[Gluster-infra] Reduce regression runs wait time - New gerrit/review work flow

[Kaushal M]

Hi all,

The recent rush of reviews being sent due to the release of 3.7 was a
cause of frustration for many of us because of the regression tests
(gerrit troubles themselves are another thing).

W.R.T regression 3 main sources of frustration were,
1. Spurious test failures
2. Long wait times
3. Regression slave troubles

We've already tackled the spurious failure issue and are quite stable
now. The trouble with the slave vms is related to the gerrit issues,
and is mainly due to the network issues we are having between the
data-centers hosting the slaves and gerrit/jenkins. People have been
looking into this, but we haven't had much success. This leaves the
issue of the long wait times.

The long wait times are because of the long queues of pending jobs,
some of which take days to get scheduled. Two things cause the long
queues,
1. Automatic regression job triggering for all submissions to gerrit
2. Long run time for regression (~2h)

The long queues coupled with the spurious failure and network
problems, meant that jobs would fail for no reason after a long wait,
and would have to be added to the back of the queue to be re-run. This
meant that developers would have to wait days for their changes to get
merged, and was one of the causes for the delay in the release of 3.7.

The solution reduce wait times for regression runs. To reduce wait
times we should,
1. Trigger runs only when required
2. Reduce regression run time.

Raghavendra Talur (rtalur/RaSTar) will soon send out a mail with his
findings on the regression run times, and we can continue discussion
on it on that thread.

Earlier, the regression runs used to be manually triggered by the
maintainers once they had determined that a change was ready for
submission. But as there were only two maintainers before (Vijay and
Avati) auto triggering was brought in to reduce their load. Auto
triggering worked fine when we had a lower volume of changes being
submitted to gerrit. But now, with the large volumes we see during the
release freeze dates, auto triggering just adds to problems.

I propose that we move back to the old model of starting regression
runs only once the maintainers are ready to merge. But instead of the
maintainers manually tiggering the runs, we could automate it.

We can model our new workflow on those of OpenStack[1] and
Wikimedia[2]. The existing Gerrit plugin for Jenkins doesn't provide
the features necessary to enable selective triggering based on Gerrit
flags. Both OpenStack and Wikimedia use a project gating tool called
Zuul[3], which provides a much better integration with Jenkins and
Gerrit and more features on top.

I propose the following work flow,

- Developer pushes change to Gerrit.
  - Zuul is notified by Gerrit of new change
- Zuul runs pre-review checks on Jenkins. This will be the current smoke tests.
  - Zuul reports back status of the checks to Gerrit.
    - If checks fail, developer will need to resend the change after
the required fixes. The process starts once more.
    - If the checks pass, the change is now ready for review
- The change is now reviewed by other developers and maintainers.
Non-maintainers will be able to give only a +1 review.
  - On a negative review, the developer will need to rework the change
and resend it. The process starts once more.
- The maintainer give a +2 review once he/she is satisfied. The
maintainers work is done here.
  - Zuul is notified of the +2 review
- Zuul runs the regression runs and reports back the status.
  - If the regression runs fail, the process starts over again.
  - If the runs pass, the change is ready for acceptance.
- Zuul will pick the change into the repository.
  - If the pick fails, Zuul will report back the failure, and the
process starts once again.

Following this flow should,
1. Reduce regression wait time
2. Improve change acceptance time
3. Reduce unnecessary  wastage of infra resources
4. Improve infra stability.

It also brings in drawbacks that we need to maintain one other piece
of infra (Zuul). This would be an additional maintenance overhead on
top of Gerrit, Jenkins and the current slaves. But I feel the
reduction in the upkeep efforts of the slaves would be enough to
offset this.

tl;dr
Current auto-triggering of regression runs is stupid and a waste of
time and resources. Bring in a project gating system, Zuul, which can
do a much more intelligent jobs triggering, and use it to
automatically trigger regression only for changes with Reviewed+2 and
automatically merge ones that pass.

What does the community think of this?

~kaushal

[1]: http://docs.openstack.org/infra/manual/developers.html#automated-testing
[2]: https://www.mediawiki.org/wiki/Continuous_integration/Workflow
[3]: http://docs.openstack.org/infra/zuul/