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By default, describe considers only 10 candidate matches, and stops
traversing when we have enough. This makes things much faster in a large
repository, where collecting all candidates requires walking all the way
down to the root (or at least to the oldest tag). This goes all the way
back to 8713ab3079 (Improve git-describe performance by reducing
revision listing., 2007-01-13).
However, we don't stop immediately when we have enough candidates. We
keep traversing and only bail when we find one more candidate that we're
ignoring. Usually this is not too expensive, if the tags are sprinkled
evenly throughout history. But if you are unlucky, you might hit the max
candidate quickly, and then have a huge swath of history before finding
the next one.
Our p6100 test has exactly this unlucky case: with a max of "1", we find
a recent tag quickly and then have to go all the way to the root to find
the old tag that will be discarded.
A more interesting real-world case is:
git describe --candidates=1 --match=v6.12-rc4 HEAD
in the linux.git repo. There we restrict the set of tags to a single
one, so there is no older candidate to find at all! But despite
--candidates=1, we keep traversing to the root only to find nothing.
So why do we keep traversing after hitting thet max? There are two
reasons I can see:
1. In theory the extra information that there was another candidate
could be useful, and we record it in the gave_up_on variable. But
we only show this information with --debug.
2. After finding the candidate, there's more processing we do in our
loop. The most important of this is propagating the "within" flags
to our parent commits, and putting them in the commit_list we'll
use for finish_depth_computation().
That function continues the traversal until we've counted all
commits reachable from the starting point but not reachable from
our best candidate tag (so essentially counting "$tag..$start", but
avoiding re-walking over the bits we've seen). If we break
immediately without putting those commits into the list, our depth
computation will be wrong (in the worst case we'll count all the
way down to the root, not realizing those commits are included in
our tag).
But we don't need to find a new candidate for (2). As soon as we finish
the loop iteration where we hit max_candidates, we can then quit on the
next iteration. This should produce the same output as the original code
(which could, after all, find a candidate on the very next commit
anyway) but ends the traversal with less pointless digging.
We still have to set "gave_up_on"; we've popped it off the list and it
has to go back. An alternative would be to re-order the loop so that it
never gets popped, but it's perhaps still useful to show in the --debug
output, so we need to know it anyway. We do have to adjust the --debug
output since it's now just a commit where we stopped traversing, and not
the max+1th candidate.
p6100 shows the speedup using linux.git:
Test HEAD^ HEAD
---------------------------------------------------------------------------------------
6100.2: describe HEAD 0.70(0.63+0.06) 0.71(0.66+0.04) +1.4%
6100.3: describe HEAD with one max candidate 0.70(0.64+0.05) 0.01(0.00+0.00) -98.6%
6100.4: describe HEAD with one tag 0.70(0.67+0.03) 0.70(0.63+0.06) +0.0%
Reported-by: Josh Poimboeuf <jpoimboe@kernel.org>
Helped-by: Rasmus Villemoes <ravi@prevas.dk>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
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