j C ij d j C ij p ij C kl C ij p ij C kj or C kj p kj C ij C ij p ij C il or C il p il C ij C ij p ij 0 (i, j) O (k, l) (i, j) PREC i, k M j ; i k (i, j), (i, l) O; j l (i, j) O Remark: also other constraints, like e.g. release dates, can be incorporated the disjunctive constraints make the problem hard (lead to an ILP formulation)

11 Shop models: General Introduction Disjunctive Graph Formulation graph representation used to represent instances and solutions of shop problems can be applied for regular objectives only

12 Shop models: General Introduction Disjunctive Graph G = (V, C, D) V set of vertices representing the operations O a vertex is labeled by the corresponding processing time; Additionally, a source node 0 and a sink node belong to V ; their weights are 0 C set of conjunctive arcs reflecting the precedence constraints: for each (k, l) (i, j) PREC a directed arc belongs to C additionally 0 O and O are added to C D set of disjunctive arcs representing conflicting operations: between each pair of operations belonging to the same job or to be processed on the same machine, for which no order follows from PREC, an undirected arc belongs to D

13 Shop models: General Introduction Disjunctive Graph - Example Job Shop Data: jobs, machines; Jobs: 1 (, 1) (2, 1) (1, 1) 2 (1, 2) (, 2) (2, ) (1, ) (, ) p 1 = 4, p 21 = 2, p 11 = 1 p 12 =, p 2 = p 2 = 2, p 1 = 4, p = 1 M1 M2 M

14 Shop models: General Introduction Disjunctive Graph - Example Job Shop Jobs: 1 (, 1) (2, 1) (1, 1) 2 (1, 2) (, 2) (2, ) (1, ) (, ) p 1 = 4, p 21 = 2, p 11 = 1 p 12 =, p 2 = p 2 = 2, p 1 = 4, p = 1 Graph:,1 2,1 1,1 0 1,2,2 Conjunctive arcs Disjunctive arcs 2, 1,,

15 Shop models: General Introduction Disjunctive Graph - Example Open Shop Data: jobs, machines; Jobs: 1 (1, 1), (2, 1), (, 1) 2 (1, 2), (2, 2), (, 2) (1, ), (2, ), (, ) p 11 = 4, p 21 = 2, p 1 = 1 p 12 =, p 22 = 1, p 2 = p 1 = 2, p 2 = 4, p = 1

16 Shop models: General Introduction Disjunctive Graph - Example Open Shop Jobs: 1 (1, 1), (2, 1), (, 1) 2 (1, 2), (2, 2), (, 2) (1, ), (2, ), (, ) p 11 = 4, p 21 = 2, p 1 = 1 p 12 =, p 22 = 1, p 2 = p 1 = 2, p 2 = 4, p = 1 Graph: 1,1 2,1,1 0 1,2 2,2,2 Conjunctive arcs Disjunctive arcs 1, 2,,

17 Shop models: General Introduction Disjunctive Graph - Selection basic scheduling decision for shop problems (see disj. formulation): define an ordering for operations connected by a disjunctive arc turn the undirected disjunctive arc into a directed arc selection S: a set of directed disjunctive arcs (i.e. S D together with a chosen direction for each a S) disjunctive arcs which have been directed are called fixed a selection is a complete selection if each disjunctive arc has been fixed the graph G(S) = (V, C S) is acyclic

18 Shop models: General Introduction Selection - Remarks a feasible schedule induces a complete selection a complete selection leads to sequences in which operations have to be processed on machines a complete selection leads to sequences in which operations of a job have to be processed Does each complete selection leads to a feasible schedule?

19 Shop models: General Introduction Calculate a Schedule for a Complete Selection S calculated longest paths from 0 to all other vertices in G(S) Technical description: length of a path i 1, i 2,..., i r = sum of the weights of the vertices i 1, i 2,..., i r calculate length l ij of the longest path from 0 to (i, j) (using e.g. Dijkstra) start operation (i, j) at time l ij p ij (i.e. C ij = l ij ) the length of a longest path from 0 to (such paths are called critical paths) is equal to the makespan of the schedule resulting schedule is the semiactive schedule which respects all precedence given by C and S

20 Shop models: General Introduction Reformulation Shop Problem find a complete selection for which the corresponding schedule minimizes the given (regular) objective function

21 Flow Shop models Makespan Minimization Lemma: For problem F C max an optimal schedule exists with the job sequence on the first two machines is the same the job sequence on the last two machines is the same (Proof as Exercise) Consequence: For F 2 C max and F C max an optimal solution exists which is a permutation solution For Fm C max, m 4, instances exist where no optimal solution exists which is a permutation solution (Exercise)

22 Flow Shop models Problem F 2 C max solution can be described by a sequence π problem was solved by Johnson in 1954 Johnson s Algorithm: 1 L = set of jobs with p 1j < p 2j ; 2 R = set of remaining jobs; sort L by SPT w.r.t. the processing times on first machine (p 1j ) 4 sort R by LPT w.r.t. the processing times on second machine (p 2j ) 5 sequence L before R (i.e. π = (L, R) where L and R are sorted)

23 Flow Shop models Example solution problem F 2 C max ( ) n = 5; p =

24 Flow Shop models Example solution problem F 2 C max ( ) n = 5; p = L = {1,, 4}; R = {2, 5} sorting leads to L = {4,, 1}; R = {5, 2}

25 Flow Shop models Example solution problem F 2 C max ( ) n = 5; p = L = {1,, 4}; R = {2, 5} sorting leads to L = {4,, 1}; R = {5, 2} π = (4,, 1, 5, 2) M M

26 Flow Shop models Problem F 2 C max Lemma 1: If min{p 1i, p 2j } < min{p 2i, p 1j } then job i is sequenced before job j by Johnson s algorithm. Lemma 2: If job j is scheduled immediately after job i and min{p 1j, p 2i } < min{p 2j, p 1i } then swapping job i and j does not increase C max. Theorem: Johnson s algorithm solves problem F 2 C max optimal in O(n log(n)) time. (Proofs on the board)

27 Flow Shop models Problem F C max F C max is NP-hard in the strong sense Reduction using -PARTITION Proof on the board

28 Open Shop models Algorithm Problem O2 C max 1 I = set of jobs with p 1j p 2j ; J = set of remaining jobs; 2 IF p 1r = max{max j I p 1j, max j J p 2j } then order on M 1 : (I \ {r}, J, r); order on M 2 : (r, I \ {r}, J) r first on M 2, than on M 1 ; all other jobs vice versa M 1 M 2 I \ {r} r I \ {r} J J r

29 Open Shop models Algorithm Problem O2 C max 1 I = set of jobs with p 1j p 2j ; J = set of remaining jobs; 2 IF p 1r = max{max j I p 1j, max j J p 2j } then order on M 1 : (I \ {r}, J, r); order on M 2 : (r, I \ {r}, J) r first on M 2, than on M 1 ; all other jobs vice versa ELSE IF p 2r = max{max j I p 1j, max j J p 2j } then order on M 1 : (r, J \ {r}, I ); order on M 2 : (J \ {r}, I, r) r first on M 1, than on M 2 ; all other jobs vice versa M 1 M 2 r J \ {r} J \ {r} I I r

30 Open Shop models Remarks Algorithm Problem O2 C max complexity: O(n) algorithm solves problem O2 C max optimally Proof builds on fact that C max is either n j=1 p 1j or n j=1 p 2j or p 1r + p 2r

31 Open Shop models Remarks Algorithm Problem O2 C max complexity: O(n) algorithm solves problem O2 C max optimally Proof builds on fact that C max is either n j=1 p 1j or n j=1 p 2j or p 1r + p 2r Problem O C max Problem O C max is NP-hard Proof as Exercise (Reduction using PARTITION)

32 Open Shop models Problem O pmtn C max define ML i := n j=1 p ij (load of machine i) define JL j := m i=1 p ij (load of job j) LB := max{max m i=1 ML i, max n j=1 JL j} is a lower bound on C max

33 Open Shop models Problem O pmtn C max define ML i := n j=1 p ij (load of machine i) define JL j := m i=1 p ij (load of job j) LB := max{max m i=1 ML i, max n j=1 JL j} is a lower bound on C max Theorem: For problem O pmtn C max a schedule with C max = LB exists. Proof of the theorem is constructive and leads to a polynomial algorithm for problem O pmtn C max

34 Open Shop models Notations for Algorithm O pmtn C max job j (machine i) is called tight if JL j = LB (ML i = LB) job j (machine i) has slack if JL j < LB (ML i < LB) a set D of operataions is called a decrementing set if it contain for each tight job and machine exactly one operation and for each job and machine with slack at most one operation Theorem: A decrementing set always exists and can be calculated in polynomial time (Proof based on maximal cardinality matchings; see e.g. P. Brucker: Scheduling Algorithms)

35 Open Shop models Algorithm O pmtn C max REPEAT 1 Calculate a decrementing set D; 2 Calculate maximum value with min (i,j) D p ij LB ML i if machine i has slack and no operation in D LB JL j if job j has slack and no operation in D; schedule the operations in D for time units in parallel; 4 Update values p, LB, JL, and ML UNTIL all operations have been completely scheduled.

36 Open Shop models Correctness Algorithm O pmtn C max after an iteration we have: LB new = LB old in each iteration a time slice of time units is scheduled the algorithm terminates after at most nm + n + m iterations since in each iteration either an operation gets completely scheduled or one additional machine or job gets tight

37 Open Shop models Example Algorithm O pmtn C max p ML p JL LB = 11

38 Open Shop models Example Algorithm O pmtn C max = p ML p JL LB = 11

39 Open Shop models Example Algorithm O pmtn C max = p ML p JL LB = 11 M M 2 M 1 2 1

40 Open Shop models Example Algorithm O pmtn C max = p ML p JL LB = 11 p ML p JL LB = 8 M M 2 M 1 2 1

41 Open Shop models Example Algorithm O pmtn C max = p ML p JL LB = 11 = 1 p ML p JL LB = 8 M M 2 M 1 M M 2 M

42 Open Shop models Example Algorithm O pmtn C max = 1 p ML p JL LB = 8 = p ML p JL LB = 7 M M 2 M 1 M M 2 M

43 Open Shop models Example Algorithm O pmtn C max = p ML p JL LB = 7 M M 2 M = 2 p ML p JL LB = 4 M M 2 M

44 Open Shop models Example Algorithm O pmtn C max = 2 p ML p JL LB = 4 M M 2 M = 1 p ML p JL LB = 2 M M 2 M

45 Open Shop models Example Algorithm O pmtn C max = 1 p ML p JL LB = 2 M M 2 M = 1 p ML p JL LB = 1 M M 2 M

46 Open Shop models Final Schedule Example Algorithm O pmtn C max p ML p JL LB = 11 M M 2 M iterations C max = 11 = LB sequence of time slices may be changed arbitrary

47 Job Shop models Problem J2 C max I 1 : set of jobs only processed on M 1 I 2 : set of jobs only processed on M 2 I 12 : set of jobs processed first on M 1 and than on M 2 I 21 : set of jobs processed first on M 2 and than on M 1 π 12 : optimal flow shop sequence for jobs from I 12 π 21 : optimal flow shop sequence for jobs from I 21

48 Job Shop models Algorithm Problem J2 C max 1 on M 1 first schedule the jobs from I 12 in order π 12, than the jobs from I 1, and last the jobs from I 21 in order π 21 2 on M 2 first schedule the jobs from I 21 in order π 21, than the jobs from I 2, and last the jobs from I 12 in order π 12 I 12 I 1 I M 2 I 21 2 I 12 M 1 21I

49 Job Shop models Algorithm Problem J2 C max 1 on M 1 first schedule the jobs from I 12 in order π 12, than the jobs from I 1, and last the jobs from I 21 in order π 21 2 on M 2 first schedule the jobs from I 21 in order π 21, than the jobs from I 2, and last the jobs from I 12 in order π 12 I 12 I 1 I M 2 I 21 2 I 12 M 1 21I Theorem: The above algorithm solves problem J2 C max optimally in O(n log(n)) time. Proof: almost straightforward!

50 Job Shop models Problem J C max as a generalization of F C max, this problem is NP-hard it is one of the most treated scheduling problems in literature we presented a branch and bound approach a heuristic approach called the Shifing Bottleneck Heuristic for problem J C max which both depend on the disjunctive graph formulation

51 Job Shop models Base of Branch and Bound The set of all active schedules contains an optimal schedule Solution method: Generate all active schedules and take the best Improvement: Use the generation scheme in a branch and bound setting Consequence: We need a generation scheme to produce all active schedules for a job shop Approach: extend partial schedules

52 Job Shop models Generation of all active schedules Notations: (assuming that already a partial schedule S is given) Ω: set of all operations which predecessors have already been scheduled in S r ij :earliest possible starting time of operation (i, j) Ω w.r.t. S Ω : subset of Ω Remark: r ij can be calculated via longest path calculations in the disjunctive graph belonging to S

53 Job Shop models Generation of all active schedules (cont.) 1 (Initial Conditions) Ω := {first operations of each job}; r ij := 0 for all (i, j) Ω; 2 (Machine selection) Compute for current partial schedule t(ω) := min (i,j) Ω {r ij + p ij }; i := machine on which minimum is achieved; (Branching) Ω := {(i, j) r i j < t(ω)} FOR ALL (i, j) Ω DO 1 extend partial schedule by scheduling (i, j) next on machine i ; 2 delete (i, j) from Ω; add job-successor of (i, j) to Ω; 4 Return to Step 2

54 Job Shop models Generation of all active schedules - example Jobs: 1 (, 1) (2, 1) (1, 1) p 1 = 4, p 21 = 2, p 11 = 1 2 (1, 2) (, 2) (2, ) (1, ) (, ) Partial Schedule: p 12 =, p 2 = p 2 = 2, p 1 = 4, p = 1 M 1 M 2 M 4 6

55 Job Shop models Generation of all active schedules - example Jobs: 1 (, 1) (2, 1) (1, 1) 2 (1, 2) (, 2) (2, ) (1, ) (, ) p 1 = 4, p 21 = 2, p 11 = 1 p 12 =, p 2 = p 2 = 2, p 1 = 4, p = 1 Partial Schedule: M 1 M 2 M 4 6 Ω={(1, 1), (, 2), (1, )}; r 11 = 6, r 2 = 4, r 1 = ; t(ω) = min{6 + 1, 4 +, + 4} = 7; i = M 1 ; Ω = {(1, 1), (1, )}

56 Job Shop models Generation of all active schedules - example (cont.) Partial Schedule: M 1 M 2 M 4 6 Ω={(1, 1), (, 2), (1, )}; r 11 = 6, r 2 = 4, r 1 = ; t(ω) = min{6 + 1, 4 +, + 4} = 7; i = M 1 ; Ω = {(1, 1), (1, )} Extended partial schedules: M 1 M 1 M 2 M 2 M M 6 6

57 Job Shop models Remarks on the generation: the given algorithm is the base of the branching nodes of the branching tree correspond to partial schedules Step branches from the node corresponding to the current partial schedule the number of branches is given by the cardinality of Ω a branch corresponds to the choice of an operation (i, j) to be schedules next on machine i a branch fixes new disjunctions

58 Job Shop models Disjunctions fixed by a branching Node v with Ω = {(i, j), (i, l)} Node v Node v selection (i, j) Add disjunctions (i, j) (i k) for all unscheduled operations (i, k) Consequence: Each node in the branch and bound tree is characterized by a set S of fixed disjunctions

59 Job Shop models Lower bounds for nodes of the branch and bound tree Consider node V with fixed disjunctions S : Simple lower bound: calculate critical path in G(S ) Lower bound LB(V )

60 Job Shop models Lower bounds for nodes of the branch and bound tree Consider node V with fixed disjunctions S : Simple lower bound: calculate critical path in G(S ) Lower bound LB(V ) Better lower bound: consider machine i allow parallel processing on all machines i solve problem on machine i

61 Job Shop models 1-machine problem resulting for better LB 1 calculate earliest starting times r ij of all operations (i, j) on machine i (longest paths from source in G(S )) 2 calculate minimum amount q ij of time between end of (i, j) and end of schedule (longest path to sink in G(S )) solve single machine problem on machine i: respect release dates no preemption minimize maximum value of C ij + q ij Result: head-body-tail problem (see Lecture )

62 Job Shop models Better lower bound solve 1-machine problem for all machines this results in values f 1,..., f m LB new (V ) = max m i=1 f i

63 Job Shop models Better lower bound solve 1-machine problem for all machines this results in values f 1,..., f m LB new (V ) = max m i=1 f i Remarks: 1-machine problem is NP-hard computational experiments have shown that it pays of to solve these m NP-hard problems per node of the search tree job-shop instances are already hard to solve by branch and bound

64 Job Shop models M 6 Better lower bound - example Partial Schedule: M 1 M 2 Corresponding graph G(S ):,1 2,1 1,1 0 1,2,2 * 2, 1,, Conjunctive arcs fixed disj.

65 Job Shop models Graph G(S ) with processing times: 4 2 1,1 2,1 1,1 0 1,2,2 * 2, 1,, LB(V )=l(0, (1, 2), (1, ), (, ), )=8

66 Job Shop models Graph G(S ) with processing times: 4 2 1,1 2,1 1,1 0 1,2,2 * 2, 1,, LB(V )=l(0, (1, 2), (1, ), (, ), )=8 green blue red Data for jobs on Machine 1: r 12 = 0 r 1 = r 11 = 6 q 12 = 5 q 1 = 1 q 11 = 0 Opt. solution: Opt = 8, LB new (V ) = 8 M 1 7 8

67 Job Shop models Change p 11 from 1 to 2! 4 2 2,1 2,1 1,1 0 1,2,2 * 2, 1,, LB(V ) = l(0, (1, 2), (1, ), (, ), ) = l(0, (, 1), (2, 1), (1, 1), ) = 8

68 Job Shop models Change p 11 from 1 to 2! 4 2 2,1 2,1 1,1 0 1,2,2 * 2, 1,, LB(V ) = l(0, (1, 2), (1, ), (, ), ) = l(0, (, 1), (2, 1), (1, 1), ) = 8 green blue red Data for jobs on Machine 1: r 12 = 0 r 1 = r 11 = 6 q 12 = 5 q 1 = 1 q 11 = 0 Opt. solution: OPT = 9, LB new (V ) = 9 M 1 7 9

69 Job Shop models The Shifting Bottleneck Heuristic successful heuristic to solve makespan minimization for job shop iterative heuristic determines in each iteration the schedule for one additional machine uses reoptimization to change already scheduled machines can be adapted to more general job shop problems other objective functions workcenters instead of machines set-up times on machines...

70 Shifting Bottleneck Heuristic for Job Shop Basic Idea Notation: M set of all machines Given: fixed schedules for a subset M 0 M of machines (i.e. a selection of disjunctive arcs for cliques corresponding to these machines) Actions in one iteration: select a machine k which has not been fixed (i.e. a machine from M \ M 0 ) determine a schedule (selection) for machine k on the base of the fixed schedules for the machines in M 0 reschedule the machines from M 0 based on the other fixed schedules

71 Shifting Bottleneck Heuristic for Job Shop Selection of a machine Idea: Chose unscheduled machine which causes the most problems (bottleneck machine) Realization: Calculate for each operation on an unscheduled machine the earliest possible starting time and the minimal delay between the end of the operation and the end of the complete schedule based on the fixed schedules on the machines in M 0 and the job orders calculate for each unscheduled machine a schedule respecting these earliest release times and delays chose a machine with maximal completion time and fix the schedule on this machine

72 Shifting Bottleneck Heuristic for Job Shop Technical realization Define graph G = (N, A ): N same node set as for the disjunctive graph A contains all conjunctive arcs and the disjunctive arcs corresponding to the selections on the machines in M 0 C max (M 0 ) is the length of a critical path in G

73 Shifting Bottleneck Heuristic for Job Shop Technical realization Define graph G = (N, A ): N same node set as for the disjunctive graph A contains all conjunctive arcs and the disjunctive arcs corresponding to the selections on the machines in M 0 C max (M 0 ) is the length of a critical path in G Comments: with respect to G operations on machines from M \ M 0 may be processed in parallel C max (M 0 ) is the makespan of a corresponding schedule

74 Shifting Bottleneck Heuristic for Job Shop Technical realization (cont.) for an operation (i, j); i M \ M 0 let r ij be the length of the longest path from 0 to (i, j) (without p ij ) in G q ij be the length of the longest path from (i, j) to (without p ij ) in G Comments: r ij is the release time of (i, j) w.r.t. G q ij is the tail (minimal time till end) of (i, j) w.r.t. G

75 Shifting Bottleneck Heuristic for Job Shop Technical realization (cont.) For each machine from M \ M 0 solve the nonpreemptive one-machine head-body-tail problem 1 r j, d j < 0 L max Result: values f (i) for all i M \ M 0 Action: Chose machine k as the machine with the largest f (i) value schedule machine k according to the optimal schedule of the one-machine problem add k to M 0 and the corresponding disjunctive arcs to G C max (M 0 k) f (k)

76 Shifting Bottleneck Heuristic for Job Shop Technical realization - Example Given: M 0 = {M} and on M sequence (, 2) (, 1) (, ) Graph G : 4 2 1,1 2,1 1,1 0 1,2,2 * , 1,, Conjunctive arcs Selection M C max (M 0 ) = 1

77 Shifting Bottleneck Heuristic for Job Shop Technical realization - Example (cont.) Machine M 1 : (i, j) (1, 1) (1, 2) (1, ) r ij q ij p ij 1 4

78 Shifting Bottleneck Heuristic for Job Shop Technical realization - Example (cont.) Machine M 1 : (i, j) (1, 1) (1, 2) (1, ) r ij q ij p ij 1 4 M 1 1,2 1, 1, f (M 1 ) = 1

79 Shifting Bottleneck Heuristic for Job Shop Technical realization - Example (cont.) Machine M 1 : (i, j) (1, 1) (1, 2) (1, ) r ij q ij p ij 1 4 Machine M 2 : (i, j) (2, 1) (2, ) r ij 10 0 q ij 1 5 p ij 2 2 M 1 1,2 1, 1, f (M 1 ) = 1

80 Shifting Bottleneck Heuristic for Job Shop Technical realization - Example (cont.) Machine M 1 : (i, j) (1, 1) (1, 2) (1, ) r ij q ij p ij 1 4 Machine M 2 : (i, j) (2, 1) (2, ) r ij 10 0 q ij 1 5 p ij 2 2 M 1 1,2 1, 1, f (M 1 ) = 1 M 2 2, 2, f (M 2 ) = 1

81 Shifting Bottleneck Heuristic for Job Shop Technical realization - Example (cont.) Choose machine M 1 as the machine to fix the schedule: add (1, 2) (1, ) (1, 1) to G M 0 = {M 1, M } 4 2 1,1 2,1 1,1 0 1,2,2 * , 1,, Conjunctive arcs Selection M 1, M C max (M 0 ) = 1

82 Shifting Bottleneck Heuristic for Job Shop Reschedule Machines try to reduce the makespan of the schedule for the machines in M 0 Realization: consider the machines from M 0 one by one remove the schedule of the chosen machine and calculate a new schedule based on the earliest starting times and delays resulting from the other machines of M 0 and the job orders

83 Shifting Bottleneck Heuristic for Job Shop Technical realization rescheduling For a chosen machine l M 0 \ {k} do: remove the arcs corresponding to the selection on machine l from G call new graph G calculate values r ij, q ij in graph G reschedule machine l according to the optimal schedule of the single machine head-body-tail problem

84 Shifting Bottleneck Heuristic for Job Shop Technical realization rescheduling - Example M 0 \ {k} = {M }, thus l = M removing arcs corresponding to M leads to graph G : 4 2 1,1 2,1 1,1 0 1,2,2 * , 1,, Conjunctive arcs Selection M 1 C max (G ) = 8;

85 Shifting Bottleneck Heuristic for Job Shop Technical realization rescheduling - Example M 0 \ {k} = {M }, thus l = M removing arcs corresponding to M leads to graph G : 4 2 1,1 2,1 1,1 0 1,2,2 * , 1,, Conjunctive arcs Selection M 1 C max (G ) = 8; (i, j) (, 1) (, 2) (, ) r ij 0 7 q ij 0 0 p ij 4 1

86 Shifting Bottleneck Heuristic for Job Shop Technical realization rescheduling - Example M 0 \ {k} = {M }, thus l = M removing arcs corresponding to M leads to graph G : 4 2 1,1 2,1 1,1 0 1,2,2 * , 1,, Conjunctive arcs Selection M 1 C max (G ) = 8; (i, j) (, 1) (, 2) (, ) r ij 0 7 q ij 0 0 p ij 4 1 M,1,2, 5 8 f (M ) = 8

87 Shifting Bottleneck Heuristic for Job Shop Technical realization rescheduling - Example (cont.) add (, 1) (, 2) (, ) to G New graph: 4 2 1,1 2,1 1,1 0 1,2,2 * , 1,, Conjunctive arcs Selection M 1, M C new max (M 0 ) = 8

88 Shifting Bottleneck Heuristic for Job Shop Heuristic: summary 1 Initialization: 1 M 0 := ; 2 G := graph with all conjunctive arcs; C max (M 0 ) := length longest path in G: 2 Analyze unscheduled machines: FOR ALL i M \ M 0 DO FOR ALL operation (i, j) DO 1 r ij := length longest path from 0 to (i, j) in G; 2 q ij := length longest path from (i, j) to in G; solve single machine head body tail problem f (i)

89 Shifting Bottleneck Heuristic for Job Shop Heuristic: summary (cont.) Bottleneck selection: 1 determine k such that f (k) = max i M\M 0 f (i); 2 schedule machine k according to the optimal solution in Step 2; add corresponding disjunctive arcs to G; 4 M 0 := M 0 {k};

90 Shifting Bottleneck Heuristic for Job Shop Heuristic: summary (cont.) 4 Resequencing of machines: FOR ALL i M 0 \ {k} DO 1 delete disjunctive arcs corresponding to machine k from G; 2 FOR ALL operation (i, j) DO 1 r ij := length longest path from 0 to (i, j) in G; 2 q ij := length longest path from (i, j) to in G; solve single machine head body tail problem f (i) 4 insert corresponding disjunctive arcs to G; 5 Stopping condition IF M 0 = M THEN Stop ELSE go to Step 2;

91 Shifting Bottleneck Heuristic for Job Shop SBH - Example (cont.) M 0 = {M 1, M }; thus M 2 is bottleneck graph G: 4 2 1,1 2,1 1,1 0 1,2,2 * , 1,, Conjunctive arcs Selection M 1, M C max (G) = 8;

92 Shifting Bottleneck Heuristic for Job Shop SBH - Example (cont.) M 0 = {M 1, M }; thus M 2 is bottleneck graph G: 4 2 1,1 2,1 1,1 0 1,2,2 * , 1,, Conjunctive arcs Selection M 1, M C max (G) = 8; (i, j) (2, 1) (2, ) r ij 4 0 q ij 1 5 p ij 2 2

93 Shifting Bottleneck Heuristic for Job Shop SBH - Example (cont.) M 0 = {M 1, M }; thus M 2 is bottleneck graph G: 4 2 1,1 2,1 1,1 0 1,2,2 * , 1,, Conjunctive arcs Selection M 1, M C max (G) = 8; (i, j) (2, 1) (2, ) r ij 4 0 q ij 1 5 p ij 2 2 M 2 2, 2, f (M 2 ) = 7

94 Shifting Bottleneck Heuristic for Job Shop SBH - Example (cont.) Situation after Step : M 0 = {M 1, M 2, M }, C max (M 0 ) = 8 Graph G: 4 2 1,1 2,1 1,1 0 1,2, , 1,, *

95 Shifting Bottleneck Heuristic for Job Shop SBH - Example (cont.) Situation after Step : M 0 = {M 1, M 2, M }, C max (M 0 ) = 8 Graph G: 4 2 1,1 2,1 1,1 0 1,2,2 * , 1,, Corresponding Schedule: M 1 M 2 M

96 Shifting Bottleneck Heuristic for Job Shop An Important Subproblem within the SBH the one-machine head-body-tail problem occurs frequently: this problem was also used within branch and bound to calculate lower bounds the problem is NP-hard (see Lecture ) there are efficient branch and bound methods for smaller instances (see also Lecture ) the actual one-machine problem is a bit more complicated than stated in Lecture (see following example)

97 Shifting Bottleneck Heuristic for Job Shop Example Delayed Precedences Jobs: (1, 1) (2, 1) (2, 2) (1, 2) (, ) (, 4)

98 Shifting Bottleneck Heuristic for Job Shop Example Delayed Precedences (cont.) after 2 iterations SBH we get: M 0 = {M, M 1 }; (, 4) (, ) and (1, 2) (1, 1) Resulting graph G: (C max (M 0 ) = 8) 1 1 1,1 2, ,2 1,2 0 * 4, 4,4

99 Shifting Bottleneck Heuristic for Job Shop Example Delayed Precedences (cont.). iteration: only M 2 unscheduled (i, j) p ij r ij q ij (2, 1) 1 0 (2, 2) 1 0 Possible schedules for M 2 : M M

100 Shifting Bottleneck Heuristic for Job Shop Example Delayed Precedences (cont.) Both schedules are feasible and might be added to current solution But: second schedule leads to 1 1 1,1 2, ,2 1,2 0 * 4, 4,4 which contains a cycle

101 Shifting Bottleneck Heuristic for Job Shop Delayed Precedences The example shows: not all solutions of the one-machine problem fit to the given selections for machines from M 0 the given selections for machines from M 0 may induce precedences for machines from M \ M 0 Example: scheduling operation (1, 2) before (1, 1) on M 1 induces a delayed precedence constraint between (2, 2) and (2, 1) of length operation (2, 1) has to start at least time units after (2, 2) this time is needed to process operations (2, 2), (1, 2), and (1, 1)

102 Shifting Bottleneck Heuristic for Job Shop Rescheduling Machines after adding a new machine to M 0, it may be worth to put more effort in rescheduling the machines: do the rescheduling in some specific order (e.g. based on their head-body-tail values) repeat the rescheduling process until no improvement is found after rescheduling one machine, make a choice which machine to reschedule next (allowing that certain machines are rescheduled more often)... practical test have shown that these extra effort often pays off

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