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diff Performance Tradeoffs
GNU diff runs quite efficiently; however, in some circumstances
you can cause it to run faster or produce a more compact set of changes.
One way to improve diff performance is to use hard or
symbolic links to files instead of copies. This improves performance
because diff normally does not need to read two hard or
symbolic links to the same file, since their contents must be
identical. For example, suppose you copy a large directory hierarchy,
make a few changes to the copy, and then often use diff -r to
compare the original to the copy. If the original files are
read-only, you can greatly improve performance by creating the copy
using hard or symbolic links (e.g., with GNU cp -lR or
cp -sR). Before editing a file in the copy for the first time,
you should break the link and replace it with a regular copy.
You can also affect the performance of GNU diff by
giving it options that change the way it compares files.
Performance has more than one dimension. These options improve one
aspect of performance at the cost of another, or they improve
performance in some cases while hurting it in others.
The way that GNU diff determines which lines have changed always
comes up with a near-minimal set of differences. Usually it is good
enough for practical purposes. If the diff output is large, you
might want diff to use a modified algorithm that sometimes
produces a smaller set of differences. The -d or
--minimal option does this; however, it can also cause
diff to run more slowly than usual, so it is not the default
behavior.
When the files you are comparing are large and have small groups of
changes scattered throughout them, you can use the
--speed-large-files option to make a different modification to
the algorithm that diff uses. If the input files have a constant
small density of changes, this option speeds up the comparisons without
changing the output. If not, diff might produce a larger set of
differences; however, the output will still be correct.
Normally diff discards the prefix and suffix that is common to
both files before it attempts to find a minimal set of differences.
This makes diff run faster, but occasionally it may produce
non-minimal output. The --horizon-lines=lines option
prevents diff from discarding the last lines lines of the
prefix and the first lines lines of the suffix. This gives
diff further opportunities to find a minimal output.
Suppose a run of changed lines includes a sequence of lines at one end
and there is an identical sequence of lines just outside the other end.
The diff command is free to choose which identical sequence is
included in the hunk. In this case, diff normally shifts the
hunk's boundaries when this merges adjacent hunks, or shifts a hunk's
lines towards the end of the file. Merging hunks can make the output
look nicer in some cases.
