On Mon, 8 Sep 2025 09:09:49 GMT, Thomas Schatzl <tscha...@openjdk.org> wrote:
>> Hi all, >> >> please review this change that implements (currently Draft) JEP: G1: >> Improve Application Throughput with a More Efficient Write-Barrier. >> >> The reason for posting this early is that this is a large change, and the >> JEP process is already taking very long with no end in sight but we would >> like to have this ready by JDK 25. >> >> ### Current situation >> >> With this change, G1 will reduce the post write barrier to much more >> resemble Parallel GC's as described in the JEP. The reason is that G1 lacks >> in throughput compared to Parallel/Serial GC due to larger barrier. >> >> The main reason for the current barrier is how g1 implements concurrent >> refinement: >> * g1 tracks dirtied cards using sets (dirty card queue set - dcqs) of >> buffers (dirty card queues - dcq) containing the location of dirtied cards. >> Refinement threads pick up their contents to re-refine. The barrier needs to >> enqueue card locations. >> * For correctness dirty card updates requires fine-grained synchronization >> between mutator and refinement threads, >> * Finally there is generic code to avoid dirtying cards altogether >> (filters), to avoid executing the synchronization and the enqueuing as much >> as possible. >> >> These tasks require the current barrier to look as follows for an assignment >> `x.a = y` in pseudo code: >> >> >> // Filtering >> if (region(@x.a) == region(y)) goto done; // same region check >> if (y == null) goto done; // null value check >> if (card(@x.a) == young_card) goto done; // write to young gen check >> StoreLoad; // synchronize >> if (card(@x.a) == dirty_card) goto done; >> >> *card(@x.a) = dirty >> >> // Card tracking >> enqueue(card-address(@x.a)) into thread-local-dcq; >> if (thread-local-dcq is not full) goto done; >> >> call runtime to move thread-local-dcq into dcqs >> >> done: >> >> >> Overall this post-write barrier alone is in the range of 40-50 total >> instructions, compared to three or four(!) for parallel and serial gc. >> >> The large size of the inlined barrier not only has a large code footprint, >> but also prevents some compiler optimizations like loop unrolling or >> inlining. >> >> There are several papers showing that this barrier alone can decrease >> throughput by 10-20% >> ([Yang12](https://dl.acm.org/doi/10.1145/2426642.2259004)), which is >> corroborated by some benchmarks (see links). >> >> The main idea for this change is to not use fine-grained synchronization >> between refinement and mutator threads, but coarse grained based on >> atomically switching c... > > Thomas Schatzl has updated the pull request incrementally with one additional > commit since the last revision: > > * sort includes src/hotspot/share/gc/g1/g1Arguments.cpp line 250: > 248: // Verify that the maximum parallelism isn't too high to eventually > overflow > 249: // the refcount in G1CardSetContainer. > 250: uint max_parallel_refinement_threads = G1ConcRefinementThreads; Don't need to local variable `max_parallel_refinement_threads` any more, can use G1ConcRefinementThreads directly. src/hotspot/share/gc/g1/g1CardTableClaimTable.inline.hpp line 95: > 93: i_card++; > 94: } > 95: assert(false, "should have early-returned"); maybe use `ShouldNotReachHere();` src/hotspot/share/gc/g1/g1ConcurrentRefine.cpp line 216: > 214: class G1SwapThreadCardTableClosure : public HandshakeClosure { > 215: public: > 216: G1SwapThreadCardTableClosure() : HandshakeClosure("G1 Swap JT card > table") { } Above on L206 we use "Refine Java Thread CT", here we use "Swap JT card table" not sure which is better "Java Thread CT" vs. "JT card table", but lets pick one. ------------- PR Review Comment: https://git.openjdk.org/jdk/pull/23739#discussion_r2332539864 PR Review Comment: https://git.openjdk.org/jdk/pull/23739#discussion_r2332881834 PR Review Comment: https://git.openjdk.org/jdk/pull/23739#discussion_r2333281435
