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 0m0.044sA 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 0m0.013sThe 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 travelsal, 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.
Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
---
Notes:
Regarding that "most common case": I know, I know, one should not
extrapolate only from his own usage patterns... But FWIW, I've been
using various versions of this patch for over two years now, and never
missed the rewritten parents. And just pulled a couple of years worth
of ~/.bash_history files from backups, and there was not a single
occasion where I used line-level log with parent rewriting (apart from
testing these changes). line-log.c | 4 ++--
line-log.h | 2 ++
revision.c | 27 ++++++++++++++++++++++++++-
t/t4211-line-log.sh | 2 +-
4 files changed, 31 insertions(+), 4 deletions(-)
Show changes to 4 files +31 −4
line-log.c, line-log.h, revision.c, t/t4211-line-log.sh
diff --git a/line-log.c b/line-log.c
index 3aff1849e7..772ab57d52 100644
--- a/line-log.c
+++ b/line-log.c
@@ -1194,7 +1194,7 @@ static int process_ranges_merge_commit(struct rev_info *rev, struct commit *comm
/* NEEDSWORK leaking like a sieve */
}
-static int process_ranges_arbitrary_commit(struct rev_info *rev, struct commit *commit)
+int line_log_process_ranges_arbitrary_commit(struct rev_info *rev, struct commit *commit)
{
struct line_log_data *range = lookup_line_range(rev, commit);
int changed = 0;
@@ -1237,7 +1237,7 @@ int line_log_filter(struct rev_info *rev)
while (list) {
struct commit_list *to_free = NULL;
commit = list->item;
- if (process_ranges_arbitrary_commit(rev, commit)) {
+ if (line_log_process_ranges_arbitrary_commit(rev, commit)) {
*pp = list;
pp = &list->next;
} else
diff --git a/line-log.h b/line-log.h
index 882c5055bb..82ae8d98a4 100644
--- a/line-log.h
+++ b/line-log.h
@@ -54,6 +54,8 @@ struct line_log_data {
void line_log_init(struct rev_info *rev, const char *prefix, struct string_list *args);
int line_log_filter(struct rev_info *rev);
+int line_log_process_ranges_arbitrary_commit(struct rev_info *rev,
+ struct commit *commit);
int line_log_print(struct rev_info *rev, struct commit *commit);
diff --git a/revision.c b/revision.c
index 07412297f0..6bdfcb38cd 100644
--- a/revision.c
+++ b/revision.c
@@ -36,6 +36,8 @@ static const char *term_good;
implement_shared_commit_slab(revision_sources, char *);
+static inline int want_ancestry(const struct rev_info *revs);
+
void show_object_with_name(FILE *out, struct object *obj, const char *name)
{
const char *p;
@@ -3359,7 +3361,14 @@ int prepare_revision_walk(struct rev_info *revs)
sort_in_topological_order(&revs->commits, revs->sort_order);
} else if (revs->topo_order)
init_topo_walk(revs);
- if (revs->line_level_traverse)
+ if (revs->line_level_traverse && want_ancestry(revs))
+ /*
+ * At the moment we can only do line-level log with parent
+ * rewriting by performing this expensive pre-filtering step.
+ * If parent rewriting is not requested, then we rather
+ * perform the line-level log filtering during the regular
+ * history traversal.
+ */
line_log_filter(revs);
if (revs->simplify_merges)
simplify_merges(revs);
@@ -3569,6 +3578,22 @@ enum commit_action get_commit_action(struct rev_info *revs, struct commit *commi
return commit_ignore;
if (commit->object.flags & UNINTERESTING)
return commit_ignore;
+ if (revs->line_level_traverse && !want_ancestry(revs)) {
+ /*
+ * In case of line-level log with parent rewriting
+ * prepare_revision_walk() already took care of all line-level
+ * log filtering, and there is nothing left to do here.
+ *
+ * If parent rewriting was not requested, then this is the
+ * place to perform the line-level log filtering. Notably,
+ * this check, though expensive, must come before the other,
+ * cheaper filtering conditions, because the tracked line
+ * ranges must be adjusted even when the commit will end up
+ * being ignored based on other conditions.
+ */
+ if (!line_log_process_ranges_arbitrary_commit(revs, commit))
+ return commit_ignore;
+ }
if (revs->min_age != -1 &&
comparison_date(revs, commit) > revs->min_age)
return commit_ignore;
diff --git a/t/t4211-line-log.sh b/t/t4211-line-log.sh
index c378a453de..8096208abb 100755
--- a/t/t4211-line-log.sh
+++ b/t/t4211-line-log.sh
@@ -180,7 +180,7 @@ test_expect_success 'setup for checking line-log and parent oids' '
'
# Parent oid should be from immediate parent.
-test_expect_failure 'parent oids without parent rewriting' '
+test_expect_success 'parent oids without parent rewriting' '
cat >expect <<-EOF &&
$head_oid $prev_oid Modify func2() in file.c
$root_oid Add func1() and func2() in file.c
SZEDER Gábor