summaries: This paper presents an approach to jointly pre-train language models and representations for knowledge graphs. In particular, natural language texts (English Wikipedia) are used to train context representations, while knowledge graphs (Wikidata) train entity representations (and both depend on each other). Experiments show that the approach outperforms baseline methods on several natural language understanding tasks: few-shot relation classification, knowledge graph question answering, and entity classification.  This work proposes a method for joint pre-training of knowledge graph and text data which embeds KG entities and relations into shared latent semantic space as entity embeddings from text. The proposed model JAKET consists of two main parts: a language module and a knowledge module. The model is pre-trained on a collection of tasks: entity category prediction, relation type prediction, masked token prediction and masked entity prediction. The proposed framework enables fine-tuning on knowledge graphs which are unseen during pre-training. This paper proposed a new language modeling pretraining method that leverages the knowledge graph information. Specifically, the paper replaces the entity embedding in one hidden layer of BERT context embedding, with the corresponding graph attention embedding that is obtained from the knowledge graph. The pretraining tasks contain not only the language related tasks (like predicting masked tokens), but also the knowledge graph tasks like entity classification or relation type prediction. Experiments on few-shot learning tasks, question answering and entity classification show better performance over other pretraining counterparts. 
abstractText: Knowledge graphs (KGs) contain rich information about world knowledge, entities, and relations. Thus, they can be great supplements to existing pre-trained language models. However, it remains a challenge to efficiently integrate information from KG into language modeling. And the understanding of a knowledge graph requires related context. We propose a novel joint pre-training framework, JAKET, to model both the knowledge graph and language. The knowledge module and language module provide essential information to mutually assist each other: the knowledge module produces embeddings for entities in text while the language module generates context-aware initial embeddings for entities and relations in the graph. Our design enables the pre-trained model to easily adapt to unseen knowledge graphs in new domains. Experimental results on several knowledge-aware NLP tasks show that our proposed framework achieves superior performance by effectively leveraging knowledge in language understanding.
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sections.heading: 1 INTRODUCTION
sections.text: Pre-trained language models (PLM) leverage large-scale unlabeled corpora to conduct selfsupervised training. They have achieved remarkable performance in various NLP tasks, exemplified by BERT (Devlin et al., 2018), RoBERTa (Liu et al., 2019b), XLNet (Yang et al., 2019), and GPT series (Radford et al., 2018; 2019; Brown et al., 2020). It has been shown that PLMs can effectively characterize linguistic patterns in text and generate high-quality context-aware representations (Liu et al., 2019a). However, these models struggle to grasp world knowledge about entities and relations (Poerner et al., 2019; Talmor et al., 2019), which are very important in language understanding.
  Knowledge graphs (KGs) represent entities and relations in a structural way. They can also solve the sparsity problem in text modeling. For instance, a language model may require tens of instances of the phrase “labrador is a kind of dog” in its training corpus before it implicitly learns this fact. In comparison, a knowledge graph can use two entity nodes “labrador”, “dog” and a relation edge “is a” between these nodes to precisely represent this fact.
  Recently, some efforts have been made to integrate knowledge graphs into PLM. Most of them combine the token representations in PLM with representations of aligned KG entities. The entity embeddings in those methods are either pre-computed based on an external source by a separate model (Zhang et al., 2019; Peters et al., 2019), which may not be easily aligned with the language representation space, or directly learned as model parameters (Févry et al., 2020; Verga et al., 2020), which often have an over-parameterization issue due to the large number of entities. Moreover, all the previous works share a common challenge: when the pre-trained model is fine-tuned in a new domain with a previously unseen knowledge graph, it struggles to adapt to the new entities, relations and structure.
  Therefore, we propose JAKET, a Joint pre-trAining framework for KnowledgE graph and Text. Our framework contains a knowledge module and a language module, which mutually assist each other by providing required information to achieve more effective semantic analysis. The knowledge module leverages a graph attention network (Veličković et al., 2017) to provide structure-aware entity embeddings for language modeling. And the language module produces contextual representations as initial embeddings for KG entities and relations given their descriptive text. Thus, in both modules, content understanding is based on related knowledge and rich context. On one hand, the joint pre-training effectively projects entities/relations and text into a shared semantic latent space,
  which eases the semantic matching between them. On the other hand, as the knowledge module produces representations from descriptive text, it solves the over-parameterization issue since entity embeddings are no longer part of the model’s parameters.
  In order to solve the cyclic dependency between the two modules, we propose a novel two-step language module LM1 and LM2, respectively. LM1 provides embeddings for both LM2 and KG. The entity embeddings from KG are also fed into LM2, which produces the final representation. LM1 and LM2 can be easily established as the first several transformer layers and the rest layers of a pre-trained language model such as BERT and RoBERTa. Furthermore, we design an entity context embedding memory with periodic update which speeds up the pre-training by 15x.
  The pre-training tasks are all self-supervised, including entity category classification and relation type prediction for the knowledge module, and masked token prediction and masked entity prediction for the language module.
  A great benefit of our framework is that it can easily adapt to unseen knowledge graphs in the finetuning phase. As the initial embeddings of entities and relations come from their descriptive text, JAKET is not confined to any fixed KG. With the learned ability to integrate structural information during pre-training, the framework is extensible to novel knowledge graphs with previously unseen entities and relations, as illustrated in Figure 1.
  We conduct empirical studies on several knowledge-aware natural language understanding (NLU) tasks, including few-shot relation classification, question answering and entity classification. The results show that JAKET achieves the best performance compared with strong baseline methods on all the tasks, including those with a previously unseen knowledge graph.
sections.heading: 2 RELATED WORK
sections.text: Pre-trained language models have been shown to be very effective in various NLP tasks, including ELMo (Peters et al., 2018), GPT (Radford et al., 2018), BERT (Devlin et al., 2018), RoBERTa (Liu et al., 2019b) and XLNet (Yang et al., 2019). Built upon large-scale corpora, these pretrained models learn effective representations for various semantic structures and linguistic relationships. They are trained on self-supervised tasks like masked language modeling and next sentence prediction.
  Recently, a lot of efforts have been made on investigating how to integrate knowledge into PLMs (Levine et al., 2019; Soares et al., 2019; Liu et al., 2020; Guu et al., 2020). These approaches can be grouped into two categories:
  1. Explicitly injecting entity representation into the language model, where the representations are either pre-computed from external sources (Zhang et al., 2019; Peters et al., 2019) or directly learned as model parameters (Févry et al., 2020; Verga et al., 2020). For example, ERNIE (THU) (Zhang et al., 2019) pre-trains the entity embeddings on a knowledge graph using TransE (Bordes et al., 2013), while EAE (Févry et al., 2020) learns the representation from pre-training objectives with all the other model parameters. K-BERT (Liu et al., 2020) represents the entities by the embeddings of surface form tokens (i.e. entity names), which contains much less semantic information compared with description text. Moreover, it only injects KG during fine-tuning phase instead of joint-pretraining KG and text.
  2. Implicitly modeling knowledge information, including entity-level masked language modeling (Sun et al., 2019b; Shen et al., 2020), entity-based replacement prediction (Xiong et al., 2019) and knowledge embedding loss as regularization (Wang et al., 2019b). For example, besides tokenlevel masked language modeling, ERNIE (Baidu) (Sun et al., 2019b) uses phrase-level and entitylevel masking to predict all the masked slots. KEPLER (Wang et al., 2019b) calculates entity embeddings using a pre-trained language model based on the description text, which is similar to our work. However, they use the entity embeddings for the knowledge graph completion task instead of injecting them into the language model.
  Some works (Ding et al., 2019; Lv et al., 2020) investigated the combination of GNN and PLM. For example, Lv et al. (2020) uses XLNet to generate initial node representation based on node context and feeds them into a GNN. However, these approaches do not integrate knowledge into language modeling, and they are designed for specific NLP tasks such as reading comprehension or commonsense reasoning. In comparison, we jointly pre-train both the knowledge graph representation and language modeling and target for general knowledge-aware NLU tasks.
sections.heading: 3 METHOD
sections.text: In this section, we introduce the JAKET framework of joint pre-training knowledge graph and language understanding. We begin by defining the mathematical notations, and then present our model architecture with the knowledge module and language module. Finally, we introduce how to pretrain our model and fine-tune it for downstream tasks. The framework is illustrated in Figure 2.
sections.heading: 3.1 DEFINITION
sections.text: A knowledge graph is denoted by KG = (E ,R, T ), where E = {e1 . . . eN} is the set of entities and R = {r1 . . . rP } is the set of relations. T = {(et1i , rt2i , et3i )|1 ≤ i ≤ T, et1i , et3i ∈ E , rt2i ∈ R} stands for the set of head-relation-tail triplets. Nv = {(r, u)|(v, r, u) ∈ T } represents the set of neighboring relations and entities of an entity v.
  We define V = {[MASK], [CLS], [EOS], w1 . . . wV } as a vocabulary of tokens and the contextual text x = [x1, x2, . . . , xL] as a sequence of tokens where xi ∈ V . In the vocabulary, [MASK] is the special token for masked language modeling (Devlin et al., 2018) and [CLS], [EOS] are the special tokens indicating the beginning and end of the sequence. We define F as the dimension of token embeddings, which is equal to the dimension of entity/relation embeddings from the KG.
  The text x has a list of entity mentions m = [m1, . . . ,mM ], where each mention mi = (emi , smi , omi): emi is the corresponding entity and smi , omi are the start and end index of this
  mention in the context. In other words, [xsmi , . . . , xomi ] is linked with entity emi 1. We assume the span of mentions are disjoint for a given text sequence.
  As entities in the knowledge graph are represented by nodes without context, we use entity description text to describe the concept and meaning of entities. For each entity ei, its description text xei describes this entity. The mention of ei in xei is denoted as mei = (ei, sei , o e i ), similarly defined as above. For instance, the description text for the entity “sun” can be “[CLS] The Sun is the star at the center of the Solar System [EOS]”. Then the mention is mSun = (Sun, 3, 3). If there are multiple mentions of ei in its description text, we choose the first one. If there’s no mention of ei in its description text, we set sei = o e i = 1. Similarly, we define relation description text as the text that can describe each relation.
sections.heading: 3.2 KNOWLEDGE MODULE
sections.text: The goal of the knowledge module (KM) is to model the knowledge graph to generate knowledgebased entity representations.
  To compute entity node embeddings, we employ the graph attention network (GAT) (Veličković et al., 2017), which uses the self-attention mechanism to specify different weights for different neighboring nodes. However, the vanilla GAT is designed for homogeneous graphs with singlerelation edges. To leverage the multi-relational information, we adopt the idea of composition operator (Vashishth et al., 2019) to compose entity embeddings and relation embeddings. In detail, in the l-th layer of LM, we update the embedding E(l)v of entity v as follows:
  E(l)v = LayerNorm  K⊕ k=1 σ  ∑ (r,u)∈Nv αkv,r,uW kf(E(l−1)u , Rr) + E(l−1)v  (1)
  αkv,r,u = exp
  ( LeakyReLU ( aT [ W kE (l−1) v ⊕W kf(E(l−1)u , Rr) ])) ∑
  (r′,u′)∈Nv exp ( LeakyReLU ( aT [ W kE (l−1) u ⊕W kf(E(l−1)u′ , Rr′) ])) (2) where LayerNorm stands for layer normalization (Ba et al., 2016). ⊕ means concatenation and K
  is the number of attention heads. W k is the model parameter and Rr is the embedding of relation r. Note that the relation embeddings are shared across different layers. The function f(·, ·) : RF × RF → RF merges a pair of entity and relation embeddings into one representation. Here, we set f(x, y) = x + y inspired by TransE (Bordes et al., 2013). More complicated functions like MLP network can also be applied.
  The initial entity embeddings E(0) and relation embeddings R are generated from our language module, which will be introduced in Section 3.4. Then, the output entity embeddings from the last GAT layer are used as the final entity representations EKM. Note that the knowledge graph can be very large, making the embedding update over all the entities in Equation (1) not tractable. Thus we follow the minibatch setting (Hamilton et al., 2017): given a set of input entities, we perform neighborhood sampling to generate their multi-hop neighbor sets and we compute representations only on the entities and relations that are necessary for the embedding update.
sections.heading: 3.3 LANGUAGE MODULE
sections.text: The goal of the language module (LM) is to model text data and learn context-aware representations. The language module can be any model for language understanding, e.g. BERT (Devlin et al., 2018). In this work, we use the pre-trained model RoBERTa-base (Liu et al., 2019b) as the language module.
sections.heading: 3.4 SOLVING THE CYCLIC DEPENDENCY
sections.text: In our framework, the knowledge and language modules mutually benefit each other: the language module LM outputs context-aware embedding to initialize the embeddings of entities and relations in the knowledge graph given the description text; the knowledge module (KM) outputs knowledgebased entity embeddings for the language module.
  1We do not consider discontinous entity mentions in this work.
  However, there exists a cyclic dependency which prevents computation and optimization in this design. To solve this problem, we propose a decomposed language module which includes two language models: LM1 and LM2. We employ the first 6 layers of RoBERTa as LM1 and the remaining 6 layers as LM2. The computation proceeds as follows:
  1. LM1 operates on the input text x and generates contextual embeddings Z.
  2. LM1 generates initial entity and relation embeddings for KM given description text.
  3. KM produces its output entity embeddings to be combined with Z and sent into LM2.
  4. LM2 produces the final embeddings of x, which includes both contextual and knowledge information.
  In detail, in step 1, suppose the context x is embedded as Xembed. LM1 takes Xembed as input and outputs hidden representations Z = LM1(Xembed).
  In step 2, suppose xej is the entity description text for entity ej , and the corresponding mention is mej = (ej , s e j , o e j). LM1 takes the embedding of x
  ej and produces the contextual embedding Zej . Then, the average of embeddings at position sej and o e j is used as the initial entity embedding of ej , i.e. E(0)j = (Z ej sej +Z ej oej )/2. The knowledge graph relation embeddings R are generated in a similar way using its description text.
  In step 3, KM computes the final entity embeddingsEKM, which is then combined with the output Z from LM1. In detail, suppose the mentions in x are m = [m1, . . . ,mM ]. Z and EKM are combined at positions of mentions:
  Zmergek =
  { Zk + E
  KM emi if ∃i s.t. smi ≤ k ≤ omi Zk otherwise
  (3)
  whereEKMemi is the output embedding of entity emi from KM. Then we apply layer normalization (Ba et al., 2016) on Zmerge: Z ′ = LayerNorm(Zmerge). Finally, Z ′ is fed into LM2.
  In step 4, LM2 operates on the input Z ′ and obtains the final embeddings ZLM = LM2(Z ′). The four steps are marked by the symbol X© in Figure 2 for better illustration.
sections.heading: 3.5 ENTITY CONTEXT EMBEDDING MEMORY
sections.text: Many knowledge graphs contain a large number of entities. Thus, even for one sentence, the number of entities plus their multi-hop neighbors can grow exponentially with the number of layers in the graph neural network. As a result, it’s very time-consuming for the language module to compute context embeddings based on the description text of all involved entities in a batch on the fly.
  To solve this problem, we construct an entity context embedding memory, Econtext, to store the initial embeddings of all KG entities. Firstly, the language module pre-computes the context embeddings for all entities and places them into the memory. The knowledge module only needs to retrieve required embeddings from the memory instead of computing them, i.e. E(0) ← Econtext. However, as embeddings in the memory are computed from the “old” (initial) language module while the token embeddings during training are computed from the updated language module, there will be an undesired discrepancy. Thus, we propose to update the whole embedding memory Econtext with the current language module every T (i) steps, where i is the number of times that the memory has been updated (starting from 0). T (i) is set as follows:
  T (i) = min(Iinit ∗ abi/rc, Imax) (4)
  where Iinit is the initial number of steps before the first update and a is the increasing ratio of updating intervals. r is the number of repeated times of the current updating interval. Imax is the maximum number of steps between updates. b·c means the operation of rounding down. In our experiments, we set Iinit = 10, a = 2, r = 3, Imax = 500, and the corresponding sequence of T is [10, 10, 10, 20, 20, 20, 40, 40, 40, . . . , 500, 500]. Note that we choose a > 1 because the model parameters usually change less as training proceeds.
  Moreover, we propose a momentum update to make Econtext evolve more smoothly. Suppose the newly calculated embedding memory by LM is Econtextnew , then the updating rule is:
  Econtext ← mEcontext + (1−m)Econtextnew , (5)
  where m ∈ [0, 1) is a momentum coefficient which is set as 0.8 in experiment. This memory design speeds up our model by about 15x during pre-training while keeping the effectiveness of entity context embeddings. For consideration of efficiency, we use relation embeddings only during fine-tuning.
sections.heading: 3.6 PRE-TRAINING
sections.text: During pre-training, both the knowledge module and language module are optimized based on several self-supervised learning tasks listed below. The examples of all the training tasks are shown in Figure 2.
  At each pre-training step, we first sample a batch of root entities and perform random-walk sampling on each root entity. The sampled entities are fed into KM for the following two tasks.
  Entity category prediction. The knowledge module is trained to predict the category label of entities based on the output entity embeddings EKM. The loss function is cross-entropy for multiclass classification, denoted as Lc. Relation type prediction. KM is also trained to predict the relation type between a given entity pair based on EKM. The loss function is cross-entropy for multi-class classification, denoted as Lr. Then, we uniformly sample a batch of text sequences and their entities for the following two tasks.
  Masked token prediction. Similar to BERT, We randomly mask tokens in the sequence and predict the original tokens based on the output ZLM of the language module. We denote the loss as Lt. Masked entity prediction. The language module is also trained to predict the corresponding entity of a given mention. For the input text, we randomly remove 15% of the mentions m. Then for each removed mention mr = (er, sr, or), the model predicts the masked entity er based on the mention’s embedding. In detail, it predicts the entity whose embedding in Econtext is closest to q = g((ZLMsr + Z LM or )/2), where g(x) = GELU(xW1)W2 is a transformation function. GELU is an activation function proposed by Hendrycks & Gimpel (2016). Since the number of entities can be very large, we use er’s neighbours and other randomly sampled entities as negative samples. The loss function Le is cross entropy based on the inner product between q and each candidate entity’s embedding. Figure 2 shows an concrete example, where the mention “Earth” is not marked in the input text since it’s masked and the task is to link the mention “Earth” to entity “Q2: Earth”.
sections.heading: 3.7 FINE-TUNING
sections.text: During fine-tuning, our model supports using either the knowledge graph employed during pretraining or a novel custom knowledge graph with previously unseen entities2. If a custom KG is used, the entity context embedding memory is recomputed by the pre-trained language module using the new entity description text. In this work, we do not update the entity context memory during fine-tuning for consideration of efficiency. We also compute the relation context embedding memory using the pre-trained language model.
sections.heading: 4 EXPERIMENT
sections.heading: 4.1 BASIC SETTINGS
sections.text: Data for Pre-training. We use the English Wikipedia as the text corpus, Wikidata (Vrandečić & Krötzsch, 2014) as the knowledge graph, and SLING (Ringgaard et al., 2017) to identify entity mentions. For each entity, we use the first 64 consecutive tokens of its Wikipedia page as its description text and we filter out entities without a corresponding Wikipedia page. We also remove entities that
  2We assume the custom domain comes with NER and entity linking tools which can annotate entity mentions in text. The training of these systems is beyond the scope of this work.
  have fewer than 5 neighbors in the Wikidata KG and fewer than 5 mentions in the Wikipedia corpus. The final knowledge graph contains 3,657,658 entities, 799 relations and 20,113,978 triplets. We use the instance of relation to find the category of each entity. In total, 3,039,909 entities have category labels of 19,901 types. The text corpus contains about 4 billion tokens.
  Implementation Details. We initialize the language module with the pre-trained RoBERTabase (Liu et al., 2019b) model. The knowledge module is initialized randomly. Our implementation is based on the HuggingFace framework (Wolf et al., 2019) and DGL (Wang et al., 2019a). For the knowledge module, we use a 2-layer graph neural network, which aggregates 2-hop neighbors. The number of sampled neighbors in each hop is 10. More details are presented in the Appendix.
  Baselines. We compare our proposed model JAKET with the pre-trained RoBERTa-base (Liu et al., 2019b) and two variants of our model: RoBERTa+GNN and RoBERTa+GNN+M. The two models have the same model structure as JAKET, but they are not pre-trained on our data. Moreover, the entity and relation context embedding memories of RoBERTa+GNN are randomly generated while the memories of RoBERTa+GNN+M are computed by RoBERTa.
sections.heading: 4.2 DOWNSTREAM TASKS
sections.text: Few-shot Relation Classification. Relation classification requires the model to predict the relation between two entities in text. Few-shot relation classification takes the N -way K-shot setting. Relations in the test set are not seen in the training set. For each query instance, N relations with K supporting examples for each relation are given. The model is required to classify the instance into one of the N relations based on the N × K samples. In this paper we evaluate our model on FewRel (Han et al., 2018), which is a widely used benchmark dataset for few-shot relation classification, containing 100 relations and 70,000 instances.
  We use the pre-trained knowledge graph for FewRel as it comes with entity mentions from Wikidata knowledge graph. To predict the relation label, we build a sequence classification layer on top of the output of LM. More specifically, we use the PAIR framework proposed by Gao et al. (2019), which pairs each query instance with all the supporting instances, concatenate each pair as one sequence, and send the concatenated sequence to our sequence classification model to get the score of the two instances expressing the same relation. We do not use relation embeddings in this task to avoid information leakage.
  As shown in Table 1, our model achieves the best results in all three few-shot settings. Comparing the results between RoBERTa and RoBERTa+GNN, we see that adding GNN with randomly generated entity features does not improve the performance. The difference between RoBERTa+GNN+M and RoBERTa+GNN demonstrates the importance of generating context embedding memory by the language module, while JAKET can further improve the performance by pre-training. We also compare with a strong knowledge-enhanced PLM KnowBERT (Peters et al., 2019), which is also pretrained on English Wikipedia and Wikidata KG. The results show that JAKET consistently outperform KnowBERT in different few-shot settings.
  KGQA. The Question Answering over KG (KGQA) task is to answer natural language questions related to a knowledge graph. The answer to each question is an entity in the KG. This task requires an understanding over the question and reasoning over multiple entities and relations.
  We use the vanilla version of the MetaQA (Zhang et al., 2017) dataset, which contains questions requiring multi-hop reasoning over a novel movie-domain knowledge graph. The KG contains 135k triplets, 43k entities and 9 relations. Each question is provided with one entity mention and the
  Model KG-Full KG-50% 1-hop 2-hop 1-hop 2-hop RoBERTa 90.2 70.8 61.5 39.3 RoB+G+M 91.4 72.6 62.5 40.8 JAKET 93.9 73.2 63.1 41.9
  Table 2: Results on the MetaQA dataset over 1- hop and 2-hop questions under KG-Full and KG50% settings. RoB+G+M is the abbreviation for the baseline model RoBERTa+GNN+M.
  Model 100% 20% 5% GNN 48.2 - - RoBERTa 33.4 - - RoB+G+M 79.1 66.7 53.5 JAKET 81.6 70.6 58.4
  Table 3: Results on the entity classification task over an unseen Wikidata knowledge graph. RoB+G+M is the abbreviation for the baseline model RoBERTa+GNN+M.
  question is named as a k-hop question if the answer entity is a k-hop neighbor of the question entity. We define all the k-hop neighbor entities of the question entity as the candidate entities for the question. We also consider a more realistic setting where we simulate an incomplete KG by randomly dropping a triplet with a probability 50%. This setting is called KG-50%, compared with the full KG setting KG-Full. For each entity, we randomly sample one question containing it as the entity’s description context. We manually write the description for each relation since the number of relations is very small. We use the output embedding of [CLS] token from LM as the question embedding, and then find the entity with the closest context embedding.
  As shown in Table 2, RoBERTa+GNN+M outperforms RoBERTa, demonstrating the effectiveness of KM+LM structure. JAKET further improves the accuracy by 0.6% to 2.5% under both KG settings, showing the benefits of our proposed joint pre-training.3
  Entity Classification. To further evaluate our model’s capability to reason over unseen knowledge graphs, we design an entity classification task. Here, the model is given a portion of the Wikidata knowledge graph unseen during pre-training, denoted as KG′. It needs to predict the category labels of these novel entities. The entity context embeddings are obtained in the same way as in pretraining. The relation context embeddings are generated by its surface text. The number of entities and relations in the KG′ are 23,046 and 316 respectively. The number of triplets is 38,060. Among them, 16,529 entities have 1,291 distinct category labels. We conduct experiments under a semisupervised transductive setting by splitting the entities in KG′ into train/dev/test splits of 20%, 20% and 60%. To test the robustness of models to the size of training data, we evaluate models when using 20% and 5% of the original training set.
  In this task, RoBERTa takes the entity description text as input for label prediction while neglecting the structure information of KG. JAKET and RoBERTa+GNN+M make predictions based on the entity representation output from the knowledge module. We also include GNN as a baseline, which uses the same GAT-based structure as our knowledge module, but with randomly initialized model parameters and context embedding memory. GNN then employs the final entity representations for entity category prediction.
  As shown in Table 3, our model achieves the best performance under all the settings. The performance of GNN or RoBERTa alone is significantly lower than JAKET and RoBERTa+GNN+M, which demonstrates the importance of integrating both context and knowledge information using our proposed framework. Also, the gap between JAKET and RoBERTa+GNN+M increases when there’s less training data, showing that the joint pre-training can reduce the model’s dependence on downstream training data.
sections.heading: 5 CONCLUSION
sections.text: This paper presents a novel framework, JAKET, to jointly pre-train models for knowledge graph and language understanding. Under our framework, the knowledge module and language module both provide essential information for each other. After pre-training, JAKET can quickly adapt to unseen knowledge graphs in new domains. Moreover, we design the entity context embedding memory which speeds up the pre-training by 15x. Experiments show that JAKET outperforms baseline methods in several knowledge-aware NLU tasks: few-shot relation classification, KGQA and entity classification. In the future, we plan to extend our framework to natural language generation tasks.
  3For fair comparison, we do not include models which incorporate a dedicated graph retrieval module (Sun et al., 2018; 2019a)
sections.heading: A APPENDIX
sections.text: A.1 IMPLEMENTATION DETAILS
  The dimension of hidden states in the knowledge module is 768, the same as RoBERTa-base, and the number of attention heads is 8. During pre-training, the batch size and length of text sequences are 1024 and 512 respectively. The batch size of KG entities is 16,384. The number of training epochs is 8. JAKET is optimized by AdamW (Loshchilov & Hutter, 2017) using the following parameters: β1 = 0.9, β2 = 0.999, = 1e-8, and weight decay of 0.01. The learning rate of the language module is warmed up over the first 3,000 steps to a peak value of 1e-5, and then linearly decayed. The learning rate of our knowledge module starts from 1e-4 and then linearly decayed.
  A.2 COMPUTATION ANALYSIS
  The computation of the KG module is much less than the LM module. For BERT-base or RoBERTabase, the number of inference computation flops (#flops) over each sequence (length 128) is over 22 billion [1, 2]. Here, we theoretically compute the number of flops of the KG module as follows: The sequence length N = 128, and hidden dimension H = 768. The number of entities in a sequence is usually less than N/5. The number of sampled neighbors per entity r = 10. And the number of layers of the GNN based KG module L = 2. It follows that the #flops of KG module is about N/5× rL × 2H2 ≈ 3 billion, less than 1/7 of LM computation. If we set r = 5, the #flops can be further reduced to about 1/30 of LM computation.
  During pre-training, another computation overhead is entity context embedding memory update (Section 3.5): Firstly, the number of entities is about 3 million and the update step interval is 500. Thus for each step on average the model processes the description text of 3e6/500 = 6e3 entities. Secondly, the length of description text is 64, much smaller than the length of input text 512, and we only use LM1 (the first half of LM module) for entity context embedding generation, which saves half of the computation time compared to using the whole LM module. Thirdly, the embedding update only requires forward propagation, costing only half of computation compared to training process which requires both forward and backward propagation. Thus, generating context embedding of 6k entities consumes about the same number of flops as training 6000× 64/(512× 2× 2) ≈ 200 input texts, much smaller than the batch size 1024. In short, the entity context embedding memory update only costs 200/1024 ≈ 1/5 additional computation. Note this computation overhead only exists during pre-training, since entity embedding memory is not updated when fine-tuning.