preform GC modifications

[Thomas Schatzl]

Hi,

On 16.04.20 12:28, Ofir Gordon wrote:
> So, when it gets to those loops from - DefNewGeneration::collect() -> 
> GenCollectedHeap::gen_process_strong_roots() -> 
> SharedHeap::process_strong_roots(
> Is it running MinorGC with mark&sweep algorithm, or MinorGC with 
> Cheney's algorithm, or maybe MajorGC?

   young collection = minor gc -> uses Cheney's algo
   old/full collection = major gc -> uses Mark-Sweep-Compact, recursive 
object graph traversal using an explicit mark stack.

> I got a little confused, doesn't minor collection serial gc entering 
> point is in DefNewGeneration? does young generation collection ==  minor 
> gc or am I confusing those two?

At some point you will end up in GenCollectedHeap::do_collection(); that 
method determines the type of collection (via a rather confusing 
generational framework, and forwards to either 
DefNewGeneration::collect() or TenuredGeneration::collect(), which again 
forwards to OneContigSpaceCardGeneration::collect(), forwarding to 
GenMarkSweep::invoke_at_safepoint() which at some point gets you to 
GenMarkSweep::mark_sweep_phase1() which does the "actual" marking, i.e. 
iterates over heap roots, appyling the closures on those which fill the 
mark stack that the closures then process.

I.e. if you add a breakpoint in that method, you should end up there in 
case of a full/major gc.

Note that "Tenured generation" is another word for what is now generally 
called "old" generation. That kind of shows how old that part of Hotspot 
code is...

The other entry point I mentioned before, (Gen-)CollectedHeap::collect() 
is only used for system.gc() like calls. However at some point it also 
ends you up in GenMarkSweep::mark_sweep_phase1(). Sorry, I forgot that 
detail.. :(

As already mentioned, jdk8 code is unnecessarily complex for serial gc. 
Latest tip at least does away with some of these abstractions or makes 
them easier to understand. GC is still complex code :)

Thanks,
   Thomas