BOUNCE: memory-efficient SIMD approach for lightweight integer compression

Abstract

Integer compression plays an important role in columnar database systems to reduce the main memory footprint as well as to speedup query processing. To keep the additional computational effort of (de)compression as low as possible, the powerful Single Instruction Multiple Data (SIMD) extensions of modern CPUs are heavily applied. While a scalar compression algorithm usually compresses a block of N consecutive integers, the state-of-the-art SIMDified implementation scales the block size to $$k \cdot N$$ k · N with k as the number of elements which could be simultaneously processed in an SIMD register. On the one hand, this scaling SIMD approach improves the performance of (de)compression. But on the other hand, it can lead to a degradation of the memory footprint of the compressed data. Within this article, we analyze this degradation effect for various integer compression algorithms and present a novel SIMD concept to overcome that effect. The core idea of our novel SIMD concept called BOUNCE is to concurrently compress k different blocks of size N within SIMD registers, guaranteeing the same compression ratio as scalar variant. As we are going to show, our proposed SIMD idea works well on various Intel CPUs and may offer a new generalized SIMD concept to optimize further algorithms.