jmlr jmlr2010 jmlr2010-80 knowledge-graph by maker-knowledge-mining

80 jmlr-2010-On-Line Sequential Bin Packing


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Author: András György, Gábor Lugosi, György Ottucsàk

Abstract: We consider a sequential version of the classical bin packing problem in which items are received one by one. Before the size of the next item is revealed, the decision maker needs to decide whether the next item is packed in the currently open bin or the bin is closed and a new bin is opened. If the new item does not fit, it is lost. If a bin is closed, the remaining free space in the bin accounts for a loss. The goal of the decision maker is to minimize the loss accumulated over n periods. We present an algorithm that has a cumulative loss not much larger than any strategy in a finite class of reference strategies for any sequence of items. Special attention is payed to reference strategies that use a fixed threshold at each step to decide whether a new bin is opened. Some positive and negative results are presented for this case. Keywords: bin packing, on-line learning, prediction with expert advice

Reference: text


Summary: the most important sentenses genereted by tfidf model

sentIndex sentText sentNum sentScore

1 Before the size of the next item is revealed, the decision maker needs to decide whether the next item is packed in the currently open bin or the bin is closed and a new bin is opened. [sent-8, score-2.113]

2 If a bin is closed, the remaining free space in the bin accounts for a loss. [sent-10, score-1.021]

3 The goal of the decision maker is to minimize the loss accumulated over n periods. [sent-11, score-0.19]

4 We present an algorithm that has a cumulative loss not much larger than any strategy in a finite class of reference strategies for any sequence of items. [sent-12, score-0.269]

5 Special attention is payed to reference strategies that use a fixed threshold at each step to decide whether a new bin is opened. [sent-13, score-0.565]

6 Keywords: bin packing, on-line learning, prediction with expert advice 1. [sent-15, score-0.763]

7 Introduction In the classical off-line bin packing problem, an algorithm receives items (also called pieces) of size x1 , x2 , . [sent-16, score-1.224]

8 We have an infinite number of bins, each with capacity 1, and every item is to be assigned to a bin. [sent-20, score-0.301]

9 Further, the sum of the sizes of the items (also denoted by xt ) assigned to any bin cannot exceed its capacity. [sent-21, score-0.932]

10 A bin is empty if no item is assigned to it, otherwise, it is used. [sent-22, score-0.816]

11 Another well-studied version of the problem is the so-called on-line bin packing problem. [sent-26, score-1.034]

12 Here items arrive one by one and each item xt must be assigned to a bin (with free space at least xt ) c 2010 Andr´ s Gy¨ rgy, G´ bor Lugosi and Gy¨ rgy Ottucs´ k. [sent-27, score-1.473]

13 In this paper we abandon this assumption and introduce a more restricted version that we call sequential bin packing. [sent-31, score-0.655]

14 , in where it = 0 means that the next item is assigned to the open bin and it = 1 means that a new bin is opened and the next item is assigned to that bin. [sent-36, score-1.618]

15 Of course, if it = 0, then it may happen that the item xt does not fit in the open bin. [sent-37, score-0.489]

16 ” If the decision is it = 1 then the remaining empty space in the last closed bin is counted as a loss. [sent-39, score-0.58]

17 The measure of performance we use is the total sum of all lost items and wasted empty space. [sent-40, score-0.279]

18 Just as in the original bin packing problem, we may distinguish off-line and on-line versions of the sequential bin packing problem. [sent-41, score-2.229]

19 In the off-line sequential bin packing problem the entire sequence x1 , . [sent-42, score-1.239]

20 Note that unlike in the classical bin packing problem, the order of the items is relevant. [sent-46, score-1.209]

21 In Section 3 we present a simple algorithm with running time of O(n2 ) that minimizes the total loss in the off-line sequential bin packing problem. [sent-48, score-1.273]

22 Much more interesting is the on-line variant of the sequential bin packing problem. [sent-49, score-1.195]

23 Here the items xt are revealed one by one, after the corresponding decision it has been made. [sent-50, score-0.454]

24 However, unlike in standard formulations of on-line prediction, here the loss the predictor suffers depends not only on the outcome xt and decision it but also on the “state” defined by the fullness of the open bin. [sent-53, score-0.363]

25 Our goal is to extend the usual bin packing problems to situations in which one can handle only one bin at a time, and items must be processed immediately so they cannot wait for bin changes. [sent-54, score-2.212]

26 To motivate the on-line sequential model, one may imagine a simple revenue management problem in which a decision maker has a unit storage capacity at his disposal. [sent-55, score-0.322]

27 Arguably the most natural comparison class contains all algorithms that use a fixed threshold to decide whether a new bin is opened. [sent-67, score-0.511]

28 An expert with parameter p simply decides to open a new bin whenever the remaining free space in the open bin is less than p. [sent-69, score-1.265]

29 We also offer some data-dependent bounds for an algorithm designed to compete with the best of all constant-threshold strategies, and show that if item sizes are jittered with a certain noise then a uniform regret bound of the order of n2/3 ln1/3 n may be achieved . [sent-73, score-0.337]

30 The principal difficulty of the problem lies in the fact that each action of the decision maker takes the problem in a new “state” (determined by the remaining empty space in the open bin) which has an effect on future losses. [sent-74, score-0.282]

31 Moreover, the state of the algorithm is typically different from the state of the experts which makes comparison difficult. [sent-75, score-0.228]

32 (2002) considered a similar setup in which the loss function has a “memory,” that is, the loss of a predictor depends on the loss of past actions. [sent-77, score-0.189]

33 However, the special properties of the bin packing problem make it possible to design a prediction strategy with small regret. [sent-87, score-1.052]

34 We also mention here the similar on-line bin packing with rejection problem where the algorithm has an opportunity to reject some items and the loss function is the sum of the number of the used bins and the “costs” of the rejected items, see He and D´ sa (2005). [sent-94, score-1.434]

35 Then the cumulative loss of the optimal off-line bin packing is 0 and it is 0. [sent-106, score-1.19]

36 4 in the case of sequential off-line bin packing (see Figure 1). [sent-107, score-1.195]

37 In sequential bin packing we assume that the cost of the items coincides with their size. [sent-111, score-1.37]

38 3 b) sequential off-line Figure 1: The difference between the optimal solutions for the off-line and sequential off-line problems. [sent-128, score-0.322]

39 In Section 3 the complexity of the off-line sequential bin packing problem is analyzed. [sent-131, score-1.195]

40 Setup We use a terminology borrowed from the theory of on-line prediction with expert advice. [sent-134, score-0.191]

41 Thus, we call the sequential decisions of the on-line algorithm predictions and we use forecaster as a synonym for algorithm. [sent-135, score-0.354]

42 We denote by It ∈ {0, 1} the action of the forecaster at time t (i. [sent-136, score-0.21]

43 Action 0 means that the next item will be assigned to the open bin and action 1 represents the fact that a new bin is opened and the next item is assigned to the next empty bin. [sent-139, score-1.744]

44 Denote by st ∈ [0, 1) the free space in the open (last) bin at time t ≥ 1, that is, after having placed the items x1 , x2 , . [sent-142, score-0.966]

45 More precisely, the state of the forecaster is defined, recursively, as follows: As at the beginning we have an empty bin, s0 = 1. [sent-147, score-0.226]

46 , n, • st = 1 − xt , when the algorithm assigns the item to the next empty bin (i. [sent-151, score-1.237]

47 , It = 1); • st = st−1 , when the assigned item does not fit in the open bin (i. [sent-153, score-1.051]

48 , It = 0 and st−1 < xt ); • st = st−1 − xt , when the assigned item fits in the open bin (i. [sent-155, score-1.483]

49 This may be written in a more compact form: st = st (It , xt , st−1 ) = It (1 − xt ) + (1 − It )(st−1 − I{st−1 ≥xt } xt ) where I{·} denotes the indicator function of the event in brackets, that is, it equals 1 if the event is true and 0 otherwise. [sent-158, score-1.122]

50 The loss suffered by the forecaster at round t is ℓ(It , xt | st−1 ), 92 O N - LINE S EQUENTIAL B IN PACKING where the loss function ℓ is defined by 0, if s ≥ x; ℓ(0, x | s) = x, otherwise (1) and ℓ(1, x | s) = s . [sent-159, score-0.524]

51 (2) The goal of the forecaster is to minimize its cumulative loss defined by t Lt = LIt ,t = ∑ ℓ(Is , xs | ss−1 ) . [sent-160, score-0.301]

52 n In ∈{0,1}n In the on-line version of the problem the forecaster does not know the size of the next items, and the sequence of items can be completely arbitrary. [sent-165, score-0.349]

53 Since we allow the forecaster to randomize, it is important to clarify that the entire sequence of items are determined before the forecaster starts making decisions, that is, x1 , . [sent-173, score-0.479]

54 ) The performance of a sequential on-line algorithm is measured by its cumulative loss. [sent-178, score-0.269]

55 (This is in contrast with the non-sequential counterpart of the bin packing problem in which there exist on-line algorithms for which the number of used bins is within a constant factor of that of the optimal solution, see Seiden 2002. [sent-181, score-1.118]

56 ) So in order to measure the performance of a sequential on-line algorithm in a meaningful way, we adopt an approach extensively used in on-line prediction (the so-called “experts” framework). [sent-182, score-0.194]

57 The performance of the algorithm is evaluated relative to this set of experts, and the goal is to perform asymptotically as well as the best expert from the reference class. [sent-184, score-0.24]

58 Formally, let fE,t ∈ {0, 1} be the decision of an expert E at round t, where E ∈ E and E is the set of the experts. [sent-185, score-0.265]

59 Similarly, we denote the state of expert E with sE,t after the t-th item has been revealed. [sent-187, score-0.452]

60 Then the loss of expert E at round t is ℓ( fE,t , xt | sE,t−1 ) and the cumulative loss of expert E is n LE,n = ∑ ℓ( fE,t , xt | sE,t−1 ). [sent-188, score-1.049]

61 t=1 93 ¨ ´ G Y ORGY, L UGOSI AND OTTUCS AK S EQUENTIAL ON - LINE BIN PACKING PROBLEM WITH EXPERT ADVICE Parameters: set E of experts, state space S = [0, 1), action space A = {0, 1}, nonnegative loss function ℓ : (A × (0, 1]|S ) → [0, 1), number n of items. [sent-189, score-0.193]

62 , n, (a) each expert forms its action fE,t ∈ A ; (b) the forecaster observes the actions of the experts and forms its own decision It ∈ A ; (c) the next item xt ∈ (0, 1] is revealed; (d) the algorithm incurs loss ℓ(It , xt | st−1 ) and each expert E ∈ E incurs loss ℓ( fE,t , xt | sE,t−1 ). [sent-194, score-1.865]

63 Figure 2: Sequential on-line bin packing problem with expert advice. [sent-196, score-1.207]

64 The goal of the algorithm is to perform almost as well as the best expert from the reference class E (determined in hindsight). [sent-197, score-0.24]

65 The model of sequential on-line bin packing with expert advice is given in Figure 2. [sent-200, score-1.446]

66 In Sections 4 and 5 we design sequential on-line bin packing algorithms. [sent-201, score-1.195]

67 However, we show that if the item sizes are jittered with certain noise, the regret of the algorithm vanishes uniformly regardless of the original sequence of items. [sent-208, score-0.381]

68 Sequential Off-line Bin Packing As it is well known, most variants of the bin packing problem are NP-hard, including bin packing with rejection, see He and D´ sa (2005), and maximum resource bin packing, see Boyar et al. [sent-211, score-2.58]

69 o 94 O N - LINE S EQUENTIAL B IN PACKING In this section we show that the sequential bin packing problem is significantly easier. [sent-213, score-1.195]

70 The key property is that after the t-th item has been received, the 2t possible sequences of decisions cannot lead to more than t different states. [sent-215, score-0.277]

71 Lemma 1 For any fixed sequence of items x1 , x2 , . [sent-216, score-0.219]

72 , xn and for every 1 ≤ t ≤ n, |St | ≤ t, where St = {s : s = sIt ,t , It ∈ {0, 1}t } and sIt ,t is the state reached after receiving items x1 , . [sent-219, score-0.242]

73 At time t, the state of every sequence of decisions with It = 0 belongs to the set St′ = {s′ : s′ = s − I{s≥xt } xt , s ∈ St−1 } and the state of those with It = 1 becomes 1 − xt . [sent-226, score-0.624]

74 To describe a computationally efficient algorithm to compute I∗ , we set up a graph with the set n of possible states as a vertex set (there are O(n2 ) of them by Lemma 1) and we show that the shortest path on this graph yields the optimal solution of the sequential off-line bin packing problem. [sent-228, score-1.287]

75 Each vertex vk = v(sk ,tk ) of the graph is defined by a time index tk and a state sk ∈ Stk and corresponds to state sk reachable after tk steps. [sent-236, score-0.26]

76 Two vertices (vi , v j ) are connected by an edge if and only if vi ∈ St−1 , v j ∈ St and state v j is reachable from state vi . [sent-238, score-0.201]

77 That is, by choosing either action 0 or action 1 in state vi , the new state becomes v j after item xt has been placed. [sent-239, score-0.734]

78 Thus, if we find a path with minimal total weight from v1 to v|V | , we also find the optimal sequence of actions for the off-line bin packing problem. [sent-248, score-1.145]

79 0/ ℓ(0 ,x 2 |s 3) Figure 3: The graph corresponding to the off-line sequential bin packing problem. [sent-265, score-1.195]

80 Sequential On-line Bin Packing In this section we study the sequential on-line bin packing problem with expert advice, as described in Section 2. [sent-267, score-1.368]

81 We construct a randomized algorithm that, with large probability, achieves a cumulative loss not larger than that of the best expert plus O(n2/3 ln1/3 N) where N = |E | is the number of experts. [sent-272, score-0.371]

82 It shows that in sequential on-line bin packing the cumulative loss is not sensitive to the initial states in the sense that the cumulative loss depends on the initial state in a minor way. [sent-274, score-1.576]

83 Then 0 m m t=1 t=1 ′ ∑ ℓ(it , xt | st−1 ) − ∑ ℓ(it , xt | st−1 ) ≤ s′ + s0 ≤ 2 . [sent-296, score-0.432]

84 Therefore, we have m m ′ ∑ ℓ(it , xt | st−1 ) − ∑ ℓ(it , xt | st−1 ) t=1 t=1 m′ = ∑ ℓ(it , xt | t=1 = m′ −1 ∑ t=1 ′ st−1 ) − m′ ∑ ℓ(it , xt | st−1 ) t=1 ′ ℓ(0, xt | st−1 ) − m′ −1 ∑ ℓ(0, xt | st−1 ) + ℓ(1, xm ′ t=1 96 | s′ ′ −1 ) − ℓ(1, xm′ | sm′ −1 ) . [sent-299, score-1.322]

85 m O N - LINE S EQUENTIAL B IN PACKING Now using the definition of the loss (see Equations 1 and 2), we write m m ′ ∑ ℓ(it , xt | st−1 ) − ∑ ℓ(it , xt | st−1 ) t=1 t=1 = m′ −1 ∑ xt (I{s ′ t−1 < s0 , and m′ = 2. [sent-300, score-0.711]

86 Then 0 m m ′ ∑ ℓ(it , xt | st−1 ) − ∑ ℓ(it , xt | st−1 ) t=1 t=1 = ℓ(0, x1 | s′ ) + ℓ(1, x2 | s′ ) − ℓ(0, x1 | s0 ) + ℓ(1, x2 | s1 ) 0 1 = ℓ(0, s0 | s′ ) + ℓ(1, x2 | s′ ) − ℓ(0, s0 | s0 ) + ℓ(1, x2 | 0) 0 0 = s0 + s′ − (0 + 0) . [sent-301, score-0.432]

87 0 Now we consider the on-line sequential bin packing problem when the goal of the algorithm is to keep its cumulative loss close to the best in a finite set of experts. [sent-302, score-1.381]

88 At the beginning of each segment, the algorithm selects an expert randomly, according to an exponentially weighted average distribution. [sent-310, score-0.188]

89 Recall that each expert E ∈ E recommends an action fE,t ∈ {0, 1} at every time instance t = 1, . [sent-316, score-0.253]

90 Recall that Ln denotes the cumulative loss of the algorithm while Li,n is that of expert i. [sent-334, score-0.344]

91 , xn ∈ (0, 1] of items, the cumulative loss Ln of the randomized strategy defined in Figure 4 satisfies for all i = 1, . [sent-339, score-0.2]

92 ,N δ ln(N/δ) 2 8m ln N/n, Proof We introduce an auxiliary quantity, the so-called hypothetical loss, defined as the loss the algorithm would suffer if it had been in the same state as the selected expert. [sent-350, score-0.235]

93 More precisely, the true loss of the algorithm at time instance t is ℓ(It , xt | st ) and its hypothetic loss is ℓ(It , xt | sJt ,t ). [sent-352, score-0.81]

94 Introducing the notation ℓi,t = ℓ( fi,t , xt | si,t ) , the hypothetical loss of the algorithm is just ℓ(It , xt | sJt ,t ) = ℓ( fJt ,t , xt | sJt ,t ) = ℓJt ,t . [sent-353, score-0.771]

95 2 in Cesa-Bianchi and Lugosi, 2006) that the hypothetical loss of the sequential on-line bin packing algorithm satisfies simultaneously for all i = 1, . [sent-358, score-1.318]

96 Now we may decompose the regret relative to expert i as follows: n Ln − Li,n = Ln − ∑ ℓJt ,t t=1 98 n + ∑ ℓJ ,t − Li,n t t=1 . [sent-362, score-0.24]

97 (c) The size of next item xt ∈ (0, 1] is revealed. [sent-382, score-0.445]

98 (d) The algorithm incurs loss ℓ(It , xt | st−1 ) and each expert i incurs loss ℓ( fi,t , xt | si,t−1 ). [sent-383, score-0.836]

99 Constant-threshold Experts In this section we address the sequential on-line bin packing problem when the goal is to perform almost as well as the best in the class of all constant-threshold strategies. [sent-393, score-1.21]

100 Recall that a constantthreshold strategy is parameterized by a number p ∈ (0, 1] and it opens a new bin if and only if the remaining empty space in the bin is less than p. [sent-394, score-1.034]


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