7-Zip engine in PA 17 not as good as in 7-Zip 17

  • Alpha Testers

    I have been doing some test on 7-Zip compression inside PA, and noticed that 7-Zip 17.00 always get better compression ratios that PA with 7-Zip format. No matter if we choose LZMA or LZMA2.

    I guess that the advantage on 7-Zip is because:

    1. Can use -myx=9 to improve ratio.
    2. Can use fastbytes=273 which is in 90% of cases best ratio
    3. Can use 128MB o 256MB dictionaries.
    4. Maybe because recent 7Z engine, is also better than the one in PA

    Would be fine if those additional options can be setup inside PA, or either they are finetunned inside PA default values.

  • conexware

    @nikkho we dont really set manual options in 7z, we could though… how does it compare to PA format?

  • Alpha Testers

    I have compressed full PowerArchiver x64 portable in my SSD disk, so it is easy for you to reproduce.

    • Uncompressed size: 176.473.679 bytes.
    • 7-Zip 17.00 Ultra LZMA 128MB fb273 myx=9: 28.330.063 bytes.
    • PowerArchiver 7z 17.00.69 LZMA Ultra: 29.060.624 bytes.
    • 7-Zip 17.00 Ultra LZMA2 128MB fb273 myx=9: 28.323.572 bytes.
    • PowerArchiver 17.00.69 7z LZMA2 Ultra: 29.062.095 bytes.
    • PowerArchiver 17.00.69 7z Optimized Ultra: 41.579.382 bytes.

    Take a look at the bad results with the Optimized profile, and also at the equivalent PA implementation, being slightly worse than the one in 7-Zip, meaning that the library used or its parameters, are subobtimal.

    By the way, same with PA format, excellent results:

    • PowerArchiver 17.00.69 pa Extreme Optimize Strong: 27.114.421 bytes.

  • Alpha Testers

    @spwolf myx was added in 7z 15.00 (https://sourceforge.net/p/sevenzip/discussion/45797/thread/fd1f6344/) and it improves results a bit at the cost of some additional analysis time.

    As for fastbytes fb=273 is mostly the better.

  • conexware

    @nikkho cool, keep in mind for .pa we use 64M as default in extreme, so result with 128M dictionary would be even better due to codecs, not just settings.

  • conexware

    as to Optimized, there is something wrong with the settings, we will check it out. Thank you very much!

  • Alpha Testers

    Confirmed that the problem is fixed in 17.00.70, but still with some room for improvement:

    • PowerArchiver 17.00.69 7z Optimized Ultra: 41.579.382 bytes.
    • PowerArchiver 17.00.70 7z Optimized Ultra: 29.057.719 bytes.

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