Falcot922/minilm-msmarco-simcse

MiniLM-L6, unsupervised SimCSE on MS MARCO passages

Control model from the embeddings and RAG step of mistral-optimisation. It shows what unsupervised contrastive learning (SimCSE) brings to a raw MiniLM, compared with training on (query, passage) pairs. It is a weak retriever, not meant for real use: for that, see the supervised version minilm-msmarco-contrastive, or better, all-MiniLM-L6-v2.

How SimCSE works here

Each passage is encoded twice with dropout active. The two slightly different vectors form a positive pair, and the other passages of the batch are the negatives. No query and no label is used: the model only learns that a passage should stay close to itself under noise.

Results

1,000 MS MARCO v2.1 validation queries; the corpus is every passage proposed for these queries (9,984). Recall@K is the share of queries whose relevant passage is in the top K (FAISS, inner product).

Model	Recall@1	Recall@5	Recall@10
Raw MiniLM (starting point)	1.0%	3.2%	3.9%
This model (SimCSE, unsupervised)	3.3%	8.6%	12.1%
Supervised, same data and settings	25.6%	73.7%	87.9%
all-MiniLM-L6-v2 (reference)	42.2%	89.2%	98.5%

SimCSE triples the raw model's scores, but stays far behind supervision: making a passage close to itself does not teach which passage answers a question.

Training

• Base: nreimers/MiniLM-L6-H384-uncased
• 20,000 MS MARCO v2.1 train passages (the relevant passage of each query), 1 epoch, batch 64, lr 3e-5
• InfoNCE loss with in-batch negatives, temperature 0.05, dropout as the only augmentation
• Embedding: mean of token vectors (padding excluded), L2-normalized, max 128 tokens

Usage

import torch
import torch.nn.functional as F
from transformers import AutoModel, AutoTokenizer

tok = AutoTokenizer.from_pretrained("Falcot922/minilm-msmarco-simcse")
model = AutoModel.from_pretrained("Falcot922/minilm-msmarco-simcse").eval()

def embed(texts):
    enc = tok(texts, padding=True, truncation=True, max_length=128, return_tensors="pt")
    with torch.no_grad():
        hidden = model(**enc).last_hidden_state
    mask = enc["attention_mask"].unsqueeze(-1)
    return F.normalize((hidden * mask).sum(1) / mask.sum(1), dim=-1)

q = embed(["what is the capital of france"])
p = embed(["Paris is the capital of France.", "Bananas are yellow."])
print(q @ p.T)