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Model Description

SentenceTransformers is a set of models and frameworks that enable training and generating sentence embeddings from given data. The generated sentence embeddings can be utilized for Clustering, Semantic Search and other tasks. We used a pretrained mpnet-base model and trained it using Siamese Network setup and contrastive learning objective. Question and answer pairs from StackExchange was used as training data to make the model robust to Question / Answer embedding similarity.

We developped this model during the Community week using JAX/Flax for NLP & CV, organized by Hugging Face. We developped this model as part of the project: Train the Best Sentence Embedding Model Ever with 1B Training Pairs. We benefited from efficient hardware infrastructure to run the project: 7 TPUs v3-8, as well as assistance from Google’s Flax, JAX, and Cloud team members about efficient deep learning frameworks.

Intended uses

Our model is intended to be used as a sentence encoder for a search engine. Given an input sentence, it ouptuts a vector which captures the sentence semantic information. The sentence vector may be used for semantic-search, clustering or sentence similarity tasks.

How to use

Here is how to use this model to get the features of a given text using SentenceTransformers library:

from sentence_transformers import SentenceTransformer

model = SentenceTransformer('flax-sentence-embeddings/mpnet_stackexchange_v1')
text = "Replace me by any question / answer you'd like."
text_embbedding = model.encode(text)
# array([-0.01559514,  0.04046123,  0.1317083 ,  0.00085931,  0.04585106,
#        -0.05607086,  0.0138078 ,  0.03569756,  0.01420381,  0.04266302 ...],
#        dtype=float32)

Training procedure


We use the pretrained Mpnet-base. Please refer to the model card for more detailed information about the pre-training procedure.


We fine-tune the model using a contrastive objective. Formally, we compute the cosine similarity from each possible sentence pairs from the batch. We then apply the cross entropy loss by comparing with true pairs.

Hyper parameters

We trained on model on a TPU v3-8. We train the model during 80k steps using a batch size of 1024 (128 per TPU core). We use a learning rate warm up of 500. The sequence length was limited to 128 tokens. We used the AdamW optimizer with a 2e-5 learning rate. The full training script is accessible in this current repository.

Training data

We used the concatenation from multiple Stackexchange Question-Answer datasets to fine-tune our model. We sampled each StackExchange given a weighted probability of following equation.

int((stackexchange_length[path] / total_stackexchange_length) * total_weight)

MSMARCO, NQ & other question-answer datasets were also used. Sampling ratio for StackExchange vs remaining : 2 vs 1.

Dataset Paper Number of training tuples
Stack Exchange QA - Title & Answer - 4,750,619
MS MARCO paper 9,144,553
GOOAQ: Open Question Answering with Diverse Answer Types paper 3,012,496
Yahoo Answers Question/Answer paper 681,164
SearchQA - 582,261
Natural Questions (NQ) paper 100,231
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