preform GC modifications

[Thomas Schatzl]

On 15.04.20 12:17, Ofir Gordon wrote:
> Hello,
> 
> I'm trying to investigate the performance of different garbage collectors,
> as part of a project I'm working on,
> and as part of the project I need to preform modifications in the existing
> GCs (working with openjdk8 hotspot).

I really recommend working with at least jdk14 or even tip if possible. 
We removed quite a bit of cruft in the last six or so years. Also, the 
majority of people are currently working on tip, so you might get even 
less feedback  than otherwise when using such an old version. While the 
general rough structure is the same, probably your audience might not 
have a checkout handy, needing to check out that version, read into it, 
double check, etc.

Particularly Serial GC got much easier to understand due to many many 
cleanups and removal of CMS.

> I tried to start with a simple task and figure out where in the code the
> call for the GC happens? I couldn't figure this out.

Implementations of CollectedHeap::collect() perform the collection. 
GenCollectedHeap::collect() is the entry point for serial gc (and CMS).

> 
> Also, I'm currently want to try only to run simple serial Mark&Sweep GC -
> can someone please help me understand:
> 1. how do I specify it when running the java program? (if I specify
> -XX:+UseSerialGC is that enough? what type of GC is running for this flag?)

Yes. Serial GC.

> 2. is there a way to produce logs in which I can see that it is actually
> working (more info than just when a GC is activated)

Also in later versions logging is probably much better (maybe not for 
Serial GC) :)

-XX:+PrintGC and -XX:PrintGCDetails, potentially with -XX:+Verbose

> 3. most important - which class/methods in the source code are relevant for
> that simple collector (basically I'm trying to understand which method
> handles the marking phase and where is it being called)? I need this to try
> modify some code and see that it is actually working.

Serial gc is kind of strewn all over the place in jdk8, it only got 
better later. Start with defNewGeneration.*pp for the young collector 
and (gen)markSweep.*pp for the full collector iirc.

Checking out jdk14+ and look for similiar files you can find in the 
src/hotspot/share/gc/serial directory in jdk8u might help here.

> 
> I know that's a lot of questions, if someone can give me a hint on how to
> start I will be very grateful.
> 

Hth,
   Thomas