This project provides a simple implementation of a bit compressed vector. This means, that in contrast ot a bit vector which only captures a series of 0s and 1s this library provides a vector that can store arbitrary numbers. However, the width of value has to be specified.

The goal is to achieve a good compression ratio by keeping up with the sequential scan speed of a std::vector.

This work is largely based on the algorithms presented by Wilhalm et al "SIMD- Scan: Ultra Fast in-Memory Table Scan using on-Chip Vector Processing Units" published in PVLDB 2(1): 385-394 (2009).

The BCV is intended to be a drop-in replacement of std::vector, however, currently it is only of fixed size and does not support any kind of iterator interface. The access methods to the vector are:

1. Index-based subscript
2. Index-based get/set()
3. Multi-get based

The index-based access allows array subscript operator access, however this is proxy access around the get() / set() methods. Especially the [] as lvalue might be more expensive than a simple set().

The multi-get method allows to extract multiple values at once. Here we differentiate between to versions of the mget() the first amget will extract one cache line of compressed values and write them out to a external storage array. The second version mget_fixed() will only extract one cache line of uncompressed values and write them to the external storage. It is important to mention that mget_fixed() will not perform any range checks on the data, so make sure you extract the right amount of data.

To increase the performance of the bit-compressed vector some parts of the bit mask lookups are generated so you have to run

make release

before continuing. Now you can copy everything from pkg/bcv to your project and use it as is.

Currently the performance of the vector is comparable to the std::vector<T> for sequential scans but allowing to save a significant amount of memory

For a vector with 100M elements the sequential scan speed is on a Intel Xeon 7560 and 5 bits stored for 32 bit integers a scan aggregating all values takes:

• get time 0.268859s
• mget time 0.095577s
• vector time 0.133813s

The memory consumption for the vector is ~ 400MB and for the bit compressed vector ~ 60MB.

Copyright (c) 2012, Martin Grund

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