Going deep on my investigations on Shenandoah there is a world of things to learn. What is interesting is that the more you learn about a GC specifically, the more you learn form the others (as consequence of the associations and correlations your mind can make). Just like taking a book on Russian Rockets (like the N1) only and at some point you will understand much simpler, and faster Spacex’s Merlin/Raptor combustion rations and stuff. Anyways.

Reviewing the videos and the actual original paper Shenandoah’s paper Shenandoah an Open source concurrent compacting garbage collector for OpenJDK, it is very interesting that they solve the problem of accessing with basically two concepts: SATB (Snapshot at the beginning, which is also used in CMS and G1) and Forwarding pointer (which adds one more word to the two already there in OpenJDK).

One of those details that I’ve understand much better learning from Shenandoah was the importance of the compaction to avoid fragmentation, which actually happens to CMS (dep in JDK 11, and bye bye in JDK 17). Learning concurrent gc helps learn STW gc <– ofc, all of them are GCs at the end of the day.

Concurrent compaction is a very big part on Shenandoah algorithm as it is one of the three main concurrent phases (other three are respectively mark – given by the init mark|final mark STW – evaluation and updating references.

Roman Kennke has been a very good mentor, and setting up stones for me to learn considerably. But also the PhD Thesis from Paul Thomas, which introduce several concepts like STAB vs Incremental Updates, and Barriers, like write/read barriers and so on. Of course forwarding pointer and how it changed from Shenandoah1 vs Shenandoah2.