On Tue, 4 Mar 2025 09:57:56 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: > > * iwalulya review 2 > * G1ConcurrentRefineWorkState -> G1ConcurrentRefineSweepState > * some additional documentation src/hotspot/share/gc/g1/g1ConcurrentRefine.cpp line 108: > 106: > 107: void G1ConcurrentRefineThreadControl::control_thread_do(ThreadClosure* > tc) { > 108: if (_control_thread != nullptr) { maybe maintain using `if (max_num_threads() > 0)` as used in `G1ConcurrentRefineThreadControl::initialize`, so that it is clear that setting `G1ConcRefinementThreads=0` effectively turns off concurrent refinement. src/hotspot/share/gc/g1/g1ConcurrentRefine.cpp line 354: > 352: if (!r->is_free()) { > 353: // Need to scan all parts of non-free regions, so reset the > claim. > 354: // No need for synchronization: we are only interested about > regions s/about/in src/hotspot/share/gc/g1/g1OopClosures.hpp line 205: > 203: G1CollectedHeap* _g1h; > 204: uint _worker_id; > 205: bool _has_to_cset_ref; Similar to `_cards_refer_to_cset` , do you mind renaming `_has_to_cset_ref` and `_has_to_old_ref` to `_has_ref_to_cset` and `_has_ref_to_old` src/hotspot/share/gc/g1/g1Policy.hpp line 105: > 103: uint _free_regions_at_end_of_collection; > 104: > 105: size_t _pending_cards_from_gc; A comment on the variable would be nice, especially on how it is set/reset both at end of GC and by refinement. Also the `_to_collection_set_cards` below could use a comment ------------- PR Review Comment: https://git.openjdk.org/jdk/pull/23739#discussion_r1979077904 PR Review Comment: https://git.openjdk.org/jdk/pull/23739#discussion_r1979102189 PR Review Comment: https://git.openjdk.org/jdk/pull/23739#discussion_r1979212854 PR Review Comment: https://git.openjdk.org/jdk/pull/23739#discussion_r1979155941
