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This brings support for multi-threaded compression. This brings close N times faster compression where N is the number of CPU cores. Because of this, liblzma now depends on libthr. Soon libarchive will be modified to use the new lzma API. Thanks to antoine@ for the exp-run. Differential Revision: https://reviews.freebsd.org/D1786 Reviewed by: bapt
112 lines
3.9 KiB
Plaintext
112 lines
3.9 KiB
Plaintext
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XZ Utils To-Do List
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===================
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Known bugs
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----------
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The test suite is too incomplete.
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If the memory usage limit is less than about 13 MiB, xz is unable to
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automatically scale down the compression settings enough even though
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it would be possible by switching from BT2/BT3/BT4 match finder to
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HC3/HC4.
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XZ Utils compress some files significantly worse than LZMA Utils.
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This is due to faster compression presets used by XZ Utils, and
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can often be worked around by using "xz --extreme". With some files
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--extreme isn't enough though: it's most likely with files that
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compress extremely well, so going from compression ratio of 0.003
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to 0.004 means big relative increase in the compressed file size.
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xz doesn't quote unprintable characters when it displays file names
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given on the command line.
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tuklib_exit() doesn't block signals => EINTR is possible.
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SIGTSTP is not handled. If xz is stopped, the estimated remaining
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time and calculated (de)compression speed won't make sense in the
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progress indicator (xz --verbose).
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If liblzma has created threads and fork() gets called, liblzma
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code will break in the child process unless it calls exec() and
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doesn't touch liblzma.
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Missing features
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----------------
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Add support for storing metadata in .xz files. A preliminary
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idea is to create a new Stream type for metadata. When both
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metadata and data are wanted in the same .xz file, two or more
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Streams would be concatenated.
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The state stored in lzma_stream should be cloneable, which would
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be mostly useful when using a preset dictionary in LZMA2, but
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it may have other uses too. Compare to deflateCopy() in zlib.
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Support LZMA_FINISH in raw decoder to indicate end of LZMA1 and
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other streams that don't have an end of payload marker.
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Adjust dictionary size when the input file size is known.
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Maybe do this only if an option is given.
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xz doesn't support copying extended attributes, access control
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lists etc. from source to target file.
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Multithreaded compression:
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- Reduce memory usage of the current method.
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- Implement threaded match finders.
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- Implement pigz-style threading in LZMA2.
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Multithreaded decompression
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Buffer-to-buffer coding could use less RAM (especially when
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decompressing LZMA1 or LZMA2).
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I/O library is not implemented (similar to gzopen() in zlib).
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It will be a separate library that supports uncompressed, .gz,
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.bz2, .lzma, and .xz files.
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Support changing lzma_options_lzma.mode with lzma_filters_update().
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Support LZMA_FULL_FLUSH for lzma_stream_decoder() to stop at
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Block and Stream boundaries.
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lzma_strerror() to convert lzma_ret to human readable form?
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This is tricky, because the same error codes are used with
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slightly different meanings, and this cannot be fixed anymore.
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Make it possible to adjust LZMA2 options in the middle of a Block
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so that the encoding speed vs. compression ratio can be optimized
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when the compressed data is streamed over network.
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Improved BCJ filters. The current filters are small but they aren't
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so great when compressing binary packages that contain various file
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types. Specifically, they make things worse if there are static
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libraries or Linux kernel modules. The filtering could also be
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more effective (without getting overly complex), for example,
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streamable variant BCJ2 from 7-Zip could be implemented.
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Filter that autodetects specific data types in the input stream
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and applies appropriate filters for the corrects parts of the input.
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Perhaps combine this with the BCJ filter improvement point above.
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Long-range LZ77 method as a separate filter or as a new LZMA2
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match finder.
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Documentation
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-------------
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More tutorial programs are needed for liblzma.
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Document the LZMA1 and LZMA2 algorithms.
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Miscellaneous
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------------
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Try to get the media type for .xz registered at IANA.
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