G1 Full GC Write Barrier Mechanism

[Kim Barrett]

> On Feb 27, 2021, at 11:04 AM, Ofir Gordon <ofirg6 at gmail.com> wrote:
> 
> Hi all,
> 
> I'm currently working on the JDK 14 version source code.
> I'm trying to understand the process of a full collection using the G1 gc.
> specifically, I can't follow the write barrier mechanism--where is it being
> activated? what exactly happens in the write barrier? (I see that there are
> both remember set and card tables, are they both being used for tracking
> changes in the heap during the collection?)
> 
> Can anyone help me understand the basic flow of this gc in the code?
> I can see that the marking phase begins in
> G1FullCollector::collect()->phase1_mark_live_objects(), is this part
> running concurrently with the program? is the barrier being activated
> beforehand? (where?)

G1FullCollector and related classes (in files g1Full*) are for the G1 STW full GC,
which doesn’t use the write barriers.

The write post-barrier is used by G1 young/mixed STW collections to track locations
that might contain inter-region references that need to be updated when objects
are moved by those collections.  This is what uses the card table and remembered
sets.

The write pre-barrier is used by the G1 concurrent oldgen collector to track values
that were reachable at the start of the concurrent collection but might no longer
be so because references have been overwritten before the concurrent collector
gets around to examining the location.  This is needed to maintain the SATB
invariant.  This doesn’t involve the card table and remembered sets.

> In addition, is there a way to verify that the barrier is being executed
> during the marking? which code is supposed to run for this part?

The barrier implementations are part of the interpreter/compilers.  The
interpreter invokes relevant functions, and the compilers insert barriers in the
generated code.  This is all managed through the Access API, which provides
an abstraction over the various GCs for use by non-GC code that needs to
read or write object locations while maintaining whatever information the
selected GC requires.

> I know that this are a lot of questions, I'm just trying to figure out the
> basic flow of the process so if someone can give me an explanation that
> will help me start I'll be really thankful.
> 
> Thanks a lot,
> Ofir