Re: [PATCH 0/3] Make add_missing_tags() linear

[Derrick Stolee <stolee@gmail.com>]

On 11/1/2018 2:52 AM, Elijah Newren wrote:
> On Wed, Oct 31, 2018 at 5:05 AM Derrick Stolee <stolee@gmail.com> wrote:
>> On 10/31/2018 2:04 AM, Elijah Newren wrote:
>>>
>>> On the original repo where the topic was brought up, with commit-graph
>>> NOT turned on and using origin/master, I see:
>>>
>>> $ time git push --dry-run --follow-tags /home/newren/repo-mirror
>>> To /home/newren/repo-mirror
>>>   * [new branch]       test5 -> test5
>>>
>>> real 1m20.081s
>>> user 1m19.688s
>>> sys 0m0.292s
>>>
>>> Merging this series in, I now get:
>>>
>>> $ time git push --dry-run --follow-tags /home/newren/repo-mirror
>>> To /home/newren/repo-mirror
>>>   * [new branch]       test5 -> test5
>>>
>>> real 0m2.857s
>>> user 0m2.580s
>>> sys 0m0.328s
>>>
>>> which provides a very nice speedup.
>>>
>>> Oddly enough, if I _also_ do the following:
>>> $ git config core.commitgraph true
>>> $ git config gc.writecommitgraph true
>>> $ git gc
>>>
>>> then my timing actually slows down just slightly:
>>> $ time git push --follow-tags --dry-run /home/newren/repo-mirror
>>> To /home/newren/repo-mirror
>>>   * [new branch]          test5 -> test5
>>>
>>> real 0m3.027s
>>> user 0m2.696s
>>> sys 0m0.400s
>> So you say that the commit-graph is off in the 2.8s case, but not here
>> in the 3.1s case? I would expect _at minimum_ that the cost of parsing
>> commits would have a speedup in the commit-graph case.  There may be
>> something else going on here, since you are timing a `push` event that
>> is doing more than the current walk.
>>
>>> (run-to-run variation seems pretty consistent, < .1s variation, so
>>> this difference is just enough to notice.)  I wouldn't be that
>>> surprised if that means there's some really old tags with very small
>>> generation numbers, meaning it's not gaining anything in this special
>>> case from the commit-graph, but it does pay the cost of loading the
>>> commit-graph.
>> While you have this test environment, do you mind applying the diff
>> below and re-running the tests? It will output a count for how many
>> commits are walked by the algorithm. This should help us determine if
>> this is another case where generation numbers are worse than commit-date,
>> or if there is something else going on. Thanks!
> I can do that, but wouldn't you want a similar patch for the old
> get_merge_bases_many() in order to compare?  Does an absolute number
> help by itself?
> It's going to have to be tomorrow, though; not enough time tonight.

No rush. I'd just like to understand how removing the commit-graph file
can make the new algorithm faster. Putting a similar count in the old
algorithm would involve giving a count for every call to
in_merge_bases_many(), which would be very noisy.

Thanks!
-Stolee