(diffutils.info)diff Performance

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6 'diff' Performance Tradeoffs
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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 '--minimal' ('-d') 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.