Am 27.05.21 um 10:37 schrieb Elijah Newren via GitGitGadget:
> From: Elijah Newren <newren@xxxxxxxxx>
>
> 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@xxxxxxxxx>
> ---
> 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);
>
