Summary
-------

Enhance G1 so that it does not rely on full GCs to perform class
unloading or any other critical operations.


Description
-----------

The goal of G1 is to minimize pause times without constraining the heap
size or the amount of live data. This is achieved by doing a large part
of the GC work concurrently and also doing partial compaction. Avoiding
doing full GCs (_i.e., stop-the-world GCs) is one of the major benefits
of G1.

However, currently G1 relies on full GCs to unload classes. This causes a
major issue with large applications that do redeployment or generate
temporary classes via reflection. We need to implement class unloading
without full GC in G1 to fix this.  This project will enhance G1 to do
class unloading at the end of concurrent marking cycles.

We will also investigate if there are other important features that
depend on full GC that might cause similar problems for G1.


Testing
-------

When testing this we need to remember to play around with all kinds of
references to class loaders. Not just plain class loading in Java, but
also JNI and reflection. Class redefinition could also exercise this code
in unusual ways.


Risks and Assumptions
---------------------

Implementing class unloading in G1 will present some new problem areas
for which we have not yet finalized a solution. One example is the
interaction between class unloading and "snapshot-at-the-beginning"
(SATB). It may turn out to be more tricky than we have anticipated,
making this a medium effort.


Dependences
-----------

This project is not immediately dependent on the removal of the permanent
generation ([JEP 122](JDK-8046112)), but the implementation of class unloading in
G1 will be simplified by that work, so it makes sense to wait with this
project until the permanent generation is gone.