Re: [PATCH] Avoid loading commits twice in log with diffs

Thomas Rast [off-list ref] writes:

Test                      with patch        before
--------------------------------------------------------------------
4000.2: log --raw -3000   0.50(0.43+0.06)   0.54(0.46+0.06) +7.0%***
4000.3: log -p -3000      2.34(2.20+0.13)   2.37(2.22+0.13) +1.2%
--------------------------------------------------------------------
Significance hints:  '.' 0.1  '*' 0.05  '**' 0.01  '***' 0.001

It may be a silly question but what is a significance hint?

Overall I think this goes in the right direction and I can see why
the changes in later two hunks are correct, but I am not sure if we
can safely assume that the caller has parsed the incoming commit and
we have a good commit->tree here already.

Right now, this static function has a single call-site in a public
function log_tree_commit(), whose existing callers may all pass an
already parsed commit, but I feel somewhat uneasy to do the above
without some mechanism in place (either parse it here or in the
caller if unparsed, or document that log_tree_commit() must be
called with a parsed commit and perhaps add an assert there) to
ensure that the invariant is not broken in the future.

Thanks.

If you run a log with diffs (such as -p, --raw, --stat etc.) the
current code ends up loading many objects twice.  For example, for
'log -3000 -p' my instrumentation said the objects loaded more than
once are distributed as follows:

  2008 blob
  2103 commit
  2678 tree

Fixing blobs and trees will be harder, because those are really used
within the diff engine and need some form of caching.

However, fixing the commits is easy at least at the band-aid level.
They are triggered by log_tree_diff() invoking diff_tree_sha1() on
commits, which duly loads the specified object to dereference it to a
tree.  Since log_tree_diff() knows that it works with commits and they
must have trees, we can simply pass through the trees.

We add some parse_commit() calls.  The ones for the parents are
required; we do not know at this stage if they have been looked at.
The one for the commit itself is pure paranoia, but has about the same
cost as an assertion on commit->object.parsed.

This has a quite dramatic effect on log --raw, though only a
negligible impact on log -p:

Test                      this tree         HEAD
--------------------------------------------------------------------
4000.2: log --raw -3000   0.50(0.43+0.06)   0.54(0.46+0.06) +7.0%***
4000.3: log -p -3000      2.34(2.20+0.13)   2.37(2.22+0.13) +1.2%
--------------------------------------------------------------------
Significance hints:  '.' 0.1  '*' 0.05  '**' 0.01  '***' 0.001

Signed-off-by: Thomas Rast <redacted>
Signed-off-by: Thomas Rast <redacted>
---

Adjusted for the concern that the commit might not be parsed yet.  I
think it's now more paranoid than the original code, since we cannot
look at commit->parents without parsing.  But it's really an almost
free check.


 log-tree.c | 13 ++++++++++---
 1 file changed, 10 insertions(+), 3 deletions(-)

diff --git a/log-tree.c b/log-tree.c
index 5dc45c4..8a34332 100644
--- a/log-tree.c
+++ b/log-tree.c

@@ -792,11 +792,14 @@ static int log_tree_diff(struct rev_info *opt, struct commit *commit, struct log
 {
 	int showed_log;
 	struct commit_list *parents;
-	unsigned const char *sha1 = commit->object.sha1;
+	unsigned const char *sha1;
 
 	if (!opt->diff && !DIFF_OPT_TST(&opt->diffopt, EXIT_WITH_STATUS))
 		return 0;
 
+	parse_commit(commit);
+	sha1 = commit->tree->object.sha1;
+
 	/* Root commit? */
 	parents = commit->parents;
 	if (!parents) {

@@ -819,7 +822,9 @@ static int log_tree_diff(struct rev_info *opt, struct commit *commit, struct log
 			 * parent, showing summary diff of the others
 			 * we merged _in_.
 			 */
-			diff_tree_sha1(parents->item->object.sha1, sha1, "", &opt->diffopt);
+			parse_commit(parents->item);
+			diff_tree_sha1(parents->item->tree->object.sha1,
+				       sha1, "", &opt->diffopt);
 			log_tree_diff_flush(opt);
 			return !opt->loginfo;
 		}

@@ -832,7 +837,9 @@ static int log_tree_diff(struct rev_info *opt, struct commit *commit, struct log
 	for (;;) {
 		struct commit *parent = parents->item;
 
-		diff_tree_sha1(parent->object.sha1, sha1, "", &opt->diffopt);
+		parse_commit(parent);
+		diff_tree_sha1(parent->tree->object.sha1,
+			       sha1, "", &opt->diffopt);
 		log_tree_diff_flush(opt);
 
 		showed_log |= !opt->loginfo;

-- 
1.8.2.351.g867a5da

On Thu, Mar 28, 2013 at 09:19:34AM +0100, Thomas Rast wrote:

However, fixing the commits is easy at least at the band-aid level.
They are triggered by log_tree_diff() invoking diff_tree_sha1() on
commits, which duly loads the specified object to dereference it to a
tree.  Since log_tree_diff() knows that it works with commits and they
must have trees, we can simply pass through the trees.

Reading this made me wonder if we could be doing the optimization at a
lower level, by re-using objects that we have in our lookup_object cache
(which would include the commit->tree peeling that you're optimizing
here).

My first instinct was to look at read_object_with_reference, but it's a
bit general; for trees, we can indeed find the buffer/size pair. But for
commits, we must infer the size from the buffer (by using strlen). And
for blobs, we do not win at all, as we do not cache the blob contents.

Another option is for diff_tree_sha1 to use parse_tree_indirect. That
will pull the commit object from the cache and avoid re-parsing it, as
well as re-use any trees (although that should not happen much in a
regular "log" traversal).

That patch looks something like:

diff --git a/tree-diff.c b/tree-diff.c
index ba01563..db454bc 100644
--- a/tree-diff.c
+++ b/tree-diff.c

@@ -269,27 +269,28 @@ int diff_tree_sha1(const unsigned char *old, const unsigned char *new, const cha
 
 int diff_tree_sha1(const unsigned char *old, const unsigned char *new, const char *base, struct diff_options *opt)
 {
-	void *tree1, *tree2;
+	struct tree *tree1, *tree2;
 	struct tree_desc t1, t2;
-	unsigned long size1, size2;
 	int retval;
 
-	tree1 = read_object_with_reference(old, tree_type, &size1, NULL);
+	tree1 = parse_tree_indirect(old);
 	if (!tree1)
 		die("unable to read source tree (%s)", sha1_to_hex(old));
-	tree2 = read_object_with_reference(new, tree_type, &size2, NULL);
+	tree2 = parse_tree_indirect(new);
 	if (!tree2)
 		die("unable to read destination tree (%s)", sha1_to_hex(new));
-	init_tree_desc(&t1, tree1, size1);
-	init_tree_desc(&t2, tree2, size2);
+	init_tree_desc(&t1, tree1->buffer, tree1->size);
+	init_tree_desc(&t2, tree2->buffer, tree2->size);
 	retval = diff_tree(&t1, &t2, base, opt);
 	if (!*base && DIFF_OPT_TST(opt, FOLLOW_RENAMES) && diff_might_be_rename()) {
-		init_tree_desc(&t1, tree1, size1);
-		init_tree_desc(&t2, tree2, size2);
+		init_tree_desc(&t1, tree1->buffer, tree1->size);
+		init_tree_desc(&t2, tree2->buffer, tree2->size);
 		try_to_follow_renames(&t1, &t2, base, opt);
 	}
-	free(tree1);
-	free(tree2);
+	/*
+	 * Note that we are now filling up our cache with extra tree data; we
+	 * could potentially unparse the tree objects.
+	 */
 	return retval;
 }
 

but it turns out to actually be slower! The problem is that parse_object
will actually re-check the sha1 on every object it reads, which
read_object_with_reference will not do.

Using this hack:

diff --git a/object.c b/object.c
index 20703f5..361eb74 100644
--- a/object.c
+++ b/object.c

@@ -195,6 +195,7 @@ struct object *parse_object_or_die(const unsigned char *sha1,
 	die(_("unable to parse object: %s"), name ? name : sha1_to_hex(sha1));
 }
 
+int parse_object_quick = 0;
 struct object *parse_object(const unsigned char *sha1)
 {
 	unsigned long size;

@@ -221,7 +222,8 @@ struct object *parse_object(const unsigned char *sha1)
 
 	buffer = read_sha1_file(sha1, &type, &size);
 	if (buffer) {
-		if (check_sha1_signature(repl, buffer, size, typename(type)) < 0) {
+		if (!parse_object_quick &&
+		    check_sha1_signature(repl, buffer, size, typename(type)) < 0) {
 			free(buffer);
 			error("sha1 mismatch %s", sha1_to_hex(repl));
 			return NULL;

diff --git a/tree-diff.c b/tree-diff.c
index db454bc..4b55a1a 100644
--- a/tree-diff.c
+++ b/tree-diff.c

@@ -267,18 +267,23 @@ int diff_tree_sha1(const unsigned char *old, const unsigned char *new, const cha
 	q->nr = 1;
 }
 
+/* hacky */
+extern int parse_object_quick;
+
 int diff_tree_sha1(const unsigned char *old, const unsigned char *new, const char *base, struct diff_options *opt)
 {
 	struct tree *tree1, *tree2;
 	struct tree_desc t1, t2;
 	int retval;
 
+	parse_object_quick = 1;
 	tree1 = parse_tree_indirect(old);
 	if (!tree1)
 		die("unable to read source tree (%s)", sha1_to_hex(old));
 	tree2 = parse_tree_indirect(new);
 	if (!tree2)
 		die("unable to read destination tree (%s)", sha1_to_hex(new));
+	parse_object_quick = 0;
 	init_tree_desc(&t1, tree1->buffer, tree1->size);
 	init_tree_desc(&t2, tree2->buffer, tree2->size);
 	retval = diff_tree(&t1, &t2, base, opt);

my best-of-five "git log --raw" on git.git went from ~4.3s to ~4.1s,
which is on par with what you saw.

-Peff