The current line-level log implementation performs a preprocessing
step in prepare_revision_walk(), during which the line_log_filter()
function filters and rewrites history to keep only commits modifying
the given line range. This preprocessing affects both responsiveness
and correctness:
- Git doesn't produce any output during this preprocessing step.
Checking whether a commit modified the given line range is
somewhat expensive, so depending on the size of the given revision
range this preprocessing can result in a significant delay before
the first commit is shown.
- Limiting the number of displayed commits (e.g. 'git log -3 -L...')
doesn't limit the amount of work during preprocessing, because
that limit is applied during history traversal. Alas, by that
point this expensive preprocessing step has already churned
through the whole revision range to find all commits modifying the
revision range, even though only a few of them need to be shown.
- It rewrites parents, with no way to turn it off. Without the user
explicitly requesting parent rewriting any parent object ID shown
should be that of the immediate parent, just like in case of a
pathspec-limited history traversal without parent rewriting.
However, after that preprocessing step rewrote history, the
subsequent "regular" history traversal (i.e. get_revision() in a
loop) only sees commits modifying the given line range.
Consequently, it can only show the object ID of the last ancestor
that modified the given line range (which might happen to be the
immediate parent, but many-many times it isn't).
This patch addresses both the correctness and, at least for the common
case, the responsiveness issues by integrating line-level log
filtering into the regular revision walking machinery:
- Make process_ranges_arbitrary_commit(), the static function in
'line-log.c' deciding whether a commit modifies the given line
range, public by removing the static keyword and adding the
'line_log_' prefix, so it can be called from other parts of the
revision walking machinery.
- If the user didn't explicitly ask for parent rewriting (which, I
believe, is the most common case):
- Call this now-public function during regular history traversal,
namely from get_commit_action() to ignore any commits not
modifying the given line range.
Note that while this check is relatively expensive, it must be
performed before other, much cheaper conditions, because the
tracked line range must be adjusted even when the commit will
end up being ignored by other conditions.
- Skip the line_log_filter() call, i.e. the expensive
preprocessing step, in prepare_revision_walk(), because, thanks
to the above points, the revision walking machinery is now able
to filter out commits not modifying the given line range while
traversing history.
This way the regular history traversal sees the unmodified
history, and is therefore able to print the object ids of the
immediate parents of the listed commits. The eliminated
preprocessing step can greatly reduce the delay before the first
commit is shown, see the numbers below.
- However, if the user did explicitly ask for parent rewriting via
'--parents' or a similar option, then stick with the current
implementation for now, i.e. perform that expensive filtering and
history rewriting in the preprocessing step just like we did
before, leaving the initial delay as long as it was.
I tried to integrate line-level log filtering with parent rewriting
into the regular history traversal, but, unfortunately, several
subtleties resisted... :) Maybe someday we'll figure out how to do
that, but until then at least the simple and common (i.e. without
parent rewriting) 'git log -L:func:file' commands can benefit from the
reduced delay.
This change makes the failing 'parent oids without parent rewriting'
test in 't4211-line-log.sh' succeed.
The reduced delay is most noticable when there's a commit modifying
the line range near the tip of a large-ish revision range:
# no parent rewriting requested, no commit-graph present
$ time git --no-pager log -L:read_alternate_refs:sha1-file.c -1 v2.23.0
Before:
real 0m9.570s
user 0m9.494s
sys 0m0.076s
After:
real 0m0.718s
user 0m0.674s
sys 0m0.044s
A significant part of the remaining delay is spent reading and parsing
commit objects in limit_list(). With the help of the commit-graph we
can eliminate most of that reading and parsing overhead, so here are
the timing results of the same command as above, but this time using
the commit-graph:
Before:
real 0m8.874s
user 0m8.816s
sys 0m0.057s
After:
real 0m0.107s
user 0m0.091s
sys 0m0.013s
The next patch will further reduce the remaining delay.
To be clear: this patch doesn't actually optimize the line-level log,
but merely moves most of the work from the preprocessing step to the
history traversal, so the commits modifying the line range can be
shown as soon as they are processed, and the traversal can be
terminated as soon as the given number of commits are shown.
Consequently, listing the full history of a line range, potentially
all the way to the root commit, will take the same time as before (but
at least the user might start reading the output earlier).
Furthermore, if the most recent commit modifying the line range is far
away from the starting revision, then that initial delay will still be
significant.
Additional testing by Derrick Stolee: In the Linux kernel repository,
the MAINTAINERS file was changed ~3,500 times across the ~915,000
commits. In addition to that edit frequency, the file itself is quite
large (~18,700 lines). This means that a significant portion of the
computation is taken up by computing the patch-diff of the file. This
patch improves the real time it takes to output the first result quite
a bit:
Command: git log -L 100,200:MAINTAINERS -n 1 >/dev/null
Before: 3.88 s
After: 0.71 s
If we drop the "-n 1" in the command, then there is no change in
end-to-end process time. This is because the command still needs to
walk the entire commit history, which negates the point of this
patch. This is expected.
As a note for future reference, the ~4.3 seconds in the old code
spends ~2.6 seconds computing the patch-diffs, and the rest of the
time is spent walking commits and computing diffs for which paths
changed at each commit. The changed-path Bloom filters could improve
the end-to-end computation time (i.e. no "-n 1" in the command).
Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Git - fast, scalable, distributed revision control system
Git is a fast, scalable, distributed revision control system with an
unusually rich command set that provides both high-level operations
and full access to internals.
Git is an Open Source project covered by the GNU General Public
License version 2 (some parts of it are under different licenses,
compatible with the GPLv2). It was originally written by Linus
Torvalds with help of a group of hackers around the net.
Please read the file INSTALL for installation instructions.
Many Git online resources are accessible from https://git-scm.com/
including full documentation and Git related tools.
See Documentation/gittutorial.txt to get started, then see
Documentation/giteveryday.txt for a useful minimum set of commands, and
Documentation/git-<commandname>.txt for documentation of each command.
If git has been correctly installed, then the tutorial can also be
read with man gittutorial or git help tutorial, and the
documentation of each command with man git-<commandname> or git help <commandname>.
CVS users may also want to read Documentation/gitcvs-migration.txt
(man gitcvs-migration or git help cvs-migration if git is
installed).
Issues which are security relevant should be disclosed privately to
the Git Security mailing list git-security@googlegroups.com.
The maintainer frequently sends the "What's cooking" reports that
list the current status of various development topics to the mailing
list. The discussion following them give a good reference for
project status, development direction and remaining tasks.
The name "git" was given by Linus Torvalds when he wrote the very
first version. He described the tool as "the stupid content tracker"
and the name as (depending on your mood):
random three-letter combination that is pronounceable, and not
actually used by any common UNIX command. The fact that it is a
mispronunciation of "get" may or may not be relevant.
stupid. contemptible and despicable. simple. Take your pick from the
dictionary of slang.
"global information tracker": you're in a good mood, and it actually
works for you. Angels sing, and a light suddenly fills the room.
"goddamn idiotic truckload of sh*t": when it breaks
3cb9d2b6f9