epm patch which reduces cpu and storage consumption for the default deb packages

[Jérôme Bouat]

Hello.

I proposed a patch for the epm external software which is integrated on master. This lowers the size of the default debian package files which are provided on https://www.libreoffice.org/ .

The modified deb.c file uses the dpkg-deb and xz environment variables in order to deal with possibly unsupported features on the older versions. However, it should work with the current build system (xz environment variables are enough).

This patch increases the memory and cpu efficiency of the compression by disabling the parallel compression of xz for the dpkg operation only. However, the compression parallelism is still used by using the make parallel processes of the default build (i.e. make -j [number of cpus]). This increases the overall memory and cpu efficiency of the compression. Finally it increases the build speed for a given compression ratio.

The dictionary size for the xz compression algorithm is adjusted to the installed size of the package up to 128MiB in order to ensure that xz will use a maximum amount of 128MiB for decompression (below the 256MiB system memory requirement for Linux here : https://www.libreoffice.org/get-help/system-requirements/ ). The compression preset "-0..-9" is chosen according to the dictionary size. This ensure that the compression algorithm has consistent parameters.

Could someone review this path here :
https://gerrit.libreoffice.org/c/core/+/179624 ?

Thanks.