Re: [PATCH v3 3/9] commit: use generations in paint_down_to_common()
- From
Derrick Stolee <stolee@gmail.com>
- Date
- Apr 18, 2018, 14:46 UTC
- Message-ID
- <d4095ab6-fca2-8ed5-0185-1f961c5d1168@gmail.com>
- In-Reply-To
- <864lk8io21.fsf@gmail.com>
On 4/18/2018 10:31 AM, Jakub Narebski wrote:
Show 20 quoted lines
> 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.
Show 29 quoted lines
>
>> 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!
Show 32 quoted lines
>
>> + 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, ...);