Re: [PATCH 1/5] merge-ort: replace string_list_df_name_compare with faster alternative

["René Scharfe" <l.s.r@web.de>]

Am 27.05.21 um 10:37 schrieb Elijah Newren via GitGitGadget:
> 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 and remove as many levels of indirection
> as possible (e.g. the old code had
>     cmp_items() ->
>       string_list_df_name_compare() ->
>         df_name_compare()
> now we just have sort_dirs_next_to_their_children()), and tweak it to be
> as optimized as possible for our specific case.  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

Interesting.

>
> Signed-off-by: Elijah Newren <newren@gmail.com>
> ---
>  merge-ort.c | 64 ++++++++++++++++++++++++++++++++++-------------------
>  1 file changed, 41 insertions(+), 23 deletions(-)
>
> diff --git a/merge-ort.c b/merge-ort.c
> index 142d44d74d63..367aec4b7def 100644
> --- a/merge-ort.c
> +++ b/merge-ort.c
> @@ -2746,31 +2746,50 @@ 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 void *a, const void *b)
>  {
> -	int onelen = strlen(one);
> -	int twolen = strlen(two);

The old code scans both strings fully, while the new one stops when it
reaches a difference and doesn't look at any further characters.  How
much does that contribute to the speedup?  (I suspect a lot.)

>  	/*
> -	 * 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.  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.
>  	 *
> -	 * 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, 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'.
> -	 */
> -	if (cmp)
> -		return cmp;
> -	return onelen - twolen;
> +	const char *one = ((struct string_list_item *)a)->string;
> +	const char *two = ((struct string_list_item *)b)->string;

Casting away const, hmm. :-/  Harmless because no actual write is
attempted, but still looks needlessly scary to me.

> +	unsigned char c1, c2;
> +
> +	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,
> @@ -3481,8 +3500,7 @@ 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;
> -	string_list_sort(&plist);
> +	QSORT(plist.items, plist.nr, sort_dirs_next_to_their_children);

How much does the direct use of QSORT instead of string_list_sort()
contribute to the speedup?  (I suspect only a bit.)

>  	trace2_region_leave("merge", "plist special sort", opt->repo);
>
>  	trace2_region_leave("merge", "process_entries setup", opt->repo);
>