Parallel Compression Algorithm

Parallel Data Compression is the application of parallel computing to the inherently sequential problem of lossless data compression — where standard algorithms like DEFLATE (gzip) and LZ4 have serial data dependencies that prevent straightforward parallelization, requiring block-level parallelism, pipelined matching, or GPU-accelerated entropy coding to achieve compression throughputs of tens to hundreds of GB/s on modern hardware.

Why Compression Is Hard to Parallelize

LZ-family compressors (LZ77, LZ4, Zstd) maintain a sliding window of recent data and search for matching sequences. Each symbol's encoding depends on ALL previous symbols (the dictionary is built incrementally). This creates a chain dependency that prevents independent processing of different parts of the input.

Block-Level Parallelism

The most practical approach: split the input into independent blocks and compress each block in parallel. Each block uses its own dictionary (no cross-block references).
- pigz (parallel gzip): Divides input into 128 KB blocks, compresses each with DEFLATE on separate threads, concatenates valid gzip streams. Decompression of each block is independent. Achieves linear speedup with cores.
- lz4mt / zstdmt: Multi-threaded LZ4 and Zstd compressors using the same block-parallel strategy. Zstd's multi-threaded mode is built into the library (`ZSTD_CCtx_setParameter(cctx, ZSTD_c_nbWorkers, N)`).
- Trade-off: Independent blocks reduce compression ratio by 1-5% (each block starts with an empty dictionary). Larger blocks improve ratio but reduce parallelism.

GPU Compression

• nvCOMP (NVIDIA): GPU-accelerated compression library supporting LZ4, Snappy, Deflate, zstd, and cascaded compression. Throughput: 100-500 GB/s decompression on A100/H100. Compression is harder to parallelize but achieves 50-200 GB/s.
• Approach: Input is divided into thousands of small chunks. Each GPU thread block compresses one chunk. The matching step uses shared memory hash tables for the sliding window. Entropy coding (Huffman/ANS) is parallelized using warp-level operations.

Pipelined and Fine-Grained Parallelism

• Parallel Huffman Decoding: Traditional Huffman decoding is serial (variable-length codes). Parallel approaches use lookup tables or finite automata that decode multiple symbols simultaneously.
• ANS (Asymmetric Numeral Systems): Modern entropy coder used in Zstd and JPEG XL. rANS (range ANS) variant can be decoded in parallel by processing multiple independent encoded streams (interleaved encoding).
• GPU-Friendly Entropy Coding: Encode data in multiple independent streams (4-32). Each GPU thread decodes one stream. Interleaved streams add minimal compression overhead while enabling massive parallelism.

Applications

• Database Query Processing: Compressed columnar storage (Apache Parquet, ORC) requires decompression in the query critical path. GPU decompression at 200+ GB/s eliminates decompression as the bottleneck.
• Scientific I/O: HDF5 datasets with compression require decompression before computation. Parallel decompression on GPU or multi-core CPU matches I/O bandwidth.
• Network: Compressed data transfer between distributed nodes. Compression throughput must exceed network bandwidth to provide net benefit.

Parallel Data Compression is the art of finding independence in an inherently sequential algorithm — exploiting block-level, stream-level, and instruction-level parallelism to achieve compression and decompression throughputs that match the bandwidth demands of modern parallel computing systems.