# Representative Data Selection for Sequence-to-Sequence Pre-training

Haiyue Song ${ }^{1,2}$ Raj Dabre ${ }^{2}$ Zhuoyuan Mao $^{1} \quad$ Chenhui Chu $^{1} \quad$ Sadao Kurohashi $^{1}$
${ }^{1}$ Kyoto University ${ }^{2}$ NICT
\{song, zhuoyuanmao, chu, kuro\}@nlp.ist.i.kyoto-u.ac.jp
$\{$ raj.dabre $\}$ @nict.go.jp
}

\begin{abstract}
Pre-trained sequence-to-sequence models such as BART [1] have helped improve natural language generation quality. However, training large models is resourceconsuming. We propose a data selection algorithm that selects a tiny but representative subset from billion-scale datasets. Experimental results show that pre-training with $0.26 \%$ data and $4.4 \%$ energy consumption achieves about 90\% BLEU scores on machine translation (MT) tasks and ROUGE scores on text summarization tasks, compared to pre-training on the entire dataset. Compared to random selection baselines, it shows lower perplexity (PPL), higher BLEU and ROUGE scores.
\end{abstract
## 1 Introduction

Pre-training and then fine-tuning is a widely-used paradigm for natural language processing $[2,3]$. However, training pre-trained models such as BART $[1,4]$, IndicBART [5], mT5 [6] usually takes hundreds to thousands of GPU days. Previous works focus on reducing the parameters of the model [7], but there are very few studies [8] related to shrinking the dataset, which can also reduce computational costs.

In this work, we propose a clustering-based representative data selection algorithm. As illustrated in Figure 1, we first convert discrete sentences into continuous embeddings. Then, we perform large-scale and efficient clustering of the sentences based on the embeddings. From each cluster, we select several centroid points according to the scale of the cluster. The centroids from each cluster are combined to form the representative subset. Furthermore, we propose to combine an unsupervised outlier detection method to remove noisy data points. We calculate the

Figure 1: Centroids of clusters as representative data. Each component stands for one cluster with close sentence embeddings. Mapped to 2-D with t-SNE [9].

center point of the entire embedding space and filter out points distant from the center. Experimental results show that with $0.26 \%$ data of the entire dataset and $4.4 \%$ energy comsumption, it can obtain relatively high performance on MT and text summarization tasks, only 2 to 4 points lower in terms of BLEU and ROUGE scores.

## 2 Related Work

Supervised Data Selection In-domain data selection $[10,11,12]$ focuses on extracting sentences from a large general-domain corpus that are most relevant to a target domain. Trusted data or clean selection $[13,14]$ aims to select trusted (clean) data from a general-domain corpus given a small trusted (clean) dataset. They all rank data according to the cross-entropy difference [10] between a general language model (LM) and a target LM, where the
target LM is trained on in-domain data, trusted data, or clean data. However, they require supervision and only solve one particular downstream task.

## Small Scale Representative Data Selection

Representative data selection finds a small subset of the original dataset that captures the most information. Previous methods include calculating the mutual information and relative entropy [15], converting to a sparse multiple measurement vector problem [16]. They are slow and require large memory, therefore, can only handle approximately 10k data points; however, billions of sentences are used in pre-training.

## 3 Representative Data Selection

We introduce the representative data selection approach to extract a fraction from a multi-million to billion scale dataset. It consists of the following steps:

Continuous Embedding Conversion In order to perform clustering, we first convert discrete data such as sentences into continuous representations in a common space. Suppose there is a set containing $n$ sentences $S=\left.\{s_{1}, \ldots, s_{n}\right.\}$. We convert $S$ into embedding set $E=\left.\{e_{1}, \ldots, e_{n}\right.\}$ as following:

$
e_{i}=f\left(s_{i} \mid \theta\right), \quad f: \mathbb{S} \rightarrow \mathbb{R}^{d}
$

where $f$ denotes the sentence-to-vector model, $\theta$ is the parameters of $f, d$ is the dimension of the output vector and $\mathbb{S}$ is a set of all the natural language sentences. Here $f$ can be sent2vec [17] or sentBERT [18].

Outlier Detection We apply an outlier detection algorithm [19] to eliminate noisy data in an unsupervised manner. We first calculate the center of the embedding space $e_{c}$ and filter outliers whose distance from $e_{c}$ is greater than two standard deviations. The de-noised embedding set contains $m$ points, $E^{\prime}=\left.\{e_{1}^{\prime}, \ldots, e_{m}^{\prime}\right.\}$. More precisely:

$
\begin{aligned}
& e_{c}=\operatorname{center}(E)=\frac{1}{n} \sum_{i} e_{i} \\
& E^{\prime}=\left.\{e_{i}^{\prime} \mid e_{i}^{\prime} \in E,\left.\|e_{i}^{\prime}-e_{c}\right.\|<2 \sigma\right.\}
\end{aligned}
$

where $\sigma$ denotes the standard deviation.

Clustering and Selection Suppose we select a subset $S^{\prime}$ containing $k$ sentences. We first apply efficient K-Means algorithm on GPUs [20] to create $k$ clusters $c_{1}, \ldots, c_{k}$ from $E^{\prime}$. For each cluster $c_{i}$ with size $\left|c_{i}\right|$, we select $\frac{k}{m} *\left|c_{i}\right|$ sentences whose embeddings are the nearest from the center of $c_{i}$, forming $S_{c_{i}}^{\prime}=\left.\{e_{1}^{\left(c_{i}\right)}, \ldots, e_{N}^{\left(c_{i}\right)}\right.\}$ :

$
S_{c_{i}}^{\prime}=\underset{\left.\{e_{j}^{\left(c_{i}\right)} \in c_{i}\right.\}}{\arg \min } \sum_{j=1}^{N}\left.\|e_{j}^{\left(c_{i}\right)}-\operatorname{center}\left(c_{i}\right)\right.\|
$

where $N=\frac{k}{m}\left|c_{i}\right|$.

The representative set $S^{\prime}$ of the entire dataset $S$ is the union of all representative sets from each cluster:

$
S^{\prime}=\bigcup_{i=1}^{k} S_{c_{i}}^{\prime}, \quad\left|S^{\prime}\right|=\sum_{i=1}^{k} \frac{k}{m} *\left|c_{i}\right|=k
$

## 4 Experiments

## 4.1 Settings

## Datasets
- Pre-train: IndicCorp [21] that contains a total of 458M sentences in 11 Indian languages and English.
- MT: PMI dataset [22] from WAT2021 MultiIndicMT task [23].
- Summarization: data in 7 Indic languages from multilingual XLSum dataset [24].

We applied script unification for all Indic languages to Devanagari, following mBART50 [4] and IndicBART [5]. Across all experiments, we used the IndicBART vocabulary of $64 \mathrm{k}$ subwords. ${ }^{1 \text { ) }}$

## - Pre-train Methods Comparison
- w/o Pre-train: directly train on downstream tasks from random parameters initialization.
- Random: pre-trained on $k$ randomly selected sentences.
- Random+RemoveOutlier (RO): first remove outliers, then apply Random.
- Repre: pre-train on $k$ sentences by representative data selection w/o outlier detection.
- Repre+RemoveOutlier (RO): first remove outliers, then apply Repre.
- Full: use $458 \mathrm{M}$ monolingual sentences in the IndicCorp dataset.

We compare proposed Repre and Repre+RO methods with two baselines Random and Random+RO. We set $k$ to $1.2 M$ and select sentences from different languages while keeping their proportions in the IndicCorp dataset. We follow fine-tuning settings in [5].

1) Download: https://github.com/AI4Bharat/indic-bart

## - Representative Data Selection Settings
- Continuous Embedding Conversion: we trained one Sent2vec [17] model for each language on IndicCorp data with default settings and sentence embedding dimension to 768 .
- Clustering Algorithm: we used GPU-implemented K-Means in the Faiss toolkit [20].

Model Hyperparameters We followed the settings of IndicBART ${ }^{2}$ and used the yanmmt toolkit ${ }^{3)}$ based on Hugging Face. ${ }^{4}$ )
- Architecture: transformer model of 6 encoder layers and 6 decoder layers with 16 attention heads.
- Training: we used 8 GPUs with a batch size of 4,096 tokens during pre-training and 2,048 tokens during fine-tuning. We trained $200 \mathrm{k}$ steps in pre-training and apply early stopping to fine-tuning.

## 4.2 Pre-trained Model Perplexity

We report our results in terms of the perplexities obtained on a mix of all dev sets from the PMI dataset that contains high-quality data from 10 Indian languages and English. As shown in Figure 2, proposed Repre method showed approximately 0.15 lower minimal PPL than Random. Furthermore, RO is effective for both Random and Repre methods.

## 4.3 Energy Consumption Comparison
- Full: trained on 48 V100 GPUs for 750k steps [5].
- Proposed: trained on 8 V100 GPUs for 200k steps.

Therefore, our approach reduces the energy consumption to $4.4 \%$ 5) compared with Full. $^{6 \text { ) }}$

## 4.4 MT Results

As presented in Table 3, proposed methods yield the highest BLEU scores for all pairs compared with baselines. With $4.4 \%$ energy consumption, our results are only 2-4 BLEU points lower than Full. Additionally, RO helps both Random and Repre.

2) https://github.com/AI4Bharat/indic-bart

3) https://github.com/prajdabre/yanmtt

4) https://huggingface.co
5) $(8 * 200 \mathrm{k}) /(48 * 750 \mathrm{k})=4.4 \%$

6) Training sent 2 vec models and clustering consumes very little energy in comparison.

Figure 2: Perplexity curves of pre-trained models on the PMI dev sets.

## 4.5 Summarization Results

As expressed by Table 1, proposed methods achieve higher ROUGE-L F-scores than baselines. Especially for low-resource bn language that contains only 80k training points, Repre is more robust than Random.

Table 1: ROUGE-L F1 scores on the summarization task.

## 4.6 Outlier Detection Examples

We show examples of normal sentences and outliers. We extract 300 English sentences from IndicCorp and apply the outlier detection algorithm to form Figure 3 together with Table 2. We can find that outlier sentences contain more proper nouns and disfluent phrases.

Table 2: The corresponding sentences in Figure 3.

 \\

Table 3: Performance on the MT task. Report sacreBLEU [25] scores on the WAT2021 MultiIndicMT test set.

Figure 3: Outlier detection. High-dimensional embeddings are mapped into 3-D points by t-SNE [9].

## 4.7 Sentence Clustering Examples

Table 4 shows the clustering results. In each cluster, the centroid is the most relevant from all other points. For example, in the first cluster, sentences are related to "Earth", "Jupiter", "ocean planet", "Life on Earth" and the sentence related to "Earth" is the centroid.
Table 4: Example of clusters. The centroids of the clusters are representative data.

 \\

## 5 Conclusion

In this paper, we propose a representative data selection algorithm together with an unsupervised outlier detection algorithm. With only $0.26 \%$ data and $4.4 \%$ energy consumption of the full model, proposed methods show reasonable performance on MT and text summarization tasks, and much higher performance compared to baselines.

## Acknowledgement

This work was supported by JSPS KAKENHI Grant Numbers JP21J23124.

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