cmix-hp

Submission for the Hutter Prize.

v1 submitted on June 10, 2021.

v2 submitted on August 1, 2021. v2 is faster than v1, but has worse compression rate.

v3 submitted on August 9, 2021. v3 gets a slightly better compression rate than v2 (but is slower). v2 was not above the 1% improvement threshold on a testing computer.

Submission Description

This submission contains some small modifications on top of the recent STARLIT Hutter Prize winner.

Below is the cmix-hp v3 result:

Metric	Value
cmix-hp compressor's executable file size (S1)	396929 bytes
cmix-hp self-extracting archive size (S2)	113733212 bytes
Total size (S)	114130141 bytes
Previous record (L)	115352938 bytes
cmix-hp improvement (1 - S/L)	1.06%

Experiment platform
Operating system	Ubuntu 20.04
Processor	Intel Core i7-7700K @ 4.20GHz (Geekbench score 1288)
Memory	32 GB DDR4
Decompression running time	42.8 hours
Decompression RAM max usage	6905 MiB
Decompression disk usage	~35GB

Time, disk, and RAM usage are approximately symmetric for compression and decompression.

Here is a comparison between different entries on this computer:

Name	running time (hours)	S1	S2
STARLIT	44.13	401264	114920105
cmix-hp v1	45.44	397242	113688192
cmix-hp v2	41.94	396779	113760510
cmix-hp v3	42.8	396929	113733212

cmix-hp algorithm description

The submission has several small tweaks on top of STARLIT. The most substantial change is to have a huge PPM model, which gets swapped to disk to improve memory usage.

Changes to STARLIT for v1

Changed mod_ppmd from v3 to v2, which supports a higher memory limit.
Used one PPM model (order-25) instead of two.
Used mmap to store PPM memory to disk (reducing RAM usage).
Added a limit to the cmix context mixer size to make it more memory efficient.
Made the PAQ8HP context mixer use a hash map, to reduce memory usage.
Memory tuning for PAQ8HP: shifted more memory to wordModel.
Reduced memory usage for some cmix models, and removed "Direct" models.
Removed some unused preprocessor code.

Speed improvements for v2

Exported fewer model predictions from PAQ8HP to the cmix mixer.
Removed some cmix context mixers.

Compression improvements for v2

Thanks to Kaido Orav for these suggested improvements:

Swapped certain byte regions during preprocessing - a trick originally used in PAQ8HP.
Improved handling of zero state in PAQ8HP mixer.
Better handling of UTF characters in PAQ8HP.

Changes for v3

Added back two of the cmix context mixers (which were removed in v2).

Instructions

The installation and usage instructions for cmix-hp are the same as for STARLIT. For convenience, most of the information below is copied from STARLIT documentation.

One important note: it is recommended to change one variable in the source code for PPM. From line 26 in src/models/ppmd.cpp:

// If mmap_to_disk is set to false (recommended setting), PPM will only use RAM
// for memory.
// If mmap_to_disk is set to true, PPM memory will be saved to disk using mmap.
// This will reduce RAM usage, but will be slower as well. *Warning*: this will
// write a *lot* of data to disk, so can reduce the lifespan of SSDs. Not
// recommended for normal usage.
bool mmap_to_disk = true;

This variable is set to true by default, to comply with the Hutter Prize RAM limit.

Installing packages required for compiling cmix-hp compressor from sources on Ubuntu

Building cmix-hp compressor from sources requires clang-12, upx-ucl, and make packages. On Ubuntu, these packages can be installed by running the following scripts:

./install_tools/install_upx.sh
./install_tools/install_clang-12.sh

Compiling cmix-hp compressor from sources

A bash script is provided for compiling cmix-hp compressor from sources on Ubuntu. This script places the cmix-hp executable file named as cmix in ./run directory. The script can be run as

./build_and_construct_comp.sh

Running cmix-hp compressor

To run the cmix-hp compressor use

cd ./run
cmix -e <PATH_TO_ENWIK9> enwik9.comp

Expected output:

78125 bytes -> 415377 bytes in 74.17 s.
199784 bytes -> 1131233 bytes in 184.88 s.
Detected block types: TEXT: 100.0%
934188796 bytes -> 113536067 bytes in 155972.59 s.
free(): invalid size
Command terminated by signal 6

The error message does not affect the compression validity.

Running cmix-hp decompressor

The compressor is expected to output an executable file named archive9 in the same directory (./run). The file archive9 when executed is expected to reproduce the original enwik9 as a file named enwik9_restored. The executable file archive9 should be launched without argments from the directory containing it.

cd ./run
./archive9