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localrepo: experimental support for non-zlib revlog compression...
localrepo: experimental support for non-zlib revlog compression The final part of integrating the compression manager APIs into revlog storage is the plumbing for repositories to advertise they are using non-zlib storage and for revlogs to instantiate a non-zlib compression engine. The main intent of the compression manager work was to zstd all of the things. Adding zstd to revlogs has proved to be more involved than other places because revlogs are... special. Very small inputs and the use of delta chains (which are themselves a form of compression) are a completely different use case from streaming compression, which bundles and the wire protocol employ. I've conducted numerous experiments with zstd in revlogs and have yet to formalize compression settings and a storage architecture that I'm confident I won't regret later. In other words, I'm not yet ready to commit to a new mechanism for using zstd - or any other compression format - in revlogs. That being said, having some support for zstd (and other compression formats) in revlogs in core is beneficial. It can allow others to conduct experiments. This patch introduces *highly experimental* support for non-zlib compression formats in revlogs. Introduced is a config option to control which compression engine to use. Also introduced is a namespace of "exp-compression-*" requirements to denote support for non-zlib compression in revlogs. I've prefixed the namespace with "exp-" (short for "experimental") because I'm not confident of the requirements "schema" and in no way want to give the illusion of supporting these requirements in the future. I fully intend to drop support for these requirements once we figure out what we're doing with zstd in revlogs. A good portion of the patch is teaching the requirements system about registered compression engines and passing the requested compression engine as an opener option so revlogs can instantiate the proper compression engine for new operations. That's a verbose way of saying "we can now use zstd in revlogs!" On an `hg pull` conversion of the mozilla-unified repo with no extra redelta settings (like aggressivemergedeltas), we can see the impact of zstd vs zlib in revlogs: $ hg perfrevlogchunks -c ! chunk ! wall 2.032052 comb 2.040000 user 1.990000 sys 0.050000 (best of 5) ! wall 1.866360 comb 1.860000 user 1.820000 sys 0.040000 (best of 6) ! chunk batch ! wall 1.877261 comb 1.870000 user 1.860000 sys 0.010000 (best of 6) ! wall 1.705410 comb 1.710000 user 1.690000 sys 0.020000 (best of 6) $ hg perfrevlogchunks -m ! chunk ! wall 2.721427 comb 2.720000 user 2.640000 sys 0.080000 (best of 4) ! wall 2.035076 comb 2.030000 user 1.950000 sys 0.080000 (best of 5) ! chunk batch ! wall 2.614561 comb 2.620000 user 2.580000 sys 0.040000 (best of 4) ! wall 1.910252 comb 1.910000 user 1.880000 sys 0.030000 (best of 6) $ hg perfrevlog -c -d 1 ! wall 4.812885 comb 4.820000 user 4.800000 sys 0.020000 (best of 3) ! wall 4.699621 comb 4.710000 user 4.700000 sys 0.010000 (best of 3) $ hg perfrevlog -m -d 1000 ! wall 34.252800 comb 34.250000 user 33.730000 sys 0.520000 (best of 3) ! wall 24.094999 comb 24.090000 user 23.320000 sys 0.770000 (best of 3) Only modest wins for the changelog. But manifest reading is significantly faster. What's going on? One reason might be data volume. zstd decompresses faster. So given more bytes, it will put more distance between it and zlib. Another reason is size. In the current design, zstd revlogs are *larger*: debugcreatestreamclonebundle (size in bytes) zlib: 1,638,852,492 zstd: 1,680,601,332 I haven't investigated this fully, but I reckon a significant cause of larger revlogs is that the zstd frame/header has more bytes than zlib's. For very small inputs or data that doesn't compress well, we'll tend to store more uncompressed chunks than with zlib (because the compressed size isn't smaller than original). This will make revlog reading faster because it is doing less decompression. Moving on to bundle performance: $ hg bundle -a -t none-v2 (total CPU time) zlib: 102.79s zstd: 97.75s So, marginal CPU decrease for reading all chunks in all revlogs (this is somewhat disappointing). $ hg bundle -a -t <engine>-v2 (total CPU time) zlib: 191.59s zstd: 115.36s This last test effectively measures the difference between zlib->zlib and zstd->zstd for revlogs to bundle. This is a rough approximation of what a server does during `hg clone`. There are some promising results for zstd. But not enough for me to feel comfortable advertising it to users. We'll get there...
Mads Kiilerich - - Load All Authors

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merge-tools.txt
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To merge files Mercurial uses merge tools.
A merge tool combines two different versions of a file into a merged
file. Merge tools are given the two files and the greatest common
ancestor of the two file versions, so they can determine the changes
made on both branches.
Merge tools are used both for :hg:`resolve`, :hg:`merge`, :hg:`update`,
:hg:`backout` and in several extensions.
Usually, the merge tool tries to automatically reconcile the files by
combining all non-overlapping changes that occurred separately in
the two different evolutions of the same initial base file. Furthermore, some
interactive merge programs make it easier to manually resolve
conflicting merges, either in a graphical way, or by inserting some
conflict markers. Mercurial does not include any interactive merge
programs but relies on external tools for that.
Available merge tools
=====================
External merge tools and their properties are configured in the
merge-tools configuration section - see hgrc(5) - but they can often just
be named by their executable.
A merge tool is generally usable if its executable can be found on the
system and if it can handle the merge. The executable is found if it
is an absolute or relative executable path or the name of an
application in the executable search path. The tool is assumed to be
able to handle the merge if it can handle symlinks if the file is a
symlink, if it can handle binary files if the file is binary, and if a
GUI is available if the tool requires a GUI.
There are some internal merge tools which can be used. The internal
merge tools are:
.. internaltoolsmarker
Internal tools are always available and do not require a GUI but will by default
not handle symlinks or binary files.
Choosing a merge tool
=====================
Mercurial uses these rules when deciding which merge tool to use:
1. If a tool has been specified with the --tool option to merge or resolve, it
is used. If it is the name of a tool in the merge-tools configuration, its
configuration is used. Otherwise the specified tool must be executable by
the shell.
2. If the ``HGMERGE`` environment variable is present, its value is used and
must be executable by the shell.
3. If the filename of the file to be merged matches any of the patterns in the
merge-patterns configuration section, the first usable merge tool
corresponding to a matching pattern is used. Here, binary capabilities of the
merge tool are not considered.
4. If ui.merge is set it will be considered next. If the value is not the name
of a configured tool, the specified value is used and must be executable by
the shell. Otherwise the named tool is used if it is usable.
5. If any usable merge tools are present in the merge-tools configuration
section, the one with the highest priority is used.
6. If a program named ``hgmerge`` can be found on the system, it is used - but
it will by default not be used for symlinks and binary files.
7. If the file to be merged is not binary and is not a symlink, then
internal ``:merge`` is used.
8. The merge of the file fails and must be resolved before commit.
.. note::
After selecting a merge program, Mercurial will by default attempt
to merge the files using a simple merge algorithm first. Only if it doesn't
succeed because of conflicting changes Mercurial will actually execute the
merge program. Whether to use the simple merge algorithm first can be
controlled by the premerge setting of the merge tool. Premerge is enabled by
default unless the file is binary or a symlink.
See the merge-tools and ui sections of hgrc(5) for details on the
configuration of merge tools.