Comparing and Merging Files - 5. diff Performance Tradeoffs

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# 5. `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. There are two ways that you can affect the performance of GNU `diff` by changing 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 ``-H'` or ``--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.

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