RFR: JDK-8264987: G1: Fill BOTs for Survivor-turned-to-Old regions in full gc

[Stefan Johansson]

On Wed, 14 Apr 2021 07:42:28 GMT, Hamlin Li <mli at openjdk.org> wrote:

>> Easiest is to mark the code and push the "Insert code" button (<>). You can also manually write ``` to start a code block. 
>> 
>> I'll get back to you about the code above later.
>
> Thanks Stefan, it works.

Looked a bit more at the code now and I agree that it didn't become as neat as I had hoped. Solving the problem you describe with dead objects at the end should be fairly easy in the closure-destructor I guess. Something like:

  ~G1UpdateBotClosure() {
    if (_hr->top() > _threshold) {
      _hr->cross_threshold(_pre_addr, _hr->top());
    }
  }


And a few comments in the apply method will help readability:

  size_t apply(oop object) {
    size_t size = object->size();
    HeapWord* addr = cast_from_oop<HeapWord*>(object);
    HeapWord* next_addr = addr + size;

    // Threshold was crossed by dead object.
    if(addr > _threshold) {
      _threshold = _hr->cross_threshold(_pre_addr, addr);
    }

    // Threshold is crossed by this object.
    if (next_addr > _threshold) {
      _threshold = _hr->cross_threshold(addr, next_addr);
    }

    _pre_addr = next_addr;
    return size;
  }


But still it might be better to instead streamline the current approach:
Suggestion:

  HeapWord* limit = hr->top();
  HeapWord* next_addr = hr->bottom();
  HeapWord* threshold = hr->initialize_threshold();
  HeapWord* pre_addr;

  while (next_addr < limit) {
    pre_addr = next_addr;
    next_addr = _bitmap->get_next_marked_addr(next_addr + 1, limit);
    
    if (next_addr > threshold) {
      threshold = hr->cross_threshold(pre_addr, next_addr);
    }
  }
  assert(next_addr == limit, "Should stop the scan at the limit.");
}

Here I removed the `Prefetch` because we do not need to touch the heap. For this to be true I also removed the small optimization to use `obj->size()`, which is probably not an optimization if we don't need to do anything with the "object".  

After doing this I agree that reusing `apply_to_marked_objects()` is probably not helping.

-------------

PR: https://git.openjdk.java.net/jdk/pull/3459