Re: Revision walking, commit dates, slop

From: Derrick Stolee <stolee@gmail.com>
Date: Jun 25, 2019, 10:54 UTC
Message-ID: <55fad895-2c18-5a91-79b9-7b958fe280c6@gmail.com>
In-Reply-To: <86mui63xwr.fsf@gmail.com>

On 6/25/2019 3:51 AM, Jakub Narebski wrote:

Show 33 quoted lines

> Jakub Narebski <jnareb@gmail.com> writes:
>> Derrick Stolee <stolee@gmail.com> writes:
>>> On 5/20/2019 7:02 AM, Jakub Narebski wrote:
>>>>
>>>> Are there any blockers that prevent the switch to this
>>>> "generation number v2"?
>>>>
>>>> - Is it a problem with insufficient data to choose the correct numbering
>>>>   as "generation number v2' (there can be only one)?
>>>> - Is it a problem with selected "generation number v2" being
>>>>   incompatibile with gen v2, and Git failing when new version of
>>>>   commit-graph is used instead of softly just not using commit-graph?
>>>> - Or is it something else?
> [...]
> 
>>>                      Using the generation number column for the corrected
>>> commit-date offsets (assuming we also guarantee the offset is strictly
>>> increasing from parent to child), these new values will be backwards-
>>> compatible _except_ for 'git commit-graph verify'.
>>
>> O.K., so the "generation number v2 (legacy)" would be incremental and
>> backward-compatibile in use (though not in generation and validation).
>>
>> Do I understand it correctly how it is calculated:
>>
>>   corrected_date(C) = max(committer_date(C),
>>                           max_{P ∈ parents(C)}(corrected_date(P)) + 1)
>>   offset(C) = corrected_date(C) - committer_date(C)
>>   gen_v2(C) = max(offset(C), max_{P ∈ parents(C)}(gen_v2(P)) + 1) 
> 
> Do you remember who first came up with this idea for backward
> compatibile corrected commit date offsets (monotonically offset
> corrected date)?

I remember saying that the "corrected commit date" that I had suggested
was weak because it was not backwards-compatible with generation numbers
if you are only looking at the offsets. I don't remember who suggested
simply increasing the offset so they do become backwards-compatible.

Show 5 quoted lines

>> Do you have benchmark for this "monotonically offset corrected commit
>> date" generation number in https://github.com/derrickstolee/git/commits/reach-perf
>> and https://github.com/derrickstolee/gen-test ?
> 
> I guess this will have to wait...

I have not had time to revisit this topic and re-run performance numbers, sorry.

-Stolee