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PQ Fast Scan

Description

The PQ Fast Scan project is a C++11 implementation of fast vector scanning techniques for nearest neighbor search in large databases of high-dimensional vectors.

PQ Fast Scan builds on Product Quantization (PQ), a widely used solution for nearest neighbor search. To find the nearest neighbor of a query vector, PQ computes the distance between the query vector and database vectors using lookup tables stored in the L1 cache. This procedure, named PQ Scan, performs many cache accesses, which limit its performance. L1 cache accesses do not parallelize well (maximum 2 concurrent accesses) and have a high latency (about 4 cycles). To avoid these issues, PQ Fast Scan uses lookup tables stored in SIMD registers, which can be queried in parallel (16 concurrent accesses) and with a low latency (1 cycle).

<p align="center"> <img src="https://storage.sbg1.cloud.ovh.net/v1/AUTH_cb25a15f19bd41a5b3bdd2b60b718b72/pq-fast-scan/pq-fast-scan-overview.png"> </p>

This novel approach allows PQ Fast Scan to perform 4-6x faster than PQ Scan, while returning the exact same results.

<p align="center"> <img src="https://storage.sbg1.cloud.ovh.net/v1/AUTH_cb25a15f19bd41a5b3bdd2b60b718b72/pq-fast-scan/pq-fast-scan-performance.png"> </p>

Contact:
Nicolas Le Souarnec: nicolas.le-scouarnec technicolor.com
Fabien André: fabien.andre technicolor.com
Please replace the space by an at sign.

Publication

F. Andre, A.-M. Kermarrec, and N. Le Scouarnec. Cache locality is not enough: High-Performance Nearest Neighbor Search with Product Quantization. PVLDB, 9(4), 2015
Publication presented at VLDB'16: Slides, Poster

License

The PQ Fast Scan project is made available under the Clear BSD license terms (See LICENSE file).

Copyright (c) 2015 – Thomson Licensing, SAS

Building

Requirements

Hardware:

Software:

On Debian-based distributions, you can install these dependencies with the following command:

$ sudo apt-get install build-essential gcc g++ libpfm4-dev cmake

PQ Fast Scan can be built with other g++ versions than g++ 4.9, but we benchmarked it with this version only. Obtaining best results with other g++ versions may require some adjustments to the source code. Therefore, if your distribution provides g++ 4.9, we recommend installing it:

$ sudo apt-get install g++-4.9 gcc-4.9

Otherwise, you may either build PQ Fast Scan with another g++ version, or install g++ 4.9 from sources. Installing g++ 4.9 from sources takes about 20 minutes on a reasonably recent system. You can do so with the following commands:

$ sudo apt-get install libgmp-dev libmpfr-dev libmpc-dev
$ wget ftp://ftp.irisa.fr/pub/mirrors/gcc.gnu.org/gcc/\
releases/gcc-4.9.3/gcc-4.9.3.tar.bz2
$ tar xvf gcc-4.9.3.tar.bz2
$ mdkir build-gcc-4.9; cd build-gcc-4.9
$ ../gcc-4.9.3/configure --enable-languages=c,c++ --program-suffix=-4.9
$ make -j8
$ sudo make install
$ which g++-4.9

Some quick tests with g++ 5.2 seemed to indicate that PQ Fast Scan performs well when compiled with this version. You may also be able to compile PQ Fast Scan with clang++ but we have not tested it.

Building PQ Fast Scan

Once you have gathered all the requirements, building PQ Fast Scan is straightforward:

$ git clone https://github.com/technicolor-research/pq-fast-scan.git
$ mkdir build-pq-fast-scan
$ cd build-pq-fast-scan

If you use g++ 4.9:

$ CC=gcc-4.9 CXX=g++-4.9 cmake ../pq-fast-scan
$ make

To use the default g++ version of your distribution:

$ cmake ../pq-fast-scan
$ make

Usage

This project provides two executables :

pqscan does not take any command-line argument. It outputs run times and performance counters for different PQ Scan implementations. You may need to run it as root to access the performance counters kernel API.

$ sudo ./pqscan
Scan: time=74758,cycles=269918483,instructions=800272344,L1-dcache-loads=
400231701,L1-dcache-load-misses=3139139 [...]
Scan +sse +prefetch: time=91214,cycles=329937021,instructions=787927217,
L1-dcache-loads=462852107,L1-dcache-load-misses=3139943 [...]
Scan +avx +prefetch: time=68490,cycles=231844613,instructions=618770051,
L1-dcache-loads=409382281,L1-dcache-load-misses=3141246 [...]
Scan +avx +vgather +prefetch: time=97841,cycles=331413774,
instructions=188461772,L1-dcache-loads=228194484, [...]

pqfastscan requires four input files: a partition to scan, a list of IDs of query vectors, a set of query vectors and a set of distance tables.

We provide the following input files:

For more information about these datasets, see the Datasets section.

Decompress the datasets with the following commands:

$ tar xvf 100M1-8-partitions.tar.xz
$ gzip -d bigann_query.bvecs.gz
$ xz -d bigann_distance_tables.fvecs.xz

You can then run pqfastscan, e.g. to scan partition 0:

$ ./pqfastscan 100M1-8-partition/100M1-partition-0.dat bigann_query.bvecs\
bigann_distance_tables.fvecs 100M1-8-partition/100M1-list-0.txt

vec_id,partition_id,partition_n,bh_size,keep,pq_us,fast_pq_us, [...]
1,0,25159451,100,125797,75392,12272,24404,93728.6
4,0,25159451,100,125797,74754,17480,435694,96652.2
[...]

You may replace 0 by another partition number [0-7]. Make sure you use the list file corresponding to the partition, e.g., 100M1-list-6.txt with 100M1-partition-6.dat.

pqfastscan outputs one line (comma separated values) for each query vector of the list with the scan time for PQ Scan (pq_us) and PQ Fast Scan (fast_pq_us).

In addition to run times, you may also want to output performance counters. To do so, use the -p option. Like pqscan, you may also need to run pqfastscan as root when using performance counters:

$ sudo ./pqfastscan -p 100M1-8-partition/100M1-partition-0.dat\
bigann_query.bvecs bigann_distance_tables.fvecs\
100M1-8-partition/100M1-list-0.txt

vec_id,partition_id,partition_n,bh_size,keep,pq_us,pq_cycles, [...]
1,0,25159451,100,125797,76247,272870815,805615127,402679306,12027, [...]
4,0,25159451,100,125797,76000,273304611,805588015,402670181,18040, [...]
[...]

For more information about pqfastscan options, see ./pqfastscan -h.

Datasets

To test pqfastscan, we provide:

Besides, we use the query set from the well-known ANN_SIFT1B dataset;

We generated the partitions, lists and distance tables files we use from the ANN_SIFT1B dataset.

You may re-generate these files yourself. To generate these files from the ANN_SIFT1B dataset, you need an implementation of product quantization. The authors of the seminal paper on product quantization distribute two implementations of product quantization: a C implementation, and a matlab implementation. We used the C implementation (libpq), which we obtained under a commercial licence. You can obtain similar files using the matlab implementation, freely available.

Once you have an implementation of product quantization, you can generate partitions, lists and distance tables following these steps:

  1. Extract the first 100 million vectors from the ANN_SIFT1B Base Set (bigann_base.bvecs). We name this extracted dataset ANN_SIFT100M1.
  2. Build an IVFADC (database) using the ANN_SIFT1B Learning Set (bigann_learn.bvecs). We used a coarse quantizer with 8 centroids and a product quantizer with 8 sub-quantizers of 256 centroids each.
  3. Modify the assignement of sub-quantizer centroid indexes. To do so, use a k-means variant which forces groups of same sizes.
  4. Add all vectors of ANN_SIFT100M1 to the IVFADC.
  5. We name partition each Voronoi cell of the coarse quantizer. Save each partition [0-7] to a different file. See below for file format.
  6. Using the coarse quantizer, map each query vector to its corresponding partition (no multiple assignement, i.e. ma=1). Save the query vectors IDs corresponding to each partition in a different list. See below for file format.
  7. Compute distance tables for each vector. Save distance tables sequentially in a single file. See below for file format.

partitions file format: [Binary file]
The number of pqcodes in the partition (32-bit int), followed by all pqcodes of the partition (8x8 bits per pqcode).
Filename: The last number before the extension is parsed as the partition number.

distance tables file format: [Binary file]
Each query vector has 8 distance tables of 256 floats associated with it. The 8 distance tables of each query vector are stored as a vector of 2048 floats (8 x 256 floats). These vectors of 2048 floats are stored as an fvecs file.
Filename: No restrictions.

lists file format: [Text file]
One vector ID per line.
Filename: The last number before the extension is parsed as the partition number. If the vector ID 42 is in the file xxxx-list-6.txt, it means that the vector number 42 from the query set was mapped to the partition 6 by the coarse quantizer.

License

This project is licensed under the terms of BSD 3-clause Clear license. by downloading this program, you commit to comply with the license as stated in the LICENSE.md file.