On Fri, 18 Sep 2020 08:03:13 GMT, Aleksey Shipilev <[email protected]> wrote:
>> Lin Zang has refreshed the contents of this pull request, and previous
>> commits have been removed. The incremental views
>> will show differences compared to the previous content of the PR.
>
> 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
-------------
PR: https://git.openjdk.java.net/jdk/pull/67
