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Title: Minimizing FLOPs to Learn Efficient Sparse Representations

Paper content:
Deep representation learning has become one of the most widely adopted approaches for visual search, recommendation, and identification.
Retrieval of such  representations from a large database is however computationally challenging.
Approximate methods based on learning compact representations, have been widely explored for this problem, such as locality sensitive hashing, product quantization, and PCA.
In this work, in contrast to learning compact representations, we propose to learn high dimensional and sparse representations that have similar representational capacity as dense embeddings while being more efficient due to sparse matrix multiplication operations which can be much faster than dense multiplication.
Following the key insight that the number of operations decreases quadratically with the sparsity of embeddings provided the non-zero entries are distributed uniformly across dimensions, we propose a novel approach to learn such distributed sparse embeddings via the use of a carefully constructed regularization function that directly minimizes a continuous relaxation of the number of floating-point operations (FLOPs) incurred during retrieval.
Our experiments show that our approach is competitive to the other baselines and yields a similar or better speed-vs-accuracy tradeoff on practical datasets.
Learning semantic representations using deep neural networks (DNN) is now a fundamental facet of applications ranging from visual search (Jing et al., 2015; Hadi Kiapour et al., 2015) , semantic text matching (Neculoiu et al., 2016) , oneshot classification (Koch et al., 2015) , clustering (Oh Song et al., 2017) , and recommendation (Shankar et al., 2017) .
The high-dimensional dense embeddings generated from DNNs however pose a computational challenge for performing nearest neighbor search in large-scale problems with millions of instances.
In particular, when the embedding dimension is high, evaluating the distance of any query to all the instances in a large database is expensive, so that efficient search without sacrificing accuracy is difficult.
Representations generated using DNNs typically have a higher dimension compared to hand-crafted features such as SIFT (Lowe, 2004) , and moreover are dense.
The key caveat with dense features is that unlike bag-of-words features they cannot be efficiently searched through an inverted index, without approximations.
Since accurate search in high dimensions is prohibitively expensive in practice (Wang, 2011) , one has to typically sacrifice accuracy for efficiency by resorting to approximate methods.
Addressing the problem of efficient approximate Nearest-Neighbor Search (NNS) (Jegou et al., 2011) or Maximum Inner-Product Search (MIPS) (Shrivastava and Li, 2014) is thus an active area of research, which we review in brief in the related work section.
Most approaches (Charikar, 2002; Jegou et al., 2011) aim to learn compact lower-dimensional representations that preserve distance information.
While there has been ample work on learning compact representations, learning sparse higher dimensional representations have been addressed only recently (Jeong and Song, 2018; Cao et al., 2018) .
As a seminal instance, Jeong and Song (2018) propose an end-to-end approach to learn sparse and high-dimensional hashes, showing significant speed-up in retrieval time on benchmark datasets compared to dense embeddings.
This approach has also been motivated from a biological viewpoint (Li et al., 2018) by relating to a fruit fly's olfactory circuit, thus suggesting the possibility of hashing using higher dimensions instead of reducing the dimensionality.
Furthermore, as suggested by Glorot et al. (2011) , sparsity can have additional advantages of linear separability and information disentanglement.
In a similar vein, in this work, we propose to learn high dimensional embeddings that are sparse and hence efficient to retrieve using sparse matrix multiplication operations.
In contrast to compact lowerdimensional ANN-esque representations that typically lead to decreased representational power, a key facet of our higher dimensional sparse embeddings is that they can have the same representational capacity as the initial dense embeddings.
The core idea behind our approach is inspired by two key observations:
(i) retrieval of d (high) dimensional sparse embeddings with fraction p of non-zero values on an average, can be sped up by a factor of 1/p.
(ii) The speed up can be further improved to a factor of 1/p 2 by ensuring that the non-zero values are evenly distributed across all the dimensions.
This indicates that sparsity alone is not sufficient to ensure maximal speedup; the distribution of the non-zero values plays a significant role as well.
This motivates us to consider the effect of sparsity on the number of floating point operations (FLOPs) required for retrieval with an inverted index.
We propose a penalty function on the embedding vectors that is a continuous relaxation of the exact number of FLOPs, and encourages an even distribution of the non-zeros across the dimensions.
We apply our approach to the large scale metric learning problem of learning embeddings for facial images.
Our training loss consists of a metric learning (Weinberger and Saul, 2009 ) loss aimed at learning embeddings that mimic a desired metric, and a FLOPs loss to minimize the number of operations.
We perform an empirical evaluation of our approach on the Megaface dataset (Kemelmacher-Shlizerman et al., 2016) , and show that our proposed method successfully learns high-dimensional sparse embeddings that are orders-of-magnitude faster.
We compare our approach to multiple baselines demonstrating an improved or similar speed-vs-accuracy trade-off.
The rest of the paper is organized as follows.
In Section 3 we analyze the expected number of FLOPs, for which we derive an exact expression.
In Section 4 we derive a continuous relaxation that can be used as a regularizer, and optimized using gradient descent.
We also provide some analytical justifications for our relaxation.
In Section 5 we then compare our method on a large metric learning task showing an improved speed-accuracy trade-off compared to the baselines.
In this paper we proposed a novel approach to learn high dimensional embeddings with the goal of improving efficiency of retrieval tasks.
Our approach integrates the FLOPs incurred during retrieval into the loss function as a regularizer and optimizes it directly through a continuous relaxation.
We provide further insight into our approach by showing that the proposed approach favors an even distribution of the non-zero activations across all the dimensions.
We experimentally showed that our approach indeed leads to a more even distribution when compared to the 1 regularizer.
We compared our approach to a number of other baselines and showed that it has a better speed-vs-accuracy trade-off.
Overall we were able to show that sparse embeddings can be around 50× faster compared to dense embeddings without a significant loss of accuracy.
Proof.
Follows directly from Lemma 3.
Lemma 5.
Proof.
Follows directly from Lemma 2.

assistant: text: We propose an approach to learn sparse high dimensional representations that are fast to search, by incorporating a surrogate of the number of operations directly into the loss function.