From: Derrick Stolee Date: Wed, 18 Apr 2018 14:46:27 GMT Subject: Re: [PATCH v3 3/9] commit: use generations in paint_down_to_common() Message-ID: In-Reply-To: <864lk8io21.fsf@gmail.com> On 4/18/2018 10:31 AM, Jakub Narebski wrote: > Derrick Stolee writes: > >> Define compare_commits_by_gen_then_commit_date(), which uses generation >> numbers as a primary comparison and commit date to break ties (or as a >> comparison when both commits do not have computed generation numbers). >> >> Since the commit-graph file is closed under reachability, we know that >> all commits in the file have generation at most GENERATION_NUMBER_MAX >> which is less than GENERATION_NUMBER_INFINITY. >> >> This change does not affect the number of commits that are walked during >> the execution of paint_down_to_common(), only the order that those >> commits are inspected. In the case that commit dates violate topological >> order (i.e. a parent is "newer" than a child), the previous code could >> walk a commit twice: if a commit is reached with the PARENT1 bit, but >> later is re-visited with the PARENT2 bit, then that PARENT2 bit must be >> propagated to its parents. Using generation numbers avoids this extra >> effort, even if it is somewhat rare. > Does it mean that it gives no measureable performance improvements for > typical test cases? Not in this commit. When we add the `min_generation` parameter in a later commit, we do get a significant performance boost (when we can supply a non-zero value to `min_generation`). This step of using generation numbers for the priority is important for that commit, but on its own has limited value outside of the clock-skew case mentioned above. > >> Signed-off-by: Derrick Stolee >> --- >> commit.c | 20 +++++++++++++++++++- >> commit.h | 1 + >> 2 files changed, 20 insertions(+), 1 deletion(-) >> >> diff --git a/commit.c b/commit.c >> index 711f674c18..a44899c733 100644 >> --- a/commit.c >> +++ b/commit.c >> @@ -640,6 +640,24 @@ static int compare_commits_by_author_date(const void *a_, const void *b_, >> return 0; >> } >> >> +int compare_commits_by_gen_then_commit_date(const void *a_, const void *b_, void *unused) >> +{ >> + const struct commit *a = a_, *b = b_; >> + >> + /* newer commits first */ >> + if (a->generation < b->generation) >> + return 1; >> + else if (a->generation > b->generation) >> + return -1; >> + >> + /* use date as a heuristic when generataions are equal */ > Very minor typo in above comment: > > s/generataions/generations/ Good catch! > >> + if (a->date < b->date) >> + return 1; >> + else if (a->date > b->date) >> + return -1; >> + return 0; >> +} >> + >> int compare_commits_by_commit_date(const void *a_, const void *b_, void *unused) >> { >> const struct commit *a = a_, *b = b_; >> @@ -789,7 +807,7 @@ static int queue_has_nonstale(struct prio_queue *queue) >> /* all input commits in one and twos[] must have been parsed! */ >> static struct commit_list *paint_down_to_common(struct commit *one, int n, struct commit **twos) >> { >> - struct prio_queue queue = { compare_commits_by_commit_date }; >> + struct prio_queue queue = { compare_commits_by_gen_then_commit_date }; >> struct commit_list *result = NULL; >> int i; >> >> diff --git a/commit.h b/commit.h >> index aac3b8c56f..64436ff44e 100644 >> --- a/commit.h >> +++ b/commit.h >> @@ -341,6 +341,7 @@ extern int remove_signature(struct strbuf *buf); >> extern int check_commit_signature(const struct commit *commit, struct signature_check *sigc); >> >> int compare_commits_by_commit_date(const void *a_, const void *b_, void *unused); >> +int compare_commits_by_gen_then_commit_date(const void *a_, const void *b_, void *unused); >> >> LAST_ARG_MUST_BE_NULL >> extern int run_commit_hook(int editor_is_used, const char *index_file, const char *name, ...);