On Fri, 18 Sep 2020 09:40:12 GMT, Lin Zang <lz...@openjdk.org> wrote:
>> src/hotspot/share/gc/shenandoah/shenandoahHeap.cpp line 1470:
>>
>>> 1468: for (uint j = 0; j < roots_num; j++) {
>>> 1469: uint stack_id = j % _num_workers;
>>> 1470: oop obj = _roots_stack.pop();
>>
>> I am not sure we can `pop()` the stack from multiple threads without any
>> sort of synchronization. It might be better to
>> just walk the same roots from multiple threads, and then let the bitmap help
>> us figure out if we have visited the root
>> already. See how `ShenandoahVerifierReachableTask` does it?
>
> Hi @shipilev,
> Thanks for your effort for reviewing this change!
> The method prepare_worker_queue() here is invoked in the constructor of
> ShenandoahParallelObjectIterator, which is
> designed to run in single thread. The logic of using ParallelObjectIterator
> is as follows: (refer to code in
> hotspot/share/memory/heapInspection.cpp)
> - create instance of ParallelObjectIterator in shenandoahHeap:
> ParallelObjectIterator* poi =
> Universe::heap()->parallel_object_iterator(gang->active_workers())
> - create heap iteration task and run (invoke ParallelObjectIterator's method
> object_iterator())
> ParHeapInspectTask task(**poi**, cit, filter);
> // Run task with the active workers.
> gang->run_task(&task);
> =======
> So the ShenandoahParallelObjectIterator iterate all roots in the single
> thread, collecting all roots in the
> roots_stack, then prepare the queues for every workers. -- processed in
> constructor. And the workers start their
> processing from their queue, and then iterate all reachable objects. --
> processed in work(uint worker_id)
> I have checked that ShenandoahVerifierReachableTask supports iterating roots
> parallelly, I didn't choose that way
> because I want to make every workers to start with nearly the same number of
> roots in their queue, hope it could help
> on inital work load balance. Lin
Ah! I missed that part. I thought stack is used by the actual parallel workers.
Nevermind my correctness comment then.
-------------
PR: https://git.openjdk.java.net/jdk/pull/67
