Experiment: Merging hs-rt and hs-gc

[Mikael Vidstedt]

All,

As you all know the JDK 9 development of Hotspot is done in three 
different mercurial forests: hs-rt[1], hs-gc[2] and hs-comp[3]. This 
division has served as a way of isolating changes from each other in 
order to get more testing done on them before they are shared to other 
forests. However, as a side effect of this the propagation time of fixes 
is also impacted, and in some cases we have seen fixes stuck for several 
weeks waiting for the respective forests to stabilize.

We would like to propose an experiment, which will merge the hs-rt and 
hs-gc forests, having hs-rt be the forest through which all the hs-rt 
and hs-gc changes are integrated. For the duration of the experiment the 
hs-gc forest will be locked down so that no accidental integrations are 
made to it. This change would mean that the combined hs-rt gets more 
testing faster, and that the fix propagation time goes to zero for 
changes between hs-rt and hs-gc. The hs-comp forest will not be affected.

We suggest that the experiment starts June 4th and goes on for at least 
two weeks (giving us some time to adapt in case of issues). Monitoring 
and evaluation of the new structure will take place continuously, with 
an option to revert back if things do not work out. This will remain an 
experiment for at least a few months, after which we will evaluate it 
and depending on the results consider making it the new standard. If so, 
the hs-gc forest will eventually be retired, with an option of looking 
at further reduction of forests going forward.

It's worth pointing out explicitly that if you have any changes based on 
the hs-gc forest those changes would have to be rebased on top of hs-rt 
instead once the hs-gc forest has been locked down.

Please let us know if you have any feedback or questions!

Cheers,
Mikael

[1] http://hg.openjdk.java.net/jdk9/hs-rt
[2] http://hg.openjdk.java.net/jdk9/hs-gc
[3] http://hg.openjdk.java.net/jdk9/hs-comp