Re: ‘core-updates’ is gone; long live ‘core-packages-team’!

[Simon Tournier]

Hi Ludo, all,

On Sat, 31 Aug 2024 at 15:03, Ludovic Courtès <ludo@gnu.org> wrote:

> To reduce world rebuilds, perhaps we’ll sometimes create “merge trains”,
> whereby we’ll merge, say, the branch upgrading CMake and that ungrafting
> ibus on top of ‘core-packages-team’, and then merge this combination in
> ‘master’.

I do not see how the world rebuild would be reduced.

Correct me if my understanding is misleading, the branch ’core-updates’
had been extended to more than core packages because the project had
(has?) not enough CPU power for continuously building any change with a
massive impact.

Therefore, this ’core-updates’ branch was built only every X months in
order to reduce the workload. As you perfectly know, all these grouped
changes is a mess and it requires a lot of effort to stabilize.  Hence,
a very boring task… i.e., an arbitrary X. ;-)

For sure, that’s a good idea to have a core-packages-team that focuses
only on core packages – the initial aim. :-)

However, it does not address the issue with changes that have a massive
impact.  This new core-packages-team just becomes one of other branches
that will also contain packages triggering world rebuilds.

And the question is how do we manage that?


>            The key being: these branches will have been developed and
> tested independently of one another by dedicated teams, and the merge
> train will be a mere formality.

In this picture of “merge train”, the CI workload and world rebuilds
will increase, no?

Consider the two teams: science and this new core-packages.  Then
science takes care of openblas (6000+ dependent packages) and
core-packages of grep (6000+ dependent packages).

When science team wants to merge to master, they ask for a rebuild.

When core-packages team wants to merge to master, they ask for a
rebuild.

Therefore, we have two world rebuilds.  Or the both teams need to
synchronize and thus the branches are indeed developed independently but
not tested independently.  Are they?

Well, my understanding of “merge train” is an automatic synchronization
workflow: science team does not ask for a rebuild but ask for a merge
and core-packages team idem – so they cannot evaluate their impact and
what needs to be fixed by their changes – then both merges are the two
wagons of the train.  Well, it’s not clear for me what CI builds: first
one wagon then the two wagons?  Or first the two wagons and then what?

Aside, the core-packages or science merges might have an impact to
packages tracked by other teams.  Other said, fixes unrelated to their
scope.  Somehow, it requires that other teams also follow – it means a
kind of synchronization.

That’s why core-updates became the big catch-all and complex mess.
That’s just a poor man way to synchronize: first it minimizes the number
of world rebuilds because they are more or less manually triggered, and
second it eases to have fixes unrelated to core changes.

For sure, I agree that having more branches will help to reduce the
complexity of merging core-updates-like changes.  However, it’s not
clear how it will work in practise.  Especially, the number of world
rebuilds will increase, IMHO.

Well, all in all, I understand the “merge train” story with “regular”
branches.  But I do not see how it would work for different branches
that trigger world rebuilds.

Could you detail a bit your perspective with “merge train”?  Or the
workflow for merging branches that contains world rebuild changes.

I know about [1] but I am lacking imagination for applying it to our
situation.  Because, if we have 3 merge requests (A, B and C), the
“merge train” workflow implies the pipeline is run 3 times:

 with A on top of master,
 with A+B (B on the top of A on the top of master),
 with A+B+C (C on the top of B on the top of A on the top of master).

If A+B fails then B is removed from the train and the pipeline runs A+C.

And you can also consider if A fails then B and B+C is built.

The “merge train” workflow makes sens when the number of changes in A, B
and C is high and thus it helps to keep the merges under control.
However, one implicit assumption is a low-cost pipeline, IMHO.

Bah the question how to merge different branches containing world
rebuilds without the big catch-all old core-updates branch is not
addressed under the constraint of reducing as much as possible the
number of world rebuilds, for what my opinion is worth.

Cheers,
simon

1: https://docs.gitlab.com/ee/ci/pipelines/merge_trains.html

PS: My understanding of Gitlab Merge Train selling point is that several
pipelines are run in parallel.  How to waste power for increasing the
throughput?