Porting request for JEP Reduce Latency of G1 Write Barriers

[Thomas Schatzl]

Hi all,

   we in the GC team have been experimenting with new post-write 
barriers for G1 to reduce the throughput difference between G1 and 
Parallel GC.

After quite a few attempts we think we found a balanced solution between 
overall complexity and impact on throughput and latency.

The JEP draft in [1] summarizes the new write barrier and other relevant 
changes; feel free to also chime in in the a public discussion thread 
[2] if there are any questions.

Summarizing, throughput is always better or the same as before, 
sometimes getting very close to Parallel GC. There are no changes to 
latency with pause times being a bit better at the cost of some native 
memory.

This email is mostly an advance notice that unfortunately the new write 
barrier is incompatible with the old one, so effort is required to make 
G1 work again on all platforms. Hence we would like to ask you to help 
us with them.

We intend to post a PR containing the implementation in the next (one or 
two) weeks (for this CR [3]); our goal would be to merge sometime for 
(early?) JDK 25 together with all barrier changes completed.

At Oracle we will as usual fully support the x64_64 and aarch64 platforms.

There are (seemingly) working implementations for riscv, aarch32/arm, 
and x86, which would need further testing/checking, but completely 
missing are barriers for PPC and S390 (I tried my luck with those, but 
gave up a after a bit ...).

Thanks in advance,
   Thomas

[1] https://bugs.openjdk.org/browse/JDK-8342382
[2] 
https://mail.openjdk.org/pipermail/hotspot-gc-dev/2024-October/049944.html
[3] https://bugs.openjdk.org/browse/JDK-8342382