As measured on linux.git, adding --date-order to a log command can
result in a significant slowdown (~25x here; I've seen ~100x on other
repositories):
$ time git log --first-parent --max-count=51 master > /dev/null
real 0m0.024s
user 0m0.006s
sys 0m0.016s
$ time git log --date-order --first-parent --max-count=51 master > /dev/null
real 0m0.652s
user 0m0.570s
sys 0m0.074s
In combination with --first-parent, --date-order (or any other
ordering option) should be a no-op, since --first-parent selects a
linear history. So it seems like there's a significant performance win
available by ignoring ordering options when --first-parent is present.
Would this change be desirable? If so, I'll see about submitting a
patch.
More generally, it seems like it might be possible to identify when
the selected commits are linear and avoid the cost of sorting.
Josiah
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