On Thu, 10 Sep 2020 14:05:25 GMT, Kim Barrett <kbarrett@openjdk.org> wrote:
src/hotspot/share/gc/g1/g1ParScanThreadState.cpp line 68:
66: _old_gen_is_full(false), 67: _partial_objarray_chunk_size(ParGCArrayScanChunk), 68: _partial_array_stepper(n_workers),
What do you think about saving the chunk size in the stepper instead? Then we don't need to pass it in to `start()` and `next()`. To avoid needing it for the call to `oop_iterate_range()` we could instead have the `Step` include both the start and end index.
See above discussion about naming and factoring. The same stepper can support multiple array types if the chunk size is external to the stepper.
It can, but I think I would prefer multiple "steppers" for that case. This is of course just a matter of taste and I'm fine with leaving the chunk size external.
src/hotspot/share/gc/g1/g1ParScanThreadState.hpp line 84:
82: bool _old_gen_is_full; 83: // Size (in elements) of a partial objArray task chunk. 84: int _partial_objarray_chunk_size;
I would prefer to skip the "obj"-part here to have more consistent naming or, as mentioned above, include it in the stepper instead.
Some of the naming and factoring I've done is forward looking.
I think we should consider splitting the copying part as well as the scanning, and chunking the copy of typeArrays. For example, JDK-8031565 suggests copying large typeArrays as part of termination waiting; I think splitting them into partial array tasks to use the normal parallelism in the framework is better than some new side channel. The chunk size for that should probably be substantially larger (and depend on the element size) than for objArray. Also, Project Valhalla is going to add new kinds of arrays that are neither objArray nor typeArray. We'll want to split them too. The same splitting calculations can apply, even though the chunk size may be different (and probably depends on the element klass).
Ok, keeping the "obj"-part sounds reasonable.
src/hotspot/share/gc/g1/g1ParScanThreadState.hpp line 165:
163: private: 164: inline void do_partial_array(PartialArrayScanTask task); 165: inline void start_partial_objArray(G1HeapRegionAttr dest_dir, oop from, oop to);
Same here, drop "obj" for consistent naming and avoiding the camel-case.
See response above about the name of _partial_objarray_chunk_size. But I should probably be consistent about objarray vs objArray. Since objArray is what it is over in runtime oop-land I'm going to go with that.
Again, we have different taste. We very seldom go with camel-case in members and especially in the middle it looks strange. Looking through the GC-code I find mostly `objarray` or `obj_array`
src/hotspot/share/gc/shared/partialArrayTaskStepper.cpp line 34:
32: // that maximizes available parallelism. 33: return n_workers; 34: }
In preparation for a more advanced logic for the limit, or why not just use the input at the call-site?
Being noncommittal about whether something more "clever" could or should be done.
👍
src/hotspot/share/gc/shared/partialArrayTaskStepper.hpp line 57:
55: int _index; // Array index for the step. 56: uint _ncreate; // Number of new tasks to create. 57: };
I wouldn't mind having getters for those, but it's not a hard request :)
Step is intended to be a trivial data carrier. A (HotSpot) Pair or std::pair (if we used stdlib) would do, except I like named data items.
I prefer them named as well 👍
src/hotspot/share/gc/shared/partialArrayTaskStepper.inline.hpp line 103:
101: uint ncreate = MIN2(_task_fannout, MIN2(remaining_tasks, _task_limit + 1) - pending); 102: Step result = { start, ncreate }; 103: return result;
Similar to my comment above, what do you think about trying to write some test for this to verify we never get to many "tasks".
The asserts in the stepper's next() after the increment of to's length verify that we haven't overrun. I haven't thought of a way to verify the algorithm doesn't generate too few tasks though. Well, other than getting crashes because some array elements didn't get processed. But maybe you mean unit tests? I will try to write some; I should have done so already.
Should have been a bit more clear. I'm talking about unit tests, so if you plan writing some that's great. ------------- PR: https://git.openjdk.java.net/jdk/pull/90