limit "contains" traversals based on commit timestamp
When looking for commits that contain other commits (e.g.,
via "git tag --contains"), we can end up traversing useless
portions of the graph. For example, if I am looking for a
tag that contains a commit made last week, there is not much
point in traversing portions of the history graph made five
years ago.
This optimization can provide massive speedups. For example,
doing "git tag --contains HEAD~200" in the linux-2.6
repository goes from:
real 0m5.302s
user 0m5.116s
sys 0m0.184s
to:
real 0m0.030s
user 0m0.020s
sys 0m0.008s
The downside is that we will no longer find some answers in
the face of extreme clock skew, as we will stop the
traversal early when seeing commits skewed too far into the
past.
Name-rev already implements a similar optimization, using a
"slop" of one day to allow for a certain amount of clock
skew in commit timestamps. This patch introduces a
"core.clockskew" variable, which allows specifying the
allowable amount of clock skew in seconds. For safety, it
defaults to "none", causing a full traversal (i.e., no
change in behavior from previous versions).
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>