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view mercurial/help/merge-tools.txt @ 26117:4dc5b51f38fe
revlog: change generaldelta delta parent heuristic
The old generaldelta heuristic was "if p1 (or p2) was closer than the last full text,
use it, otherwise use prev". This was problematic when a repo contained multiple
branches that were very different. If commits to branch A were pushed, and the
last full text was branch B, it would generate a fulltext. Then if branch B was
pushed, it would generate another fulltext. The problem is that the last
fulltext (and delta'ing against `prev` in general) has no correlation with the
contents of the incoming revision, and therefore will always have degenerate
cases.
According to the blame, that algorithm was chosen to minimize the chain length.
Since there is already code that protects against that (the delta-vs-fulltext
code), and since it has been improved since the original generaldelta algorithm
went in (2011), I believe the chain length criteria will still be preserved.
The new algorithm always diffs against p1 (or p2 if it's closer), unless the
resulting delta will fail the delta-vs-fulltext check, in which case we delta
against prev.
Some before and after stats on manifest.d size.
internal large repo
old heuristic - 2.0 GB
new heuristic - 1.2 GB
mozilla-central
old heuristic - 242 MB
new heuristic - 261 MB
The regression in mozilla central is due to the new heuristic choosing p2r as
the delta when it's closer to the tip. Switching the algorithm to always prefer
p1r brings the size back down (242 MB). This is result of the way in which
mozilla does merges and pushes, and the result could easily swing the other
direction in other repos (depending on if they merge X into Y or Y into X), but
will never be as degenerate as before.
I future patch will address the regression by introducing an optional, even more
aggressive delta heuristic which will knock the mozilla manifest size down
dramatically.
| author | Durham Goode <durham@fb.com> |
|---|---|
| date | Sun, 30 Aug 2015 13:58:11 -0700 |
| parents | 38e0363dcbe0 |
| children | 247bb7a2c492 |
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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.