182 lines
8.8 KiB
Markdown
182 lines
8.8 KiB
Markdown
Pcompress
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=========
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Copyright (C) 2012 Moinak Ghosh. All rights reserved.
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Use is subject to license terms.
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Pcompress is a utility to do compression and decompression in parallel by
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splitting input data into chunks. It has a modular structure and includes
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support for multiple algorithms like LZMA, Bzip2, PPMD, etc, with SKEIN
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checksums for data integrity. It can also do Lempel-Ziv pre-compression
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(derived from libbsc) to improve compression ratios across the board. SSE
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optimizations for the bundled LZMA are included. It also implements
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chunk-level Content-Aware Deduplication and Delta Compression features
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based on a Semi-Rabin Fingerprinting scheme. Delta Compression is done
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via the widely popular bsdiff algorithm. Similarity is detected using a
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custom hashing of maximal features of a block. When doing chunk-level
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dedupe it attempts to merge adjacent non-duplicate blocks index entries
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into a single larger entry to reduce metadata. In addition to all these it
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can internally split chunks at rabin boundaries to help dedupe and
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compression.
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It has low metadata overhead and overlaps I/O and compression to achieve
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maximum parallelism. It also bundles a simple slab allocator to speed
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repeated allocation of similar chunks. It can work in pipe mode, reading
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from stdin and writing to stdout. It also provides some adaptive compression
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modes in which multiple algorithms are tried per chunk to determine the best
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one for the given chunk. Finally it supports 14 compression levels to allow
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for ultra compression modes in some algorithms.
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NOTE: This utility is Not an archiver. It compresses only single files or
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datastreams. To archive use something else like tar, cpio or pax.
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Usage
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=====
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To compress a file:
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pcompress -c <algorithm> [-l <compress level>] [-s <chunk size>] <file>
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Where <algorithm> can be the folowing:
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lzfx - Very fast and small algorithm based on LZF.
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lz4 - Ultra fast, high-throughput algorithm reaching RAM B/W at level1.
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zlib - The base Zlib format compression (not Gzip).
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lzma - The LZMA (Lempel-Ziv Markov) algorithm from 7Zip.
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lzmaMt - Multithreaded version of LZMA. This is a faster version but
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uses more memory for the dictionary. Thread count is balanced
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between chunk processing threads and algorithm threads.
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bzip2 - Bzip2 Algorithm from libbzip2.
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ppmd - The PPMd algorithm excellent for textual data. PPMd requires
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at least 64MB X CPUs more memory than the other modes.
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libbsc - A Block Sorting Compressor using the Burrows Wheeler Transform
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like Bzip2 but runs faster and gives better compression than
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Bzip2 (See: libbsc.com).
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adapt - Adaptive mode where ppmd or bzip2 will be used per chunk,
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depending on which one produces better compression. This mode
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is obviously fairly slow and requires lots of memory.
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adapt2 - Adaptive mode which includes ppmd and lzma. This requires
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more memory than adapt mode, is slower and potentially gives
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the best compression.
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none - No compression. This is only meaningful with -D and -E so Dedupe
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can be done for post-processing with an external utility.
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<chunk_size> - This can be in bytes or can use the following suffixes:
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g - Gigabyte, m - Megabyte, k - Kilobyte.
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Larger chunks produce better compression at the cost of memory.
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<compress_level> - Can be a number from 0 meaning minimum and 14 meaning
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maximum compression.
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NOTE: The option "libbsc" uses Ilya Grebnov's block sorting compression library
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from http://libbsc.com/ . It is only available if pcompress in built with
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that library. See INSTALL file for details.
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To decompress a file compressed using above command:
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pcompress -d <compressed file> <target file>
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To operate as a pipe, read from stdin and write to stdout:
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pcompress -p ...
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Attempt Rabin fingerprinting based deduplication on chunks:
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pcompress -D ...
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pcompress -D -r ... - Do NOT split chunks at a rabin boundary. Default
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is to split.
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Perform Delta Encoding in addition to Exact Dedup:
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pcompress -E ... - This also implies '-D'.
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Number of threads can optionally be specified: -t <1 - 256 count>
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Other flags:
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'-L' - Enable LZP pre-compression. This improves compression ratio of all
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algorithms with some extra CPU and very low RAM overhead. Using
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delta encoding in conjunction with this may not always be beneficial.
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'-S' <cksum>
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- Specify chunk checksum to use: CRC64, SKEIN256, SKEIN512
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Default one is SKEIN256. The implementation actually uses SKEIN
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512-256. This is 25% slower than simple CRC64 but is many times more
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robust than CRC64 in detecting data integrity errors. SKEIN is a
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finalist in the NIST SHA-3 standard selection process and is one of
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the fastest in the group, especially on x86 platforms. BLAKE is faster
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than SKEIN on a few platforms.
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SKEIN 512-256 is about 60% faster than SHA 512-256 on x64 platforms.
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'-M' - Display memory allocator statistics
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'-C' - Display compression statistics
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NOTE: It is recommended not to use '-L' with libbsc compression since libbsc uses
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LZP internally as well.
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Environment Variables
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=====================
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Set ALLOCATOR_BYPASS=1 in the environment to avoid using the the built-in
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allocator. Due to the the way it rounds up an allocation request to the nearest
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slab the built-in allocator can allocate extra unused memory. In addition you
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may want to use a different allocator in your environment.
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Examples
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========
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Compress "file.tar" using bzip2 level 6, 64MB chunk size and use 4 threads. In
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addition perform exact deduplication and delta compression prior to compression.
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pcompress -D -E -c bzip2 -l6 -s64m -t4 file.tar
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Compress "file.tar" using extreme compression mode of LZMA and a chunk size of
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of 1GB. Allow pcompress to detect the number of CPU cores and use as many threads.
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pcompress -c lzma -l14 -s1g file.tar
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Compression Algorithms
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======================
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LZFX - Ultra Fast, average compression. This algorithm is the fastest overall.
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Levels: 1 - 5
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LZ4 - Very Fast, better compression than LZFX.
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Levels: 1 - 3
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Zlib - Fast, better compression.
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Levels: 1 - 9
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Bzip2 - Slow, much better compression than Zlib.
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Levels: 1 - 9
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LZMA - Very slow. Extreme compression.
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Levels: 1 - 14
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Till level 9 it is standard LZMA parameters. Levels 10 - 12 use
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more memory and higher match iterations so are slower. Levels
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13 and 14 use larger dictionaries upto 256MB and really suck up
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RAM. Use these levels only if you have at the minimum 4GB RAM on
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your system.
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PPMD - Slow. Extreme compression for Text, average compression for binary.
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In addition PPMD decompression time is also high for large chunks.
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This requires lots of RAM similar to LZMA.
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Levels: 1 - 14.
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Adapt - Very slow synthetic mode. Both Bzip2 and PPMD are tried per chunk and
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better result selected.
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Levels: 1 - 14
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Adapt2 - Ultra slow synthetic mode. Both LZMA and PPMD are tried per chunk and
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better result selected. Can give best compression ratio when splitting
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file into multiple chunks.
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Levels: 1 - 14
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Since both LZMA and PPMD are used together memory requirements are
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quite extensive especially if you are also using extreme levels above
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10. For example with 64MB chunk, Level 14, 2 threads and with or without
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dedupe, it uses upto 3.5GB physical RAM and requires 6GB of virtual
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memory space.
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It is possible for a single chunk to span the entire file if enough RAM is
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available. However for adaptive modes to be effective for large files, especially
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multi-file archives splitting into chunks is required so that best compression
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algorithm can be selected for textual and binary portions.
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Caveats
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=======
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This utility is not meant for resource constrained environments. Minimum memory
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usage (RES/RSS) with barely meaningful settings is around 10MB. This occurs when
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using the minimal LZFX compression algorithm at level 2 with a 1MB chunk size and
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running 2 threads.
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Normally this utility requires lots of RAM depending on compression algorithm,
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compression level, and dedupe being enabled. Larger chunk sizes can give
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better compression ratio but at the same time use more RAM.
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In some cases for files less than a gigabyte. Using Delta Compression in addition
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to exact Dedupe can have a slight negative impact on LZMA compression ratio
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especially when using the large-window ultra compression levels above 10.
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