bug#43564: cuirass: Contention while registering new builds.

[Mathieu Othacehe]

Hello,

> I'm trying locally to spawn database workers dedicated to registration,
> that execute the evaluation queries in a single batch. Let's see if it
> helps.

I added 4 registration dedicated database workers and removed from
registration all accesses to the store. Now the evaluation returns
objects that contain all required information.

This seem to improve the situation quite a lot. For example, those three
registrations were done simultaneously in ~200 seconds:

--8<---------------cut here---------------start------------->8---
2020-10-04T12:46:19 registration took 205.429772 seconds
2020-10-04T12:46:19 evaluation 17099 registered 156 new derivations
2020-10-04T12:46:19 building 156 derivations in batches of 200
2020-10-04T12:46:19 building batch of 200 derivations (0/156)
2020-10-04T12:46:19 registration took 212.892482 seconds
2020-10-04T12:46:19 evaluation 17105 registered 164 new derivations
2020-10-04T12:46:19 building 164 derivations in batches of 200
2020-10-04T12:46:19 building batch of 200 derivations (0/164)
2020-10-04T12:46:19 registration took 205.009416 seconds
2020-10-04T12:46:19 evaluation 17102 registered 160 new derivations
2020-10-04T12:46:19 building 160 derivations in batches of 200
2020-10-04T12:46:19 building batch of 200 derivations (0/160)
--8<---------------cut here---------------end--------------->8---

Thanks,

Mathieu