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From: Jeff King <>
To: Derrick Stolee <>
Cc: Constantine <>,
	Junio C Hamano <>,
	Christian Couder <>,
	Mike Hommey <>, git <>
Subject: [PATCH] revision: quit pruning diff more quickly when possible
Date: Fri, 13 Oct 2017 11:27:45 -0400	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

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 <>
 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)
@@ -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;

  parent reply	other threads:[~2017-10-13 15:27 UTC|newest]

Thread overview: 23+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2017-10-13  9:51 git-clone causes out of memory Constantine
2017-10-13 10:06 ` Mike Hommey
2017-10-13 10:26   ` Christian Couder
2017-10-13 10:37     ` Mike Hommey
2017-10-13 10:44       ` Christian Couder
2017-10-13 12:04         ` Junio C Hamano
2017-10-13 12:12           ` Constantine
2017-10-13 12:44             ` Jeff King
2017-10-13 13:15               ` Derrick Stolee
2017-10-13 13:39                 ` Derrick Stolee
2017-10-13 13:50                   ` Jeff King
2017-10-13 13:55                     ` Derrick Stolee
2017-10-13 13:56                       ` Jeff King
2017-10-13 14:10                         ` Jeff King
2017-10-13 14:20                           ` Jeff King
2017-10-13 14:25                             ` Derrick Stolee
2017-10-13 14:26                               ` Jeff King
2017-10-13 14:30                                 ` Derrick Stolee
2017-10-13 15:27                                 ` Jeff King [this message]
2017-10-13 15:37                                   ` [PATCH] revision: quit pruning diff more quickly when possible Derrick Stolee
2017-10-13 15:44                                     ` Jeff King
2017-10-14  2:43                                   ` Junio C Hamano
2017-10-13 12:35   ` git-clone causes out of memory Jeff King

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