RFR: JDK-8264987: G1: Fill BOTs for Survivor-turned-to-Old regions in full gc [v2]

[Hamlin Li]

On Wed, 14 Apr 2021 08:25:28 GMT, Stefan Johansson <sjohanss at openjdk.org> wrote:

>> 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.

Hi Stefan, I think the refactoring you suggested is great, I just modified as you suggested and the pre-submit tests run successfully.

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