Subject: Re: Revision walking, commit dates, slop

From: Jakub Narebski <[email-masked]>
Date: Mon, 20 May 2019 13:02:36 +0200

Derrick Stolee <[email-masked]> writes:
> On 5/18/2019 12:17 AM, Mike Hommey wrote:
>> On Sat, May 18, 2019 at 12:58:28PM +0900, Mike Hommey wrote:
>>> On Sat, May 18, 2019 at 03:50:05AM +0200, SZEDER Gábor wrote:
>>>>
>>>> All the above is without commit-graph, I presume?  If so, then you
>>>> should give it a try, as it might bring immediate help in your
>>>> pathological repo.  With 5k commit in the same second (enforced via
>>>> 'export GIT_COMMITTER_DATE=$(date); for i in {1..5000} ...') I get:
>>>>
>>>>   $ best-of-five -q git rev-list HEAD~..HEAD
>>>>   0.069
>>>>   $ git commit-graph write --reachableComputing commit graph generation
>>>>   numbers: 100% (5000/5000), done.
>>>>   $ best-of-five -q git rev-list HEAD~..HEAD
>>>>   0.004
>>>
>>> I'm not observing any difference from using commit-graph, whether in
>>> time or in the number of commits that are looked at in limit_list().
>> 
>> -c core.commitGraph=true does make a difference in time, but not in the
>> number of commits looked at in limit_list(). So it's only faster because
>> each iteration of the loop is faster. It means it's still dependent on
>> the depth of the dag, and the larger the repo will grow, the slower it
>> will get.
>
> The plan is to use the commit-graph's generation numbers for these A..B
> queries, but due to some cases when commit date is a _better_ heuristic
> than generation numbers, we have not enabled them for A..B. You'll see
> that 'git rev-list --topo-order -n 1 HEAD` will be much faster with the
> commit-graph, but adding '--topo-order' to your 'HEAD~1..HEAD' query
> should not change the time at all.
>
> See [1] for the discussion about "generation number v2" which will allow
> us to use a better heuristic in these cases.
>
> [1] https://public-inbox.org/git/[email-masked]/

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?

Best,
--
Jakub Narębski