Performance regression in 2.7.0 / 3.0.0 (maybe caused by #626?)

[sratz]

I've noticed that in 2.7.0 the dependency and classpath resolution phases are slower than in 2.6.0.

For a non-parallelized build this adds about 7-10 minutes to our build time (727 artifacts):
2.6.0: ~ 25 minutes
2.7.0: ~ 35 minutes

I am currently trying to get some more concrete numbers with hot caches and populated ~/.m2/repository.

First bisecting show that this could be caused by #626.

[sratz]

So I did some testing using 2.7.0 vs. 2.7.0 + reverted 774e135 from #626:
Same machine, first a build using mvn clean verify to warm up, then a 2nd run using
mvn clean verify -o to force offline mode and work on caches only:

Here are the numbers from the 2nd runs:

                              2.7.0       2.7.1-SNAPSHOT (2.7.0 + reverted 774e135844b20b46ef71b6de4463126aa0f6da9b)
dependency resolution phase   00:08:17    00:03:33
build phase (clean verify)    00:19:25    00:18:58
--
total                         00:27:42    00:22:31

[laeubi]

Can you set a breakpoints to see how many units are queried? I think we might use some caching because most of the units should be the same for all projects.

[laeubi]

@sratz can you check if that reduces the execution times in your case?

Beside that, you should really try to enable parallel execution, as we have enabled parallel compile again in Tycho 2.7 this should reduce the build times significantly if building a lot of modules.

[sratz]

Already better with #721, but it still bottlenecks in the loop over addMatchingFragments().

I've added another improvement on top of your change in #724.

With that, he performance is almost on par again with 2.6.0.