All of lore.kernel.org
 help / color / mirror / Atom feed
From: "Elijah Newren via GitGitGadget" <gitgitgadget@gmail.com>
To: git@vger.kernel.org
Cc: "Derrick Stolee" <dstolee@microsoft.com>,
	"Jonathan Tan" <jonathantanmy@google.com>,
	"Taylor Blau" <me@ttaylorr.com>, "René Scharfe" <l.s.r@web.de>,
	"Elijah Newren" <newren@gmail.com>,
	"Derrick Stolee" <stolee@gmail.com>,
	"Elijah Newren" <newren@gmail.com>,
	"Elijah Newren" <newren@gmail.com>
Subject: [PATCH v3 1/4] merge-ort: replace string_list_df_name_compare with faster alternative
Date: Fri, 04 Jun 2021 04:39:35 +0000	[thread overview]
Message-ID: <f63ffc2a7c22faea9ffea9abbd95595b6a1196de.1622781578.git.gitgitgadget@gmail.com> (raw)
In-Reply-To: <pull.962.v3.git.1622781578.gitgitgadget@gmail.com>

From: Elijah Newren <newren@gmail.com>

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 <newren@gmail.com>
Acked-by: Derrick Stolee <dstolee@microsoft.com>
---
 merge-ort.c | 67 +++++++++++++++++++++++++++++++++++++----------------
 1 file changed, 47 insertions(+), 20 deletions(-)

diff --git a/merge-ort.c b/merge-ort.c
index 142d44d74d63..061f15701359 100644
--- a/merge-ort.c
+++ b/merge-ort.c
@@ -2746,31 +2746,58 @@ 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
+	 * it was reimplemented.
 	 */
-	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 ? *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;
 }
 
 static int read_oid_strbuf(struct merge_options *opt,
@@ -3481,7 +3508,7 @@ 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);
 
-- 
gitgitgadget


  reply	other threads:[~2021-06-04  4:39 UTC|newest]

Thread overview: 39+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-05-27  8:37 [PATCH 0/5] Optimization batch 12: miscellaneous unthemed stuff Elijah Newren via GitGitGadget
2021-05-27  8:37 ` [PATCH 1/5] merge-ort: replace string_list_df_name_compare with faster alternative Elijah Newren via GitGitGadget
2021-05-27 21:00   ` René Scharfe
2021-05-27 22:47     ` Elijah Newren
2021-05-28 16:12       ` René Scharfe
2021-05-28 18:09         ` Elijah Newren
2021-05-28  1:32   ` Taylor Blau
2021-05-28  4:10     ` Elijah Newren
2021-05-27  8:37 ` [PATCH 2/5] diffcore-rename: avoid unnecessary strdup'ing in break_idx Elijah Newren via GitGitGadget
2021-05-27  8:37 ` [PATCH 3/5] diffcore-rename: enable limiting rename detection to relevant destinations Elijah Newren via GitGitGadget
2021-05-27  8:37 ` [PATCH 4/5] Fix various issues found in comments Elijah Newren via GitGitGadget
2021-05-27  8:37 ` [PATCH 5/5] merge-ort: miscellaneous touch-ups Elijah Newren via GitGitGadget
2021-06-01 14:58 ` [PATCH v2 0/5] Optimization batch 12: miscellaneous unthemed stuff Elijah Newren via GitGitGadget
2021-06-01 14:58   ` [PATCH v2 1/5] merge-ort: replace string_list_df_name_compare with faster alternative Elijah Newren via GitGitGadget
2021-06-02 11:29     ` Derrick Stolee
2021-06-01 14:58   ` [PATCH v2 2/5] diffcore-rename: avoid unnecessary strdup'ing in break_idx Elijah Newren via GitGitGadget
2021-06-01 14:58   ` [PATCH v2 3/5] diffcore-rename: enable limiting rename detection to relevant destinations Elijah Newren via GitGitGadget
2021-06-03 12:54     ` Derrick Stolee
2021-06-03 14:13       ` Elijah Newren
2021-06-01 14:58   ` [PATCH v2 4/5] Fix various issues found in comments Elijah Newren via GitGitGadget
2021-06-01 14:58   ` [PATCH v2 5/5] merge-ort: miscellaneous touch-ups Elijah Newren via GitGitGadget
2021-06-03 12:55   ` [PATCH v2 0/5] Optimization batch 12: miscellaneous unthemed stuff Derrick Stolee
2021-06-04  4:39   ` [PATCH v3 0/4] " Elijah Newren via GitGitGadget
2021-06-04  4:39     ` Elijah Newren via GitGitGadget [this message]
2021-06-04  4:39     ` [PATCH v3 2/4] diffcore-rename: avoid unnecessary strdup'ing in break_idx Elijah Newren via GitGitGadget
2021-06-04  4:39     ` [PATCH v3 3/4] Fix various issues found in comments Elijah Newren via GitGitGadget
2021-06-04  4:39     ` [PATCH v3 4/4] merge-ort: miscellaneous touch-ups Elijah Newren via GitGitGadget
2021-06-04 13:11     ` [PATCH v3 0/4] Optimization batch 12: miscellaneous unthemed stuff Derrick Stolee
2021-06-04 15:48       ` Elijah Newren
2021-06-04 16:30         ` Elijah Newren
2021-06-04 16:35         ` Jeff King
2021-06-04 18:42           ` Derrick Stolee
2021-06-04 19:43             ` Elijah Newren
2021-06-04 19:53             ` Jeff King
2021-06-08 16:11     ` [PATCH v4 " Elijah Newren via GitGitGadget
2021-06-08 16:11       ` [PATCH v4 1/4] merge-ort: replace string_list_df_name_compare with faster alternative Elijah Newren via GitGitGadget
2021-06-08 16:11       ` [PATCH v4 2/4] diffcore-rename: avoid unnecessary strdup'ing in break_idx Elijah Newren via GitGitGadget
2021-06-08 16:11       ` [PATCH v4 3/4] Fix various issues found in comments Elijah Newren via GitGitGadget
2021-06-08 16:11       ` [PATCH v4 4/4] merge-ort: miscellaneous touch-ups Elijah Newren via GitGitGadget

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=f63ffc2a7c22faea9ffea9abbd95595b6a1196de.1622781578.git.gitgitgadget@gmail.com \
    --to=gitgitgadget@gmail.com \
    --cc=dstolee@microsoft.com \
    --cc=git@vger.kernel.org \
    --cc=jonathantanmy@google.com \
    --cc=l.s.r@web.de \
    --cc=me@ttaylorr.com \
    --cc=newren@gmail.com \
    --cc=stolee@gmail.com \
    --subject='Re: [PATCH v3 1/4] merge-ort: replace string_list_df_name_compare with faster alternative' \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

This is an external index of several public inboxes,
see mirroring instructions on how to clone and mirror
all data and code used by this external index.