nips nips2013 nips2013-187 knowledge-graph by maker-knowledge-mining

187 nips-2013-Memoized Online Variational Inference for Dirichlet Process Mixture Models


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Author: Michael Hughes, Erik Sudderth

Abstract: Variational inference algorithms provide the most effective framework for largescale training of Bayesian nonparametric models. Stochastic online approaches are promising, but are sensitive to the chosen learning rate and often converge to poor local optima. We present a new algorithm, memoized online variational inference, which scales to very large (yet finite) datasets while avoiding the complexities of stochastic gradient. Our algorithm maintains finite-dimensional sufficient statistics from batches of the full dataset, requiring some additional memory but still scaling to millions of examples. Exploiting nested families of variational bounds for infinite nonparametric models, we develop principled birth and merge moves allowing non-local optimization. Births adaptively add components to the model to escape local optima, while merges remove redundancy and improve speed. Using Dirichlet process mixture models for image clustering and denoising, we demonstrate major improvements in robustness and accuracy.

Reference: text


Summary: the most important sentenses genereted by tfidf model

sentIndex sentText sentNum sentScore

1 We present a new algorithm, memoized online variational inference, which scales to very large (yet finite) datasets while avoiding the complexities of stochastic gradient. [sent-9, score-0.806]

2 Exploiting nested families of variational bounds for infinite nonparametric models, we develop principled birth and merge moves allowing non-local optimization. [sent-11, score-0.672]

3 Births adaptively add components to the model to escape local optima, while merges remove redundancy and improve speed. [sent-12, score-0.359]

4 Unfortunately, contemporary inference algorithms do not live up to this promise, scaling poorly and yielding solutions that represent poor local optima of the true posterior. [sent-16, score-0.235]

5 Stochastic online variational inference is a promising general-purpose approach to Bayesian nonparametric learning from streaming data [1]. [sent-19, score-0.37]

6 While individual steps of stochastic optimization algorithms are by design scalable, they are extremely vulnerable to local optima for non-convex unsupervised learning problems, frequently yielding poor solutions (see Fig. [sent-20, score-0.2]

7 Recent work has proposed methods for automatically adapting learning rates [2], but these algorithms’ progress on the overall variational objective remains local and non-monotonic. [sent-24, score-0.225]

8 In this paper, we present an alternative algorithm, memoized online variational inference, which avoids noisy gradient steps and learning rates altogether. [sent-25, score-0.759]

9 The algorithm visits each batch in turn and updates a cached set of sufficient statistics which accurately reflect the entire dataset. [sent-27, score-0.264]

10 Our memoized approach is generally applicable in any case batch or stochastic online methods are useful, including topic models [1] and relational models [3], though we do not explore these here. [sent-29, score-0.792]

11 1 We further develop a principled framework for escaping local optima in the online setting, by integrating birth and merge moves within our algorithm’s coordinate ascent steps. [sent-30, score-0.693]

12 Our birth and merge moves, together with a nested variational approximation to the posterior, enable adaptive creation and pruning of clusters on-the-fly. [sent-32, score-0.605]

13 Because these moves are validated by an exactly tracked global variational objective, we avoid potential instabilities of stochastic online split-merge proposals [4]. [sent-33, score-0.516]

14 The structure of our moves is very different from split-merge MCMC methods [5, 6]; applications of these algorithms have been limited to hundreds of data points, while our experiments show scaling of memoized split-merge proposals to millions of examples. [sent-34, score-0.669]

15 We review the Dirichlet process mixture model and variational inference in Sec. [sent-35, score-0.33]

16 2 Variational inference for Dirichlet process mixture models The Dirichlet process (DP) provides a nonparametric prior for partitioning exchangeable datasets into discrete clusters [7]. [sent-39, score-0.228]

17 Component k has mixture weight wk sampled as follows: ∞ G ∼ DP(α0 H), G k−1 wk δφk , vk ∼ Beta(1, α0 ), wk = vk k=1 (1 − vℓ ). [sent-41, score-0.392]

18 Each data item n chooses an assignment zn ∼ Cat(w), and then draws observations xn ∼ F (φzn ). [sent-43, score-0.178]

19 The goal of inference is to recover stick-breaking proportions vk and data-generating parameters φk for each global mixture component k, as well as discrete cluster assignments z = {zn }N for each observation. [sent-47, score-0.426]

20 ˆ To ease notation, we mark variables with hats to distinguish parameters θ of variational factors q from ˆk always have equal dimension. [sent-58, score-0.197]

21 In this way, θk and θ 1 2 Crucially, our truncation is nested: any learned q with truncation K can be represented exactly under truncation K +1 by setting the final component to have zero mass. [sent-60, score-0.255]

22 For each component k, ˆ we store its expected mass Nk and expected sufficient statistic sk (x). [sent-65, score-0.198]

23 We next review variational algorithms for optimizing q via coordinate ascent, iteratively updating individual factors of q. [sent-68, score-0.197]

24 We describe algorithms in terms of two updates [1]: global parameters (stick-breaking proportions vk and datagenerating parameters φk ), and local parameters (assignments of data to components zn ). [sent-69, score-0.472]

25 2 Full-dataset variational inference Standard full-dataset variational inference [7] updates local factors q(zn | rn ) for all observations ˆ n = 1, . [sent-71, score-0.628]

26 , N by visiting each item n and computing the fraction rnk explained by component k: ˆ rnk = exp Eq [log wk (v)] + Eq [log p(xn | φk )] , ˜ rnk = ˆ rnk ˜ . [sent-74, score-1.173]

27 After computing α ˆ ˆk , sk (x) given the new rnk via Eq. [sent-76, score-0.41]

28 (7), the update equations become summary statistics N ˆ K ˆ αk1 = α1 + Nk , ˆ ˆ Nℓ , αk0 = α0 + ˆ ˆ λk = λ0 + sk (x). [sent-77, score-0.186]

29 3 Stochastic online variational inference Stochastic online (SO) variational inference scales to huge datasets [1]. [sent-81, score-0.654]

30 For example, to update the variational paramˆ eter λk from (5) at iteration t, we first compute the global update given only data in the current batch, 3 ˆ amplified to be at full-dataset scale: λ∗ = λ0 + k N |Bt | sk (Bt ). [sent-89, score-0.393]

31 This online approach has clear computational advantages and can sometimes yield higher quality solutions than the full-data algorithm, since it conveys information between local and global parameters more frequently. [sent-92, score-0.225]

32 3 Memoized online variational inference Generalizing previous incremental variants of the expectation maximization (EM) algorithm [10], we now develop our memoized online variational inference algorithm. [sent-94, score-1.168]

33 For each batch, we maintain memoized sufficient statistics Sk = [Nk (Bb ), sk (Bb )] b=1 0 ˆk , sk (x)]. [sent-96, score-0.82]

34 We also track the full-dataset statistics Sk = [N summary statistics allow guarantees of correct full-dataset analysis while processing only one small batch at a time. [sent-98, score-0.19]

35 Our approach hinges on the fact that these sufficient statistics are additive: summaries of an entire dataset can be written exactly as the addition of summaries of distinct batches. [sent-99, score-0.25]

36 Note that our memoization of deterministic analyses of batches of data is distinct from the stochastic memoization, or “lazy” instantiation, of random variables in some Monte Carlo methods [11, 12]. [sent-100, score-0.216]

37 Memoized inference proceeds by visiting (in random order) each distinct batch once in a full pass through the data, incrementally updating the local and global parameters related to that batch b. [sent-101, score-0.684]

38 First, we update local parameters for the current batch (q(zn | rn ) for n ∈ Bb ) via Eq. [sent-102, score-0.211]

39 Next, we ˆ update cached global sufficient statistics for each component: we subtract the old (cached) summary of batch b, compute a new batch-level summary, and add the result to the full-dataset summary: 0 0 b Sk ← Sk − Sk , b Sk ← rnk , ˆ n∈Bb rnk t(xn ) , ˆ 0 0 b Sk ← Sk + Sk . [sent-104, score-0.889]

40 Unlike stochastic online algorithms, memoized inference is guaranteed to improve the full-dataset ELBO at every step. [sent-107, score-0.717]

41 In the limit where B = 1, memoized inference reduces to standard full-dataset updates. [sent-110, score-0.596]

42 However, given many batches it is far more scalable, while maintaining all guarantees of batch inference. [sent-111, score-0.292]

43 Provided we can store memoized sufficient statistics for each batch (not each observation), memoized inference has the same computational complexity as stochastic methods while avoiding noise and sensitivity to learning rates. [sent-113, score-1.346]

44 Recent analysis of convex optimization algorithms [13] demonstrated theoretical and practical advantages for methods that use cached fulldataset summaries to update parameters, as we do, instead of stochastic current-batch-only updates. [sent-114, score-0.265]

45 This memoized algorithm can compute the full-dataset objective L(q) exactly at any point (after visiting all items once). [sent-115, score-0.571]

46 To do so efficiently, we need to compute and store the assignment entropy b Hk = − n∈Bb rnk log rnk after visiting each batch b. [sent-116, score-0.762]

47 1 Birth moves to escape local optima We now propose additional birth moves that, when interleaved with conventional coordinate ascent parameter updates, can add useful new components to the model and escape local optima. [sent-121, score-0.833]

48 Previous methods [14, 9, 4] for changing variational truncations create just one extra component via a “split” move that is highly-specialized to particular likelihoods. [sent-122, score-0.216]

49 In contrast, our births add many components at once and apply to any exponential family mixture model. [sent-124, score-0.331]

50 1 6 1 7 ˆb 1) Create new Memoized summary Nk components expected count of each component 2) Adopt in one pass thru data 1 2 3 4 5 6 7 Batch b current position Batch b+1 Learn fresh DP-GMM on subsample via VB Subsample data explained by 1 0 0 0 0 0 0 0 0 0 0 ! [sent-126, score-0.456]

51 Left: Scatter plot of 2D observed data, and a subsample targeted via the first mixture component. [sent-128, score-0.178]

52 ˆb Right: Bar plots of memoized counts Nk for each batch. [sent-130, score-0.514]

53 k Creating new components in the online setting is challenging. [sent-132, score-0.183]

54 Each small batch may not have enough examples of a missing component to inspire a good proposal, even if that component is wellsupported by the full dataset. [sent-133, score-0.278]

55 We thus advocate birth moves that happen in three phases (collection, creation, and adoption) over two passes of the data. [sent-134, score-0.338]

56 1, creates new components and then updates every batch with the expanded model. [sent-137, score-0.334]

57 Successive births are interleaved; at each pass there are proposals both active and in preparation. [sent-138, score-0.242]

58 Collection During pass 1, we collect a targeted subsample x′ of the data, of size at most N ′ = 104 . [sent-141, score-0.178]

59 Creation Before pass 2, we create new components by fitting a DP mixture model with K ′ (we take K ′ = 10) components to x′ , running variational inference for a limited budget of iterations. [sent-147, score-0.625]

60 Adoption During pass 2, we visit each batch and perform local and global parameter updates for the expanded (K + K ′ )-component mixture. [sent-154, score-0.425]

61 These updates use expanded global summaries S 0 that include summaries S ′ from the targeted analysis of x′ . [sent-155, score-0.433]

62 By adding many components at once, our birth move allows rapid escape from poor local optima. [sent-160, score-0.436]

63 However, in practice merge moves reliably reject poor births and restore original configurations. [sent-162, score-0.444]

64 4, births are so effective that runs started at K = 1 recover necessary components on-the-fly. [sent-164, score-0.265]

65 2 Merge moves that optimize the full data objective The computational cost of inference grows with the number of components K. [sent-166, score-0.332]

66 To keep K small, we develop merge moves that replace two components with a single merged one. [sent-167, score-0.459]

67 Merge moves were first explored for batch variational methods [16, 14]. [sent-168, score-0.438]

68 passes thru data (N=100000) Data: 5x5 patches worst MO-BM 1. [sent-175, score-0.291]

69 passes thru data (N=100000) worst MO worst Full best SOb 0. [sent-181, score-0.228]

70 00 Figure 2: Comparison of full-data, stochastic (SO), and memoized (MO) on toy data with K = 8 true components (Sec. [sent-213, score-0.67]

71 variational inference methods have been augmented to evaluate merge proposals based on noisy, single-batch estimates of the ELBO [4]. [sent-221, score-0.479]

72 In contrast, our algorithm accurately computes the full ELBO for each merge proposal, ensuring only useful merges are accepted. [sent-225, score-0.312]

73 Given two components ka , kb to merge, we form a candidate q ′ with K − 1 components, where ˆ ˆ merged component km takes over all assignments to ka , kb : rnkm = rnka + rnkb . [sent-226, score-0.515]

74 Our merge move has three steps: select components, form the candidate configuration q ′ , and accept q ′ if the ELBO improves. [sent-230, score-0.208]

75 Selecting ka , kb to merge at random is unlikely to yield an improved configuration. [sent-231, score-0.29]

76 After choosing ka at random, we select kb using a ratio of marginal likelihoods M which compares the merged and separated configurations, easily computed with cached summaries: M (Ska + Skb ) , M (Sk ) = exp a0 (λ0 + sk (x)) . [sent-232, score-0.386]

77 (12) M (Ska )M (Skb ) Our memoized approach allows exact evaluation of the full-data ELBO to compare the existing q to merge candidate q ′ . [sent-233, score-0.695]

78 (8), evaluating L(q ′ ) is a linear function of merged sufficient N ˆ ˆ r statistics, except for the assignment entropy term: Hab = − n=1 (ˆnka + rnkb ) log(ˆnka + rnkb ). [sent-235, score-0.195]

79 r We compute this term in advance for all possible merge pairs. [sent-236, score-0.181]

80 This modest precomputation allows rapid and exact merge moves, which improve model quality and speed-up post-merge iterations. [sent-238, score-0.213]

81 p(kb | ka ) ∝ In one pass of the data, our algorithm performs a birth, memoized ascent steps for all batches, and several merges after the final batch. [sent-239, score-0.772]

82 4 Experimental results We now compare algorithms for learning DP-Gaussian mixture models (DP-GMM), using our own implementations of full-dataset, stochastic online (SO), and memoized online (MO) inference, as well as our new birth-merge memoized algorithm (MO-BM). [sent-241, score-1.3]

83 1 SOa SOb SOc Full 20 batches 100 batches MO MO−BM Kuri −3 −3. [sent-259, score-0.27]

84 components K Full 20 batches 100 batches MO MO−BM Kuri 100 80 60 40 Full MO MO−BM 0 0 40 80 120 160 200 num. [sent-270, score-0.379]

85 The ELBO trace plots show this method escaping local optima, with slight drops indicating addition of new components followed by rapid increases as these are adopted. [sent-281, score-0.24]

86 They further suggest that our fixed-truncation memoized method competes favorably with full-data inference, often converging to similar or better solutions after fewer passes through the data. [sent-282, score-0.605]

87 2 shows trace plots of GreedyMerge, a memoized online variant that instead uses only the current-batch ELBO to assess a proposed merge, as done in [4]. [sent-285, score-0.588]

88 All 5 runs of this GreedyMerge algorithm ruinously accept merges that decrease the full objective, consistently collapsing down to just one component. [sent-288, score-0.194]

89 Our memoized approach ensures merges are always globally beneficial. [sent-289, score-0.614]

90 Here, we also compare to Kurihara’s public implementation of variational inference with split moves [9]. [sent-293, score-0.363]

91 In contrast, our memoized approach consistently delivers solutions on par with full inference, with no apparent sensitivity to the number of batches. [sent-301, score-0.577]

92 With births and merges enabled, MO-BM expands from K = 1 to over 80 components, finding better solutions than every smart K = 100 initialization. [sent-302, score-0.266]

93 passes thru data (N=1880200) 300 MO−BM K=1 250 MO K=100 200 SOa K=100 150 100 50 0 0 10 20 30 40 50 60 70 80 90 100 num. [sent-326, score-0.228]

94 passes thru data (N=1880200) log evidence x109 4. [sent-327, score-0.263]

95 passes thru data (N=8640000) Figure 5: 8 × 8 image patches. [sent-339, score-0.256]

96 5 shows that our birth-merge memoized algorithm started at K = 1 can consistently add useful components and reach better solutions than alternatives. [sent-362, score-0.648]

97 5 Conclusions Our novel memoized online variational algorithm avoids noisiness and sensitivity inherent in stochastic methods. [sent-371, score-0.838]

98 Our birth and merge moves successfully escape local optima. [sent-372, score-0.553]

99 Truly nonparametric online variational inference for hierarchical Dirichlet processes. [sent-408, score-0.37]

100 Bayesian model search for mixture models based on optimizing variational bounds. [sent-472, score-0.248]


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