[v2,1/5] merge-ort: replace string_list_df_name_compare with faster alternative

Commit Message

Elijah Newren June 1, 2021, 2:58 p.m. UTC

From: Elijah Newren <[e-mail]>

Gathering accumulated times from trace2 output on the mega-renames
testcase, I saw the following timings (where I'm only showing a few
lines to highlight the portions of interest):

    10.120 : label:incore_nonrecursive
        4.462 : ..label:process_entries
           3.143 : ....label:process_entries setup
              2.988 : ......label:plist special sort
           1.305 : ....label:processing
        2.604 : ..label:collect_merge_info
        2.018 : ..label:merge_start
        1.018 : ..label:renames

In the above output, note that the 4.462 seconds for process_entries was
split as 3.143 seconds for "process_entries setup" and 1.305 seconds for
"processing" (and a little time for other stuff removed from the
highlight).  Most of the "process_entries setup" time was spent on
"plist special sort" which corresponds to the following code:

    trace2_region_enter("merge", "plist special sort", opt->repo);
    plist.cmp = string_list_df_name_compare;
    string_list_sort(&plist);
    trace2_region_leave("merge", "plist special sort", opt->repo);

In other words, in a merge strategy that would be invoked by passing
"-sort" to either rebase or merge, sorting an array takes more time than
anything else.  Serves me right for naming my merge strategy this way.

Rewrite the comparison function in a way that does not require finding
out the lengths of the strings when comparing them.  While at it, tweak
the code for our specific case -- no need to handle a variety of modes,
for example.  The combination of these changes reduced the time spent in
"plist special sort" by ~25% in the mega-renames case.

For the testcases mentioned in commit 557ac0350d ("merge-ort: begin
performance work; instrument with trace2_region_* calls", 2020-10-28),
this change improves the performance as follows:

                            Before                  After
    no-renames:        5.622 s ±  0.059 s     5.235 s ±  0.042 s
    mega-renames:     10.127 s ±  0.073 s     9.419 s ±  0.107 s
    just-one-mega:   500.3  ms ±  3.8  ms   480.1  ms ±  3.9  ms

Signed-off-by: Elijah Newren <[e-mail]>
---
 merge-ort.c | 72 ++++++++++++++++++++++++++++++++++++++---------------
 1 file changed, 52 insertions(+), 20 deletions(-)

Comments

Derrick Stolee June 2, 2021, 11:29 a.m. UTC | #1

On 6/1/21 10:58 AM, Elijah Newren via GitGitGadget wrote:
> From: Elijah Newren <[e-mail]>
> 
...
>  	/*
> -	 * Here we only care that entries for D/F conflicts are
> -	 * adjacent, in particular with the file of the D/F conflict
> -	 * appearing before files below the corresponding directory.
> -	 * The order of the rest of the list is irrelevant for us.
> +	 * Here we only care that entries for directories appear adjacent
> +	 * to and before files underneath the directory.  We can achieve
> +	 * that by pretending to add a trailing slash to every file and
> +	 * then sorting.  In other words, we do not want the natural
> +	 * sorting of
> +	 *     foo
> +	 *     foo.txt
> +	 *     foo/bar
> +	 * Instead, we want "foo" to sort as though it were "foo/", so that
> +	 * we instead get
> +	 *     foo.txt
> +	 *     foo
> +	 *     foo/bar
> +	 * To achieve this, we basically implement our own strcmp, except that
> +	 * if we get to the end of either string instead of comparing NUL to
> +	 * another character, we compare '/' to it.
> +	 *
> +	 * If this unusual "sort as though '/' were appended" perplexes
> +	 * you, perhaps it will help to note that this is not the final
> +	 * sort.  write_tree() will sort again without the trailing slash
> +	 * magic, but just on paths immediately under a given tree.

I find this comment to be helpful.

> +	 * The reason to not use df_name_compare directly was that it was
> +	 * just too expensive (we don't have the string lengths handy), so
> +	 * I had to reimplement it.

Just a hyper-nit I think first person works great in commit
messages, as the author's name is clearly listed in context.
Here, I'd prefer "so it was reimplemented" over "so I had to
reimplement it" as the author will not be present in context.

> +
> +	while (*one && (*one == *two)) {
> +		one++;
> +		two++;
> +	}
> +
> +	c1 = *one;
> +	if (!c1)
> +		c1 = '/';
> +
> +	c2 = *two;
> +	if (!c2)
> +		c2 = '/';

While I'm making other nits on this patch, perhaps this is
an appropriate place for some ternary operators to compress
the vertical space in this method:

	c1 = *one ? *one : '/';
	c2 = *two ? *two : '/';

> +	if (c1 == c2) {
> +		/* Getting here means one is a leading directory of the other */
> +		return (*one) ? 1 : -1;
> +	} else
> +		return c1-c2;

nit: "c1 - c2"

Thanks,
-Stolee

Patch

diff --git a/merge-ort.c b/merge-ort.c
index 142d44d74d63..0fe2eaf02eb2 100644
--- a/merge-ort.c
+++ b/merge-ort.c
@@ -2746,31 +2746,63 @@  static int detect_and_process_renames(struct merge_options *opt,
 
 /*** Function Grouping: functions related to process_entries() ***/
 
-static int string_list_df_name_compare(const char *one, const char *two)
+static int sort_dirs_next_to_their_children(const char *one, const char *two)
 {
-	int onelen = strlen(one);
-	int twolen = strlen(two);
+	unsigned char c1, c2;
+
 	/*
-	 * Here we only care that entries for D/F conflicts are
-	 * adjacent, in particular with the file of the D/F conflict
-	 * appearing before files below the corresponding directory.
-	 * The order of the rest of the list is irrelevant for us.
+	 * Here we only care that entries for directories appear adjacent
+	 * to and before files underneath the directory.  We can achieve
+	 * that by pretending to add a trailing slash to every file and
+	 * then sorting.  In other words, we do not want the natural
+	 * sorting of
+	 *     foo
+	 *     foo.txt
+	 *     foo/bar
+	 * Instead, we want "foo" to sort as though it were "foo/", so that
+	 * we instead get
+	 *     foo.txt
+	 *     foo
+	 *     foo/bar
+	 * To achieve this, we basically implement our own strcmp, except that
+	 * if we get to the end of either string instead of comparing NUL to
+	 * another character, we compare '/' to it.
+	 *
+	 * If this unusual "sort as though '/' were appended" perplexes
+	 * you, perhaps it will help to note that this is not the final
+	 * sort.  write_tree() will sort again without the trailing slash
+	 * magic, but just on paths immediately under a given tree.
 	 *
-	 * To achieve this, we sort with df_name_compare and provide
-	 * the mode S_IFDIR so that D/F conflicts will sort correctly.
-	 * We use the mode S_IFDIR for everything else for simplicity,
-	 * since in other cases any changes in their order due to
-	 * sorting cause no problems for us.
+	 * The reason to not use df_name_compare directly was that it was
+	 * just too expensive (we don't have the string lengths handy), so
+	 * I had to reimplement it.
 	 */
-	int cmp = df_name_compare(one, onelen, S_IFDIR,
-				  two, twolen, S_IFDIR);
+
 	/*
-	 * Now that 'foo' and 'foo/bar' compare equal, we have to make sure
-	 * that 'foo' comes before 'foo/bar'.
+	 * NOTE: This function will never be called with two equal strings,
+	 * because it is used to sort the keys of a strmap, and strmaps have
+	 * unique keys by construction.  That simplifies our c1==c2 handling
+	 * below.
 	 */
-	if (cmp)
-		return cmp;
-	return onelen - twolen;
+
+	while (*one && (*one == *two)) {
+		one++;
+		two++;
+	}
+
+	c1 = *one;
+	if (!c1)
+		c1 = '/';
+
+	c2 = *two;
+	if (!c2)
+		c2 = '/';
+
+	if (c1 == c2) {
+		/* Getting here means one is a leading directory of the other */
+		return (*one) ? 1 : -1;
+	} else
+		return c1-c2;
 }
 
 static int read_oid_strbuf(struct merge_options *opt,
@@ -3481,7 +3513,7 @@  static void process_entries(struct merge_options *opt,
 	trace2_region_leave("merge", "plist copy", opt->repo);
 
 	trace2_region_enter("merge", "plist special sort", opt->repo);
-	plist.cmp = string_list_df_name_compare;
+	plist.cmp = sort_dirs_next_to_their_children;
 	string_list_sort(&plist);
 	trace2_region_leave("merge", "plist special sort", opt->repo);