Update the FAQ.

1 files changed, 100 insertions, 4 deletions

diff --git a/doc/faq.txt b/doc/faq.txt
index 8e87457a..36182c2c 100644
--- a/doc/faq.txt
+++ b/doc/faq.txt
@@ -6,7 +6,7 @@ Q:  What do the letters XZ mean?
 
 A:  Nothing. They are just two letters, which come from the file format
     suffix .xz. The .xz suffix was selected, because it seemed to be
-    pretty much unused. It is no deeper meaning.
+    pretty much unused. It has no deeper meaning.
 
 
 Q:  What are LZMA and LZMA2?
@@ -33,7 +33,18 @@ A:  7-Zip and LZMA SDK are the original projects. LZMA SDK is roughly
     LZMA Utils.
 
     There are several other projects using LZMA. Most are more or less
-    based on LZMA SDK.
+    based on LZMA SDK. See <http://7-zip.org/links.html>.
+
+
+Q:  Why is liblzma named liblzma if its primary file format is .xz?
+    Shouldn't it be e.g. libxz?
+
+A:  When the designing of the .xz format began, the idea was to replace
+    the .lzma format and use the same .lzma suffix. It would have been
+    quite OK to reuse the suffix when there were very few .lzma files
+    around. However, the old .lzma format become popular before the
+    new format was finished. The new format was renamed to .xz but the
+    name of liblzma wasn't changed.
 
 
 Q:  Do XZ Utils support the .7z format?
@@ -96,7 +107,7 @@ A:  The .xz format is documented in xz-file-format.txt. It is a container
     Documenting LZMA and LZMA2 is planned, but for now, there is no other
     documentation that the source code. Before you begin, you should know
     the basics of LZ77 and range coding algorithms. LZMA is based on LZ77,
-    but LZMA is *a lot* more complex. Range coding is used to compress
+    but LZMA is a lot more complex. Range coding is used to compress
     the final bitstream like Huffman coding is used in Deflate.
 
 
@@ -104,6 +115,90 @@ 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,
     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
+    of RAM, xz says that it cannot allocate memory. Can I make the
+    script work without modifying it?
+
+A:  Set a default memory usage limit for compression. You can do it e.g.
+    in a shell initialization script such as ~/.bashrc or /etc/profile:
+
+        XZ_DEFAULTS=--memlimit-compress=150MiB
+        export XZ_DEFAULTS
+
+    xz will then scale the compression settings down so that the given
+    memory usage limit is not reached. This way xz shouldn't run out
+    of memory.
+
+    Check also that memory-related resource limits are high enough.
+    On most systems, "ulimit -a" will show the current resource limits.
+
+
+Q:  How do I create files that can be decompressed with XZ Embedded?
+
+A:  See the documentation in XZ Embedded. In short, something like
+    this is a good start:
+
+        xz --check=crc32 --lzma2=preset=6e,dict=64KiB
+
+    Or if a BCJ filter is needed too, e.g. if compressing
+    a kernel image for PowerPC:
+
+        xz --check=crc32 --powerpc --lzma2=preset=6e,dict=64KiB
+
+    Adjust dictionary size to get a good compromise between
+    compression ratio and decompressor memory usage. Note that
+    in single-call decompression mode of XZ Embedded, a big
+    dictionary doesn't increase memory usage.
+
+
+Q:  Will xz support threaded compression?
+
+A:  It is planned and has been taken into account when designing
+    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.
+    Since the blocks are compressed independently, they can also be
+    decompressed independently. Together with the index feature in .xz,
+    this allows using threads to create .xz files for random-access
+    reading. This also makes threaded decompression possible, although
+    it is not clear if threaded decompression will ever be implemented.
+
+    The independent blocks method has a couple of disadvantages too. It
+    will compress worse than a single-block method. Often the difference
+    is not too big (maybe 1-2 %) but sometimes it can be too big. Also,
+    the memory usage of the compressor increases linearly when adding
+    threads.
+
+    Match finder parallelization is another threading method. It has
+    been in 7-Zip for ages. It doesn't affect compression ratio or
+    memory usage significantly. Among the 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
+    pigz <http://www.zlib.net/pigz/> uses that method). It doesn't
+    affect compression ratio significantly and scales to many cores.
+    The memory usage scales linearly when threads are added. It isn't
+    significant with pigz, because Deflate uses only 32 KiB dictionary,
+    but with LZMA2 the memory usage will increase dramatically just like
+    with the independent blocks method. There is also a constant
+    computational overhead, which may make pigz-method a bit dull on
+    dual-core compared to the parallel match finder method, but with more
+    cores the overhead is not a big deal anymore.
+
+    Combining the threading methods will be possible and also useful.
+    E.g. combining match finder parallelization with pigz-style threading
+    can cut the memory usage by 50 %.
+
+    It is possible that the single-threaded method will be modified to
+    create files indentical to the pigz-style method. We'll see once
+    pigz-style threading has been implemented in liblzma.
 
 
 Q:  How do I build a program that needs liblzmadec (lzmadec.h)?
@@ -124,5 +219,6 @@ A:  Give --enable-small to the configure script. Use also appropriate
 
     If the result is still too big, take a look at XZ Embedded. It is
     a separate project, which provides a limited but significantly
-    smaller XZ decoder implementation than XZ Utils.
+    smaller XZ decoder implementation than XZ Utils. You can find it
+    at <http://tukaani.org/xz/embedded.html>.