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Docs: Update faq.txt a little.

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Lasse Collin 2022-11-08 22:26:54 +02:00
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@ -33,7 +33,7 @@ A: 7-Zip and LZMA SDK are the original projects. LZMA SDK is roughly
LZMA Utils. LZMA Utils.
There are several other projects using LZMA. Most are more or less There are several other projects using LZMA. Most are more or less
based on LZMA SDK. See <http://7-zip.org/links.html>. based on LZMA SDK. See <https://7-zip.org/links.html>.
Q: Why is liblzma named liblzma if its primary file format is .xz? Q: Why is liblzma named liblzma if its primary file format is .xz?
@ -115,7 +115,6 @@ Q: I cannot find BCJ and BCJ2 filters. Don't they exist in liblzma?
A: BCJ filter is called "x86" in liblzma. BCJ2 is not included, A: BCJ filter is called "x86" in liblzma. BCJ2 is not included,
because it requires using more than one encoded output stream. because it requires using more than one encoded output stream.
A streamable version of BCJ2-style filtering is planned.
Q: I need to use a script that runs "xz -9". On a system with 256 MiB Q: I need to use a script that runs "xz -9". On a system with 256 MiB
@ -154,19 +153,15 @@ A: See the documentation in XZ Embedded. In short, something like
dictionary doesn't increase memory usage. dictionary doesn't increase memory usage.
Q: Will xz support threaded compression? Q: How is multi-threaded compression implemented in XZ Utils?
A: It is planned and has been taken into account when designing A: The simplest method is splitting the uncompressed data into blocks
the .xz file format. Eventually there will probably be three types
of threading, each method having its own advantages and disadvantages.
The simplest method is splitting the uncompressed data into blocks
and compressing them in parallel independent from each other. and compressing them in parallel independent from each other.
This is currently the only threading method supported in XZ Utils.
Since the blocks are compressed independently, they can also be Since the blocks are compressed independently, they can also be
decompressed independently. Together with the index feature in .xz, decompressed independently. Together with the index feature in .xz,
this allows using threads to create .xz files for random-access this allows using threads to create .xz files for random-access
reading. This also makes threaded decompression possible, although reading. This also makes threaded decompression possible.
it is not clear if threaded decompression will ever be implemented.
The independent blocks method has a couple of disadvantages too. It The independent blocks method has a couple of disadvantages too. It
will compress worse than a single-block method. Often the difference will compress worse than a single-block method. Often the difference
@ -174,15 +169,17 @@ A: It is planned and has been taken into account when designing
the memory usage of the compressor increases linearly when adding the memory usage of the compressor increases linearly when adding
threads. threads.
Match finder parallelization is another threading method. It has At least two other threading methods are possible but these haven't
been in 7-Zip for ages. It doesn't affect compression ratio or been implemented in XZ Utils:
memory usage significantly. Among the three threading methods, only
this is useful when compressing small files (files that are not Match finder parallelization has been in 7-Zip for ages. It doesn't
significantly bigger than the dictionary). Unfortunately this method affect compression ratio or memory usage significantly. Among the
scales only to about two CPU cores. three threading methods, only this is useful when compressing small
files (files that are not significantly bigger than the dictionary).
Unfortunately this method scales only to about two CPU cores.
The third method is pigz-style threading (I use that name, because The third method is pigz-style threading (I use that name, because
pigz <http://www.zlib.net/pigz/> uses that method). It doesn't pigz <https://www.zlib.net/pigz/> uses that method). It doesn't
affect compression ratio significantly and scales to many cores. affect compression ratio significantly and scales to many cores.
The memory usage scales linearly when threads are added. This isn't The memory usage scales linearly when threads are added. This isn't
significant with pigz, because Deflate uses only a 32 KiB dictionary, significant with pigz, because Deflate uses only a 32 KiB dictionary,
@ -193,12 +190,35 @@ A: It is planned and has been taken into account when designing
cores the overhead is not a big deal anymore. cores the overhead is not a big deal anymore.
Combining the threading methods will be possible and also useful. Combining the threading methods will be possible and also useful.
E.g. combining match finder parallelization with pigz-style threading For example, combining match finder parallelization with pigz-style
can cut the memory usage by 50 %. threading or independent-blocks-threading can cut the memory usage
by 50 %.
It is possible that the single-threaded method will be modified to
create files identical to the pigz-style method. We'll see once Q: I told xz to use many threads but it is using only one or two
pigz-style threading has been implemented in liblzma. processor cores. What is wrong?
A: Since multi-threaded compression is done by splitting the data into
blocks that are compressed individually, if the input file is too
small for the block size, then many threads cannot be used. The
default block size increases when the compression level is
increased. For example, xz -6 uses 8 MiB LZMA2 dictionary and
24 MiB blocks, and xz -9 uses 64 MiB LZMA dictionary and 192 MiB
blocks. If the input file is 100 MiB, xz -6 can use five threads
of which one will finish quickly as it has only 4 MiB to compress.
However, for the same file, xz -9 can only use one thread.
One can adjust block size with --block-size=SIZE but making the
block size smaller than LZMA2 dictionary is waste of RAM: using
xz -9 with 6 MiB blocks isn't any better than using xz -6 with
6 MiB blocks. The default settings use a block size bigger than
the LZMA2 dictionary size because this was seen as a reasonable
compromise between RAM usage and compression ratio.
When decompressing, the ability to use threads depends on how the
file was created. If it was created in multi-threaded mode then
it can be decompressed in multi-threaded mode too if there are
multiple blocks in the file.
Q: How do I build a program that needs liblzmadec (lzmadec.h)? Q: How do I build a program that needs liblzmadec (lzmadec.h)?