Re: [PATCH v3 3/9] commit: use generations in paint_down_to_common()

[Derrick Stolee <stolee@gmail.com>]

On 4/18/2018 10:31 AM, Jakub Narebski wrote:
> Derrick Stolee <dstolee@microsoft.com> 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 <dstolee@microsoft.com>
>> ---
>>   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, ...);