# TorchPQ

TorchPQ is a python library for **Approximate Nearest Neighbor Search** (ANNS) and **Maximum Inner Product Search** (MIPS) on GPU using **Product Quantization** (PQ) algorithm. TorchPQ is implemented mainly with PyTorch, with some extra CUDA kernels to accelerate clustering, indexing and searching.

## Install

First install a version of CuPy library that matches your CUDA version

```
pip install cupy-cuda90
pip install cupy-cuda100
pip install cupy-cuda101
...
```

Then install TorchPQ

```
pip install torchpq
```

for a full list of cupy-cuda versions, please go to Installation Guide

## Quick Start

### IVFPQ

**I**n**V**erted **F**ile **P**roduct **Q**uantization (IVFPQ) is a type of ANN search algorithm that is designed to do fast and efficient vector search in million, or even billion scale vector sets. check the original paper for more details.

#### Training

```
from torchpq import IVFPQ
n_data = 1000000 # number of data points
d_vector = 128 # dimentionality / number of features
index = IVFPQ(
d_vector=d_vector,
n_subvectors=64,
n_cq_clusters=1024,
n_pq_clusters=256,
blocksize=128,
distance="euclidean",
)
x = torch.randn(d_vector, n_data, device="cuda:0")
index.train(x)
```

There are some important parameters that need to be explained:

**d_vector**: dimentionality of input vectors. there are 2 constraints on`d_vector`

: (1) it needs to be divisible by`n_subvectors`

; (2) it needs to be a multiple of 4.***n_subvectors**: number of subquantizers, essentially this is the byte size of each quantized vector, 64 byte per vector in the above example.****n_cq_clusters**: number of coarse quantizer clusters**n_pq_clusters**: number of product quantizer clusters, this is assumed to be 256 throughout the entire project, and should**NOT**be changed.**blocksize**: initial capacity assigned to each voronoi cell of coarse quantizer.`n_cq_clusters * blocksize`

is the number of vectors that can be stored initially. if any cell has reached its capacity, that cell will be automatically expanded. If you need to add vectors frequently, a larger value for`blocksize`

is recommended.

Remember that the shape of any tensor that contains data points has to be `[d_vector, n_data]`

.

* the second constraint could be removed in the future

** actual byte size would be (n_subvectors+9) bytes, 8 bytes for ID and 1 byte for is_empty

#### Adding new vectors

```
ids = torch.arange(n_data, device="cuda")
index.add(x, input_ids=ids)
```

Each ID in `ids`

needs to be a unique int64 (`torch.long`

) value that identifies a vector in `x`

. if `input_ids`

is not provided, it will be set to `torch.arange(n_data, device="cuda") + previous_max_id`

#### Removing vectors

`index.remove(ids)`

`index.remove(ids)`

will virtually remove vectors with specified `ids`

from storage. It ignores ids that doesn't exist.

#### Topk search

```
index.n_probe = 32
n_query = 10000
query = torch.randn(d_vector, n_query, device="cuda:0")
topk_values, topk_ids = index.topk(query, k=100)
```

- when
`distance="inner"`

,`topk_values`

are**inner product**of queries and topk closest data points. - when
`distance="euclidean"`

,`topk_values`

are**negative squared L2 distance**between queries and topk closest data points. - when
`distance="manhattan"`

,`topk_values`

are**negative L1 distance**between queries and topk closest data points. - when
`distance="cosine"`

,`topk_values`

are**cosine similarity**between queries and topk closest data points.

#### Encode and Decode

you can use IVFPQ as a vector codec for lossy compression of vectors

```
code = index.encode(query) # compression
reconstruction = index.decode(code) # reconstruction
```

#### Save and Load

Most of the TorchPQ modules are inherited from `torch.nn.Module`

, this means you can save and load them just like a regular pytorch model.

```
# Save to PATH
torch.save(index.state_dict(), PATH)
# Load from PATH
index.load_state_dict(torch.load(PATH))
```

### Clustering

#### K-means

```
from torchpq.kmeans import KMeans
import torch
n_data = 1000000 # number of data points
d_vector = 128 # dimentionality / number of features
x = torch.randn(d_vector, n_data, device="cuda")
kmeans = KMeans(n_clusters=4096, distance="euclidean")
labels = kmeans.fit(x)
```

Notice that the shape of the tensor that contains data points has to be `[d_vector, n_data]`

, this is consistant in TorchPQ.

#### Multiple concurrent K-means

Sometimes, we have multiple independent datasets that need to be clustered, instead of running multiple KMeans sequentianlly, we can perform multiple kmeans concurrently with **MultiKMeans**

```
from torchpq.kmeans import MultiKMeans
import torch
n_data = 1000000
n_kmeans = 16
d_vector = 64
x = torch.randn(n_kmeans, d_vector, n_data, device="cuda")
kmeans = MultiKMeans(n_clusters=256, distance="euclidean")
labels = kmeans.fit(x)
```

#### Prediction with K-means

```
labels = kmeans.predict(x)
```

## Benchmark

All experiments were performed with a Tesla T4 GPU.

### SIFT1M

#### IVFPQ

Faiss is one of the most well known ANN search libraries, and it also has a GPU implementation of IVFPQ, so we did some comparison experiments with faiss.

**How to read the plot:**

- the plot format follows the style of ann-benchmarks
- X axis is [email protected], Y axis is queries/second
- the closer to the top right corner the better
- indexes with same parameters from different libraries have similar colors.
- different libraries have different line styles (TorchPQ is solid line with circle marker, faiss is dashed line with triangle marker)
- each node on the line represents a different n_probe, starting from 1 at the left most node, and multiplied by 2 at the next node. (n_probe = 1,2,4,8,16,...)

**Summary:**

- for all the IVF16384 variants, torchpq outperforms faiss when n_probe > 16.
- for IVF4096, torchpq has lower [email protected] compared to faiss, this could be caused by not encoding residuals. An option to encode residuals will be added soon.

#### IVFPQ+R

### GIST1M

coming soon...