On 10/13/2017 11:27 AM, Jeff King wrote:
On Fri, Oct 13, 2017 at 10:26:46AM -0400, Jeff King wrote:
On Fri, Oct 13, 2017 at 10:25:10AM -0400, Derrick Stolee wrote:
This does appear to be the problem. The missing DIFF_OPT_HAS_CHANGES is
causing diff_can_quit_early() to return false. Due to the corner-case of the
bug it seems it will not be a huge performance improvement in most cases.
Still worth fixing and I'm looking at your suggestions to try and learn this
area better.
Yeah, I just timed some pathspec limits on linux.git, and it makes at
best a fraction of a percent improvement (but any improvement is well
within run-to-run noise). Which is not surprising.
I agree it's worth fixing, though.
Here it is cleaned up and with a commit message. There's another case
that can be optimized, too: --remove-empty with an all-deletions commit.
That's probably even more obscure and pathological, but it was easy to
cover in the same breath.
I didn't bother making a perf script, since this really isn't indicative
of real-world performance. If we wanted to do perf regression tests
here, I think the best path forward would be:
1. Make sure there the perf tests cover pathspecs (maybe in p0001?).
2. Make it easy to run the whole perf suite against a "bomb" repo.
This surely isn't the only slow thing of interest.
-- >8 --
Subject: revision: quit pruning diff more quickly when possible
When the revision traversal machinery is given a pathspec,
we must compute the parent-diff for each commit to determine
which ones are TREESAME. We set the QUICK diff flag to avoid
looking at more entries than we need; we really just care
whether there are any changes at all.
But there is one case where we want to know a bit more: if
--remove-empty is set, we care about finding cases where the
change consists only of added entries (in which case we may
prune the parent in try_to_simplify_commit()). To cover that
case, our file_add_remove() callback does not quit the diff
upon seeing an added entry; it keeps looking for other types
of entries.
But this means when --remove-empty is not set (and it is not
by default), we compute more of the diff than is necessary.
You can see this in a pathological case where a commit adds
a very large number of entries, and we limit based on a
broad pathspec. E.g.:
perl -e '
chomp(my $blob = `git hash-object -w --stdin </dev/null`);
for my $a (1..1000) {
for my $b (1..1000) {
print "100644 $blob\t$a/$b\n";
}
}
' | git update-index --index-info
git commit -qm add
git rev-list HEAD -- .
This case takes about 100ms now, but after this patch only
needs 6ms. That's not a huge improvement, but it's easy to
get and it protects us against even more pathological cases
(e.g., going from 1 million to 10 million files would take
ten times as long with the current code, but not increase at
all after this patch).
This is reported to minorly speed-up pathspec limiting in
real world repositories (like the 100-million-file Windows
repository), but probably won't make a noticeable difference
outside of pathological setups.
This patch actually covers the case without --remove-empty,
and the case where we see only deletions. See the in-code
comment for details.
Note that we have to add a new member to the diff_options
struct so that our callback can see the value of
revs->remove_empty_trees. This callback parameter could be
passed to the "add_remove" and "change" callbacks, but
there's not much point. They already receive the
diff_options struct, and doing it this way avoids having to
update the function signature of the other callbacks
(arguably the format_callback and output_prefix functions
could benefit from the same simplification).
Signed-off-by: Jeff King <p...@peff.net>
---
diff.h | 1 +
revision.c | 16 +++++++++++++---
2 files changed, 14 insertions(+), 3 deletions(-)
diff --git a/diff.h b/diff.h
index 7dcfcfbef7..4a34d256f1 100644
--- a/diff.h
+++ b/diff.h
@@ -180,6 +180,7 @@ struct diff_options {
pathchange_fn_t pathchange;
change_fn_t change;
add_remove_fn_t add_remove;
+ void *change_fn_data;
diff_format_fn_t format_callback;
void *format_callback_data;
diff_prefix_fn_t output_prefix;
diff --git a/revision.c b/revision.c
index 8fd222f3bf..a3f245e2cc 100644
--- a/revision.c
+++ b/revision.c
@@ -399,8 +399,16 @@ static struct commit *one_relevant_parent(const struct
rev_info *revs,
* if the whole diff is removal of old data, and otherwise
* REV_TREE_DIFFERENT (of course if the trees are the same we
* want REV_TREE_SAME).
- * That means that once we get to REV_TREE_DIFFERENT, we do not
- * have to look any further.
+ *
+ * The only time we care about the distinction is when
+ * remove_empty_trees is in effect, in which case we care only about
+ * whether the whole change is REV_TREE_NEW, or if there's another type
+ * of change. Which means we can stop the diff early in either of these
+ * cases:
+ *
+ * 1. We're not using remove_empty_trees at all.
+ *
+ * 2. We saw anything except REV_TREE_NEW.
*/
static int tree_difference = REV_TREE_SAME;
@@ -411,9 +419,10 @@ static void file_add_remove(struct diff_options *options,
const char *fullpath, unsigned dirty_submodule)
{
int diff = addremove == '+' ? REV_TREE_NEW : REV_TREE_OLD;
+ struct rev_info *revs = options->change_fn_data;
tree_difference |= diff;
- if (tree_difference == REV_TREE_DIFFERENT)
+ if (!revs->remove_empty_trees || tree_difference != REV_TREE_NEW)
DIFF_OPT_SET(options, HAS_CHANGES);
}
@@ -1351,6 +1360,7 @@ void init_revisions(struct rev_info *revs, const char *prefix)
DIFF_OPT_SET(&revs->pruning, QUICK);
revs->pruning.add_remove = file_add_remove;
revs->pruning.change = file_change;
+ revs->pruning.change_fn_data = revs;
revs->sort_order = REV_SORT_IN_GRAPH_ORDER;
revs->dense = 1;
revs->prefix = prefix;
Thanks, Peff. This patch looks good to me.
I tried a few other things like adding a flag DIFF_OPT_HAS_ANY_CHANGE
next to DIFF_OPT_HAS_CHANGES that we could check in
diff_can_quit_early() but it had side-effects that broke existing tests.
From this exploration, it does seem necessary to be aware of
'remove_empty_trees'.
Thanks,
-Stolee
