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 I got an error while testing java/foreign/TestUpcallStress.java on linuxaarch64 with this PR: # Internal Error (/openjdk-jdk-linux_aarch64-dbg/jdk/src/hotspot/share/gc/g1/g1CardTable.cpp:56), pid=19044, tid=19159 # guarantee(!failures) failed: there should not have been any failures ... V [libjvm.so+0xb6e988] G1CardTable::verify_region(MemRegion, unsigned char, bool)+0x3b8 (g1CardTable.cpp:56) V [libjvm.so+0xc3a10c] G1MergeHeapRootsTask::G1ClearBitmapClosure::do_heap_region(G1HeapRegion*)+0x13c (g1RemSet.cpp:1048) V [libjvm.so+0xb7a80c] G1CollectedHeap::par_iterate_regions_array(G1HeapRegionClosure*, G1HeapRegionClaimer*, unsigned int const*, unsigned long, unsigned int) const+0x9c (g1CollectedHeap.cpp:2059) V [libjvm.so+0xc49fe8] G1MergeHeapRootsTask::work(unsigned int)+0x708 (g1RemSet.cpp:1225) V [libjvm.so+0x19597bc] WorkerThread::run()+0x98 (workerThread.cpp:69) V [libjvm.so+0x1824510] Thread::call_run()+0xac (thread.cpp:231) V [libjvm.so+0x13b3994] thread_native_entry(Thread*)+0x130 (os_linux.cpp:877) C [libpthread.so.0+0x875c] start_thread+0x18c ------------- PR Comment: https://git.openjdk.org/jdk/pull/23739#issuecomment-2697024679
